# Hopi

> Free online calculators, converters, generators and tools. Every tool runs entirely in your browser (no sign-up, nothing you enter leaves your device), UK-focused where it matters. Agents can call the same tools through the remote MCP server at https://mcp.hopi.co.uk/mcp (Streamable HTTP, no auth; guide at https://hopi.co.uk/mcp/) or in-browser via WebMCP - see https://hopi.co.uk/webmcp/ and https://hopi.co.uk/webmcp.json.

Full reference with each tool's FAQs: https://hopi.co.uk/llms-full.txt

Machine-readable discovery: OpenAPI spec at https://hopi.co.uk/openapi.json, WebMCP tool catalogue at https://hopi.co.uk/webmcp.json, agent manifest at https://hopi.co.uk/.well-known/ai-plugin.json.

## Maths & percentages
- [Birthday magic square generator](https://hopi.co.uk/birthday-magic-square/): Any date as a Ramanujan 4 × 4 magic square.
- [Collatz conjecture calculator (3n + 1)](https://hopi.co.uk/collatz-conjecture-calculator/): 3n + 1 steps, peak and path to 1.
- [Discount & sale price calculator](https://hopi.co.uk/discount-calculator/): Sale price, savings and real discount.
- [Fourier epicycle drawer](https://hopi.co.uk/fourier-epicycle-drawer/): Spinning circles redraw any shape you draw.
- [Fraction calculator](https://hopi.co.uk/fraction-calculator/): Add, subtract, multiply or divide fractions.
- [Fraction to decimal converter](https://hopi.co.uk/fraction-to-decimal/): Fractions to decimals and back, simplified.
- [Mean, median & mode calculator](https://hopi.co.uk/average-calculator/): Mean, median, mode and more from any list.
- [Number facts finder](https://hopi.co.uk/number-facts-finder/): Primes, factors and 30 special properties.
- [Percentage calculator](https://hopi.co.uk/percentage-calculator/): Work out any percentage, three ways.
- [Percentage change calculator](https://hopi.co.uk/percentage-change-calculator/): Increase or decrease between two values.
- [Percentage decrease calculator](https://hopi.co.uk/percentage-decrease-calculator/): Take a percentage off any starting number.
- [Percentage difference calculator](https://hopi.co.uk/percentage-difference-calculator/): Symmetric difference between two values.
- [Percentage increase calculator](https://hopi.co.uk/percentage-increase-calculator/): Add a percentage rise to any starting number.
- [Prime number & factor calculator](https://hopi.co.uk/prime-factorisation-calculator/): Is it prime? Factors and divisors.
- [Ratio calculator](https://hopi.co.uk/ratio-calculator/): Simplify ratios or find the missing value.
- [Right-angle triangle calculator](https://hopi.co.uk/triangle-calculator/): Solve a right triangle from any two values.
- [Roman numerals converter](https://hopi.co.uk/roman-numerals-converter/): Numbers to Roman numerals and back.
- [Rounding & significant figures calculator](https://hopi.co.uk/rounding-calculator/): Round to decimal places or sig figs.
- [Scientific calculator](https://hopi.co.uk/scientific-calculator/): Trig, powers, logs and more.
- [Sequence solver](https://hopi.co.uk/sequence-solver/): Find the rule and the next terms.
- [Spiral art generator](https://hopi.co.uk/spiral-art-generator/): Rolling-wheel patterns with petal maths.
- [Standard deviation calculator](https://hopi.co.uk/standard-deviation-calculator/): Sample and population sd from your data.
- [String art generator](https://hopi.co.uk/string-art-generator/): Times-table curves and photo thread portraits.

## Money & UK
- [Bill splitter](https://hopi.co.uk/bill-splitter/): Split a bill fairly, item by item.
- [CAGR calculator](https://hopi.co.uk/cagr-calculator/): Annual growth rate from start and end.
- [Compound interest calculator](https://hopi.co.uk/compound-interest-calculator/): Watch savings grow year by year.
- [Currency converter](https://hopi.co.uk/currency-converter/): 150+ currencies at live hourly mid-market rates.
- [Day rate to salary calculator](https://hopi.co.uk/day-rate-calculator/): Day rate to annual salary and back.
- [Energy running cost calculator](https://hopi.co.uk/energy-cost-calculator/): Appliance running costs per day and year.
- [Fuel cost calculator](https://hopi.co.uk/fuel-cost-calculator/): What any journey costs in fuel.
- [Gold coin value calculator](https://hopi.co.uk/gold-coin-value-calculator/): Melt value and dealer premium for gold coins and bars.
- [Gold return calculator](https://hopi.co.uk/gold-return-calculator/): Your gain on gold before and after UK inflation.
- [Gold value calculator](https://hopi.co.uk/gold-value-calculator/): Melt value of gold jewellery and scrap at today's price.
- [Holiday entitlement calculator](https://hopi.co.uk/holiday-entitlement-calculator/): Statutory annual leave for your pattern.
- [Hourly wage to salary converter](https://hopi.co.uk/hourly-to-salary/): Hourly rate to annual salary and back.
- [Inheritance tax calculator](https://hopi.co.uk/inheritance-tax-calculator/): IHT on your estate, with pensions from 2027.
- [Invoice generator](https://hopi.co.uk/invoice-generator/): Make UK invoices and VAT invoices as PDFs.
- [Loan repayment calculator](https://hopi.co.uk/loan-calculator/): Monthly payments and true cost of a loan.
- [Margin & markup calculator](https://hopi.co.uk/margin-calculator/): Profit, margin and markup from two numbers.
- [Mortgage overpayment calculator](https://hopi.co.uk/mortgage-overpayment-calculator/): Time and interest saved by paying extra.
- [Mortgage repayment calculator](https://hopi.co.uk/mortgage-calculator/): Monthly payment, total interest, total cost.
- [National Insurance calculator](https://hopi.co.uk/national-insurance-calculator/): Employee NI on your salary, 2026/27.
- [Net to gross salary calculator](https://hopi.co.uk/net-to-gross/): The gross salary behind your take-home target.
- [Overtime pay calculator](https://hopi.co.uk/overtime-pay-calculator/): Pay at standard, time and a half and double.
- [Payment link button generator](https://hopi.co.uk/payment-link-button-generator/): Pay buttons for PayPal.Me, Revolut and more.
- [Pro-rata salary calculator](https://hopi.co.uk/pro-rata-salary-calculator/): Full-time salary scaled to your hours.
- [Redundancy pay calculator](https://hopi.co.uk/redundancy-pay-calculator/): Statutory redundancy pay, 2026 caps.
- [Savings goal calculator](https://hopi.co.uk/savings-goal-calculator/): How long to reach your savings goal.
- [Stamp duty calculator](https://hopi.co.uk/stamp-duty-calculator/): SDLT for your next move, band by band.
- [Student loan repayment calculator](https://hopi.co.uk/student-loan-calculator/): Monthly student loan repayments by plan.
- [Take-home pay calculator](https://hopi.co.uk/take-home-pay-calculator/): Your salary after tax and NI, 2026/27.
- [Tip calculator](https://hopi.co.uk/tip-calculator/): Tip, total and each person's share.
- [UCAS points calculator](https://hopi.co.uk/ucas-points-calculator/): A-level grades to UCAS tariff points.
- [UK inflation calculator](https://hopi.co.uk/inflation-calculator/): What past money is worth today, via CPI.
- [Unit price comparison](https://hopi.co.uk/unit-price-calculator/): Compare price per kg, litre or item.
- [VAT calculator](https://hopi.co.uk/vat-calculator/): Add or remove 20% VAT from any amount.

## Text & writing
- [AI prompt anonymiser](https://hopi.co.uk/ai-prompt-anonymiser/): Hide personal details before you prompt an AI.
- [AI prompt builder](https://hopi.co.uk/ai-prompt-builder/): Build clear prompts for ChatGPT and Claude.
- [Alphabetical order sorter](https://hopi.co.uk/alphabetical-order/): Sort any list A to Z or Z to A.
- [Caesar cipher & ROT13](https://hopi.co.uk/caesar-cipher/): Encode or decode with a Caesar shift.
- [Case converter](https://hopi.co.uk/case-converter/): Switch text between four cases in a click.
- [Character counter](https://hopi.co.uk/character-counter/): Live character counts and limit checks.
- [Duplicate line remover](https://hopi.co.uk/remove-duplicate-lines/): Delete repeated lines, keep the order.
- [Emoji remover](https://hopi.co.uk/emoji-remover/): Strip every emoji and see how many went.
- [Find and replace](https://hopi.co.uk/find-and-replace/): Search and replace with regex and live counts.
- [Hidden character and AI watermark checker](https://hopi.co.uk/hidden-character-checker/): Find AI text artefacts and hidden characters.
- [Leak tracer](https://hopi.co.uk/leak-tracer/): Watermark each copy, trace the one that leaks.
- [Lorem ipsum generator](https://hopi.co.uk/lorem-ipsum-generator/): Classic placeholder text in one click.
- [Morse code translator](https://hopi.co.uk/morse-code-translator/): Text to Morse and back, live.
- [NATO phonetic alphabet translator](https://hopi.co.uk/nato-phonetic-alphabet/): Spell words as Alfa Bravo Charlie.
- [Online notepad](https://hopi.co.uk/online-notepad/): Jot notes that autosave in your browser.
- [Reading time calculator](https://hopi.co.uk/reading-time-calculator/): How long your text takes to read or speak.
- [Remove extra spaces](https://hopi.co.uk/remove-extra-spaces/): Collapse repeated spaces and tidy lines.
- [Remove line breaks](https://hopi.co.uk/remove-line-breaks/): Turn broken lines back into flowing text.
- [Reverse text generator](https://hopi.co.uk/reverse-text/): Flip characters, word order or letters.
- [Small text generator](https://hopi.co.uk/small-text-generator/): Tiny superscript, small caps and wide text.
- [Special characters to copy](https://hopi.co.uk/special-characters/): One-tap copy for symbols and accents.
- [Text repeater](https://hopi.co.uk/text-repeater/): Repeat text with separators and numbers.
- [Text splitter](https://hopi.co.uk/text-splitter/): Split text into chunks or at a delimiter.
- [Unicode inspector](https://hopi.co.uk/unicode-inspector/): See the code point behind every character.
- [Upside down text](https://hopi.co.uk/upside-down-text/): Flip your text upside down to paste anywhere.
- [Vigenere cipher](https://hopi.co.uk/vigenere-cipher/): Encode or decode with a keyword.
- [Word counter](https://hopi.co.uk/word-counter/): Words, characters and reading time, live.
- [Word frequency counter](https://hopi.co.uk/word-frequency-counter/): Your most used words, counted live.
- [Zero-width text hider](https://hopi.co.uk/zero-width-text-hider/): Hide a secret message in invisible characters.

## Colour & design
- [CMYK to RGB converter](https://hopi.co.uk/cmyk-to-rgb/): CMYK values to RGB and hex, both ways.
- [CSS gradient generator](https://hopi.co.uk/css-gradient-generator/): Two colours to ready-made gradient CSS.
- [Colour name finder](https://hopi.co.uk/colour-name-finder/): Nearest CSS colour name for any hex code.
- [Colour palette from image](https://hopi.co.uk/image-palette/): Get 6 dominant colours from any photo.
- [Colour palette generator](https://hopi.co.uk/colour-palette-generator/): Five-swatch palettes from one base colour.
- [Colour picker](https://hopi.co.uk/colour-picker/): Pick a colour, get hex, RGB and HSL.
- [Contrast checker](https://hopi.co.uk/contrast-checker/): WCAG contrast ratios with pass badges.
- [HSL converter](https://hopi.co.uk/hsl-converter/): Hex to HSL, or HSL sliders to hex.
- [Hex to RGB converter](https://hopi.co.uk/hex-to-rgb/): Hex codes to RGB and HSL, with preview.
- [Image colour picker](https://hopi.co.uk/image-colour-picker/): Click any pixel in a photo to get its colour.
- [RGB to hex converter](https://hopi.co.uk/rgb-to-hex/): RGB values to a hex code, with preview.
- [Random colour generator](https://hopi.co.uk/random-colour-generator/): Truly random colours with hex and RGB.
- [Shades & tints generator](https://hopi.co.uk/colour-shades-generator/): Five tints and five shades from any colour.

## Date & time
- [Add working days](https://hopi.co.uk/add-working-days/): Find the date N working days away.
- [Age calculator](https://hopi.co.uk/age-calculator/): Exact age, and days to the next birthday.
- [Cat age calculator](https://hopi.co.uk/cat-age-calculator/): Cat years to human years, the 15/24/+4 chart.
- [Countdown to a date](https://hopi.co.uk/countdown-timer/): Live countdown to any date and time.
- [Date calculator](https://hopi.co.uk/date-calculator/): Add or subtract time from any date.
- [Day of the week finder](https://hopi.co.uk/day-of-week-calculator/): Find the weekday for any date.
- [Days between dates calculator](https://hopi.co.uk/days-between-dates/): Count the days between any two dates.
- [Days until Christmas](https://hopi.co.uk/days-until-christmas/): Live sleeps countdown to 25 December.
- [Dog age calculator](https://hopi.co.uk/dog-age-calculator/): Dog years to human years, by size.
- [Hours calculator](https://hopi.co.uk/hours-calculator/): Hours between two times, minus breaks.
- [Leap year checker](https://hopi.co.uk/leap-year-checker/): Yes or no, with the 4/100/400 rule shown.
- [Minutes to hours converter](https://hopi.co.uk/minutes-to-hours/): Minutes to h:mm and decimal hours.
- [Moon phase calculator](https://hopi.co.uk/moon-phase-calculator/): Moon phase and illumination for any date.
- [New Year countdown](https://hopi.co.uk/new-year-countdown/): Live countdown to midnight on 1 January.
- [Pomodoro timer](https://hopi.co.uk/pomodoro-timer/): 25/5/15 focus timer with session dots.
- [Same calendar year finder](https://hopi.co.uk/same-calendar-year-finder/): Find years with exactly the same calendar.
- [Star sign calculator](https://hopi.co.uk/star-sign-calculator/): Find the zodiac sign for any birthday.
- [Stopwatch](https://hopi.co.uk/stopwatch/): Laps with split and total, to the hundredth.
- [Time calculator](https://hopi.co.uk/time-calculator/): Add or subtract hours and minutes.
- [Time zone converter](https://hopi.co.uk/time-zone-converter/): Convert times between world zones.
- [What week number is it?](https://hopi.co.uk/week-number/): Find the ISO week number for any date.
- [Working days calculator](https://hopi.co.uk/working-days-calculator/): Count working days between two dates.

## Random & generators
- [Barcode generator](https://hopi.co.uk/barcode-generator/): Make EAN, UPC and Code 128 barcodes.
- [Bingo card generator](https://hopi.co.uk/bingo-card-generator/): Generate and print UK 90-ball bingo cards.
- [Bulk QR code generator](https://hopi.co.uk/bulk-qr-code-generator/): Up to 1,000 QR codes in one ZIP.
- [Coin flip](https://hopi.co.uk/coin-flip/): Heads or tails with a running tally.
- [Combination generator](https://hopi.co.uk/combination-generator/): Every way of choosing r items from n.
- [Dice passphrase generator](https://hopi.co.uk/dice-passphrase-generator/): Diceware passphrases from real or virtual dice.
- [Dice roller](https://hopi.co.uk/dice-roller/): Roll d4 to d20, totals included.
- [EuroMillions number generator](https://hopi.co.uk/euromillions-generator/): Random EuroMillions lines, drawn fairly.
- [Fortune cookie](https://hopi.co.uk/fortune-cookie/): Crack a cookie for an original fortune.
- [Lottery number generator](https://hopi.co.uk/lottery-number-generator/): Random lines for UK lottery games.
- [Magic 8 ball](https://hopi.co.uk/magic-8-ball/): Shake the ball and get one of 20 answers.
- [Passphrase generator](https://hopi.co.uk/passphrase-generator/): Memorable random word passphrases.
- [Password generator](https://hopi.co.uk/password-generator/): Strong random passwords, made locally.
- [Powerball number generator](https://hopi.co.uk/powerball-generator/): Random UK Powerball lines, drawn fairly.
- [QR code generator](https://hopi.co.uk/qr-code-generator/): Turn any link or text into a QR code.
- [Random coordinates generator](https://hopi.co.uk/random-coordinates/): Fair random points across the globe.
- [Random letter generator](https://hopi.co.uk/random-letter-generator/): Random letters, vowels or consonants.
- [Random number generator](https://hopi.co.uk/random-number-generator/): Fair picks between any two numbers.
- [Random team generator](https://hopi.co.uk/random-team-generator/): Split any list of names into fair teams.
- [Random word generator](https://hopi.co.uk/random-word-generator/): Random words for games and prompts.
- [Reaction time test](https://hopi.co.uk/reaction-time-test/): Time your reflexes in milliseconds.
- [Scratch card](https://hopi.co.uk/scratch-card/): Scratch the foil to reveal a random outcome.
- [Secret Santa generator](https://hopi.co.uk/secret-santa-generator/): Draw names so nobody gets themselves.
- [Set For Life number generator](https://hopi.co.uk/set-for-life-generator/): Random Set For Life lines, drawn fairly.
- [Slot machine name picker](https://hopi.co.uk/slot-machine-picker/): Spin a reel of names to pick a winner.
- [Spin the bottle](https://hopi.co.uk/spin-the-bottle/): Spin a bottle to pick a player at random.
- [Tarot card reader](https://hopi.co.uk/tarot-reader/): Draw a tarot spread from all 78 cards.
- [Thunderball number generator](https://hopi.co.uk/thunderball-generator/): Random Thunderball lines, drawn fairly.
- [Typing speed test](https://hopi.co.uk/typing-speed-test/): Measure your words per minute.
- [Username generator](https://hopi.co.uk/username-generator/): Fresh username ideas, click to copy.
- [Wheel spinner](https://hopi.co.uk/wheel-spinner/): Spin a wheel of names to pick a winner.
- [Would you rather and icebreaker generator](https://hopi.co.uk/would-you-rather-generator/): Balanced questions for groups, with an A/B vote.
- [Yes or no](https://hopi.co.uk/yes-no-wheel/): Let chance decide, yes or no.

## Converters & units
- [Angle converter](https://hopi.co.uk/angle-converter/): Degrees, radians, gradians and turns.
- [Area converter](https://hopi.co.uk/area-converter/): sq m, sq ft, acres, hectares, sq miles.
- [Aspect ratio calculator](https://hopi.co.uk/aspect-ratio-calculator/): Simplify or solve W:H, presets built in.
- [Cm to feet converter](https://hopi.co.uk/cm-to-feet/): Centimetres to feet and inches, both ways.
- [Cups to grams converter](https://hopi.co.uk/cups-to-grams/): US cups to grams for common ingredients.
- [Data size converter](https://hopi.co.uk/data-size-converter/): KB, MB, GB and TB, decimal or binary.
- [Download time calculator](https://hopi.co.uk/download-time-calculator/): Time a download from size and speed.
- [Event wine calculator](https://hopi.co.uk/event-wine-calculator/): How many bottles of red, white and fizz your event needs.
- [Kg to lbs converter](https://hopi.co.uk/kg-to-lbs/): Kilograms to pounds and ounces, both ways.
- [Kg to stone converter](https://hopi.co.uk/kg-to-stone/): Kilograms to stones and pounds, both ways.
- [Lbs to kg converter](https://hopi.co.uk/lbs-to-kg/): Pounds to kilograms, with grams and stones.
- [Length converter](https://hopi.co.uk/length-converter/): mm, cm, m, km, inches, feet, yards, miles.
- [Litres to pints converter](https://hopi.co.uk/litres-to-pints/): Litres to UK pints, both ways.
- [MPG converter](https://hopi.co.uk/mpg-converter/): UK mpg to litres per 100 km, both ways.
- [Miles to km converter](https://hopi.co.uk/miles-to-km/): Miles and kilometres, both ways, live.
- [Military time converter](https://hopi.co.uk/military-time-converter/): 24-hour to 12-hour am/pm and back.
- [Number to words converter](https://hopi.co.uk/number-to-words/): Figures written out in British English words.
- [Oven temperature converter](https://hopi.co.uk/oven-temperature-converter/): Celsius, fan, Fahrenheit and gas mark.
- [Power converter](https://hopi.co.uk/power-converter/): Watts, kW, horsepower and PS, both ways.
- [Pressure converter](https://hopi.co.uk/pressure-converter/): Bar, psi, kPa and atm for tyres and more.
- [Ring size converter](https://hopi.co.uk/ring-size-converter/): UK letters to US numbers and mm, both ways.
- [Shoe size converter](https://hopi.co.uk/shoe-size-converter/): UK shoe sizes to EU and US, men and women.
- [Speed converter](https://hopi.co.uk/speed-converter/): mph, km/h, m/s, knots and Mach.
- [Stone to kg converter](https://hopi.co.uk/stone-to-kg/): Stones and pounds to kilograms, with a chart.
- [Temperature converter](https://hopi.co.uk/celsius-to-fahrenheit/): Celsius and Fahrenheit, both ways, live.
- [Volume converter](https://hopi.co.uk/volume-converter/): ml, litres, pints, gallons, UK and US.
- [Weight converter](https://hopi.co.uk/weight-converter/): Grams, kg, ounces, pounds and stones.

## Health & fitness
- [Alcohol unit calculator](https://hopi.co.uk/alcohol-unit-calculator/): UK units from ABV and volume.
- [BMI calculator](https://hopi.co.uk/bmi-calculator/): Body mass index with the NHS categories.
- [Body fat percentage calculator](https://hopi.co.uk/body-fat-calculator/): Body fat from the US Navy tape method.
- [Calories to kJ converter](https://hopi.co.uk/calories-to-kj/): kcal to kJ and back, exact factor 4.184.
- [Daily water intake calculator](https://hopi.co.uk/water-intake-calculator/): A rough daily fluid guide from your weight.
- [Heart rate zone calculator](https://hopi.co.uk/heart-rate-zones/): Max heart rate and five training zones.
- [Pregnancy due date calculator](https://hopi.co.uk/due-date-calculator/): Due date from your last period, plus weeks.
- [Running pace calculator](https://hopi.co.uk/running-pace-calculator/): Pace, time or distance for any run.
- [Sleep cycle calculator](https://hopi.co.uk/sleep-calculator/): Bedtimes and wake times in 90-minute cycles.
- [TDEE & calorie calculator](https://hopi.co.uk/tdee-calculator/): Daily calories from the Mifflin-St Jeor formula.

## Dev & data
- [AES text encrypter](https://hopi.co.uk/aes-text-encrypter/): Password-encrypt text with AES-256-GCM.
- [Base32 encoder and decoder](https://hopi.co.uk/base32-encoder/): Encode and decode RFC 4648 Base32.
- [Base64 encode & decode](https://hopi.co.uk/base64/): Encode and decode Base64, UTF-8 safe.
- [CAPTCHA embed code generator](https://hopi.co.uk/captcha-embed-code-generator/): Turnstile, hCaptcha and reCAPTCHA code.
- [CSS minifier](https://hopi.co.uk/css-minifier/): Strip CSS whitespace, see bytes saved.
- [CSV to JSON converter](https://hopi.co.uk/csv-to-json/): Turn CSV rows into a clean JSON array.
- [Cipher identifier and auto-decoder](https://hopi.co.uk/cipher-identifier/): Detect and decode unknown encoded text.
- [Cron expression translator](https://hopi.co.uk/cron-expression-parser/): Cron schedules explained in English.
- [Diff checker](https://hopi.co.uk/diff-checker/): Compare two texts, see added and removed lines.
- [Dummy data generator](https://hopi.co.uk/dummy-data-generator/): Fake but safe test records, CSV or JSON.
- [Email header analyser](https://hopi.co.uk/email-header-analyser/): Trace an email's path and check its sender.
- [Email link generator](https://hopi.co.uk/email-link-generator/): Build correctly encoded mailto email links.
- [Encrypted QR code maker](https://hopi.co.uk/encrypted-qr-code/): Password-locked message inside a QR code.
- [Encrypted link maker](https://hopi.co.uk/encrypted-link-maker/): Hide a password-locked message in a link.
- [HMAC generator](https://hopi.co.uk/hmac-generator/): Keyed hash of a message with a secret key.
- [HTML escape & unescape](https://hopi.co.uk/html-escape/): Escape and unescape HTML entities.
- [HTML text & link extractor](https://hopi.co.uk/html-text-extractor/): Pull text, links and emails from HTML.
- [HTTP status code reference](https://hopi.co.uk/http-status-codes/): Search every standard HTTP status code.
- [Hash generator](https://hopi.co.uk/hash-generator/): SHA-256, SHA-1 and SHA-512 hex digests.
- [Hash identifier](https://hopi.co.uk/hash-identifier/): Identify the algorithm behind a hash.
- [Honeypot field generator](https://hopi.co.uk/honeypot-field-generator/): Hidden form fields that catch spam bots.
- [IBAN validator](https://hopi.co.uk/iban-validator/): Validate IBANs and build test ones.
- [Inline CSS converter](https://hopi.co.uk/inline-css-converter/): Turn style blocks into inline styles for email.
- [JSON formatter](https://hopi.co.uk/json-formatter/): Pretty print, minify and validate JSON.
- [JSON to CSV converter](https://hopi.co.uk/json-to-csv/): Turn a JSON array into clean CSV.
- [JSON to TypeScript converter](https://hopi.co.uk/json-to-typescript/): Turn a JSON sample into TypeScript interfaces.
- [JSON to YAML converter](https://hopi.co.uk/json-yaml-converter/): Convert JSON to YAML and back.
- [JWT decoder](https://hopi.co.uk/jwt-decoder/): Decode JWT header, payload and expiry.
- [JWT generator](https://hopi.co.uk/jwt-generator/): Sign a test JWT with HS256, HS384 or HS512.
- [Key pair generator](https://hopi.co.uk/key-pair-generator/): Generate RSA or EC keys as PEM and JWK.
- [Link cleaner and inspector](https://hopi.co.uk/link-cleaner/): Strip tracking and check where a link goes.
- [Lookalike domain generator](https://hopi.co.uk/lookalike-domain-generator/): Every typo and homoglyph of your domain.
- [Markdown previewer](https://hopi.co.uk/markdown-previewer/): Preview markdown as you type.
- [Meta tag generator](https://hopi.co.uk/meta-tag-generator/): Build title, OG and Twitter meta tags.
- [Number base converter](https://hopi.co.uk/number-base-converter/): Binary, octal, decimal, hex and text.
- [Password strength checker](https://hopi.co.uk/password-strength-checker/): Test a password without sending it anywhere.
- [Public key message encrypter](https://hopi.co.uk/public-key-message-encrypter/): Encrypt a message to an RSA public key.
- [Regex tester](https://hopi.co.uk/regex-tester/): Test regex patterns with live matches.
- [Robots.txt generator](https://hopi.co.uk/robots-txt-generator/): Build and download a robots.txt file.
- [SEO meta tag analyser](https://hopi.co.uk/seo-analyzer/): Check a page's SEO meta tags.
- [SQL formatter](https://hopi.co.uk/sql-formatter/): Tidy SQL with clauses on their own lines.
- [Schema markup generator](https://hopi.co.uk/schema-markup-generator/): Build JSON-LD structured data blocks.
- [Slug generator](https://hopi.co.uk/slug-generator/): Titles into clean hyphenated URL slugs.
- [TOTP generator](https://hopi.co.uk/totp-generator/): Live time-based 2FA codes from a secret.
- [Test card number generator](https://hopi.co.uk/test-card-number-generator/): Generate Luhn-valid test card numbers.
- [URL encode & decode](https://hopi.co.uk/url-encoder/): Percent-encode and decode URLs and text.
- [URL parser](https://hopi.co.uk/url-parser/): Split a URL into its decoded parts.
- [UTM link generator](https://hopi.co.uk/utm-link-generator/): Build tagged campaign URLs for GA4.
- [UUID generator](https://hopi.co.uk/uuid-generator/): Random v4 UUIDs, up to 100 at a time.
- [Unix timestamp converter](https://hopi.co.uk/unix-timestamp-converter/): Epoch timestamps to dates and back.
- [User agent parser](https://hopi.co.uk/user-agent-parser/): Read browser, OS and device from a UA string.
- [What your browser reveals](https://hopi.co.uk/browser-fingerprint/): See what any website can read about you.
- [XML formatter](https://hopi.co.uk/xml-formatter/): Pretty print, minify and check XML.
- [env to JSON converter](https://hopi.co.uk/env-to-json/): Convert .env files to JSON and back.

## Crypto
- [Bitcoin address validator](https://hopi.co.uk/bitcoin-address-validator/): Check BTC addresses: legacy, SegWit, Taproot.
- [Crypto DCA calculator](https://hopi.co.uk/dca-calculator/): Average cost across all your buys.
- [Crypto address derivation](https://hopi.co.uk/crypto-address-derivation/): Private key to address, every step.
- [Crypto address generator](https://hopi.co.uk/crypto-address-generator/): Random key to address, 5 chains.
- [Crypto profit calculator](https://hopi.co.uk/crypto-profit-calculator/): Profit, fees and breakeven for any coin.
- [ETH unit converter](https://hopi.co.uk/eth-unit-converter/): Wei, gwei and ETH without rounding errors.
- [Ethereum address checksum](https://hopi.co.uk/eth-address-checksum/): Validate and checksum ETH addresses.
- [Market cap calculator](https://hopi.co.uk/market-cap-calculator/): Price times supply, and the reverse.
- [Position size calculator](https://hopi.co.uk/position-size-calculator/): Units to buy for the risk you choose.
- [Satoshi to BTC converter](https://hopi.co.uk/satoshi-converter/): Sats, BTC, mBTC and bits, both ways.
- [Secret splitter (Shamir's Secret Sharing)](https://hopi.co.uk/secret-splitter/): Split a secret into K-of-N shares.
- [Seed phrase generator](https://hopi.co.uk/seed-phrase-generator/): Generate or check a BIP39 recovery phrase.
- [Staking rewards calculator](https://hopi.co.uk/staking-calculator/): Final balance and true APY from staking.
- [WIF key converter](https://hopi.co.uk/wif-converter/): Bitcoin private key: hex to WIF and back.

## Files & data
- [AI image credentials inspector](https://hopi.co.uk/ai-image-credentials/): See and strip C2PA, prompts, XMP, hidden files.
- [Add to calendar file generator](https://hopi.co.uk/ics-generator/): Make a .ics calendar file for any event.
- [CSV to Excel converter](https://hopi.co.uk/csv-to-excel/): CSV to .xlsx without mangled zeros or dates.
- [Dummy file generator](https://hopi.co.uk/dummy-file-generator/): Test files of an exact size, locally.
- [Dummy image generator](https://hopi.co.uk/dummy-image-generator/): Placeholder images at any size, locally.
- [EXIF remover](https://hopi.co.uk/exif-remover/): Strip GPS and metadata from photos.
- [Excel to CSV converter](https://hopi.co.uk/excel-to-csv/): Turn Excel sheets into clean CSV files.
- [Excel to JSON converter](https://hopi.co.uk/excel-to-json/): Turn Excel sheets into clean JSON.
- [Excel to XML converter](https://hopi.co.uk/excel-to-xml/): Turn Excel sheets into structured XML.
- [Face and number plate blur](https://hopi.co.uk/face-blur/): Pixelate faces and plates in a photo.
- [Favicon generator](https://hopi.co.uk/favicon-generator/): Make a real .ico favicon from any image.
- [File encrypter](https://hopi.co.uk/file-encrypter/): Password-encrypt any file with AES-256.
- [File hash checker](https://hopi.co.uk/file-hash-checker/): Check a file's SHA-256 checksum locally.
- [HEIC to JPG converter](https://hopi.co.uk/heic-to-jpg/): iPhone HEIC photos to JPG, in your browser.
- [ID document redactor](https://hopi.co.uk/id-document-redactor/): Black out and stamp ID before uploading.
- [Image compressor](https://hopi.co.uk/image-compressor/): Shrink photo file sizes with a slider.
- [Image cropper](https://hopi.co.uk/image-cropper/): Crop images with a draggable box, locally.
- [Image resizer](https://hopi.co.uk/image-resizer/): Resize images to exact pixels, locally.
- [Image steganography](https://hopi.co.uk/image-steganography/): Hide a secret message inside a photo.
- [Image to ASCII art converter](https://hopi.co.uk/image-to-ascii-art/): Photos into text art you can copy or download.
- [Image to Base64 converter](https://hopi.co.uk/image-to-base64/): Images to data URIs with copy-ready snippets.
- [Image to SVG vectoriser](https://hopi.co.uk/image-to-svg/): Trace PNG or JPG images into scalable SVG.
- [Image to code converter](https://hopi.co.uk/image-to-code/): Embed images as code, with a size budget.
- [JPG to PDF converter](https://hopi.co.uk/jpg-to-pdf/): Images become PDF pages, in your browser.
- [JPG to PNG converter](https://hopi.co.uk/jpg-to-png/): JPG to lossless PNG in one click.
- [JSON to Excel converter](https://hopi.co.uk/json-to-excel/): Turn a JSON array into a .xlsx file.
- [Merge PDF files](https://hopi.co.uk/merge-pdf/): Combine PDFs into one, in your browser.
- [PDF metadata viewer and remover](https://hopi.co.uk/pdf-metadata-remover/): View and strip hidden PDF metadata.
- [PNG to JPG converter](https://hopi.co.uk/png-to-jpg/): PNG to JPG, white background, sizes compared.
- [Photo metadata (EXIF) viewer](https://hopi.co.uk/exif-viewer/): See a photo's camera, date and GPS data.
- [Product feed generator](https://hopi.co.uk/product-feed-generator/): Spreadsheet to Meta or Google product feed.
- [QR code reader](https://hopi.co.uk/qr-code-reader/): Decode a QR code from an image.
- [Rotate & flip image](https://hopi.co.uk/rotate-image/): Rotate or flip a photo in your browser.
- [SVG to PNG converter](https://hopi.co.uk/svg-to-png/): SVG files to sharp PNGs at 1x, 2x, 4x.
- [Signature generator](https://hopi.co.uk/signature-generator/): Draw and download a transparent signature.
- [Split PDF](https://hopi.co.uk/split-pdf/): Extract pages or split a PDF to a ZIP.
- [WebP to JPG converter](https://hopi.co.uk/webp-to-jpg/): WebP to JPG with a quality slider.
- [XML to Excel converter](https://hopi.co.uk/xml-to-excel/): Turn repeated XML records into .xlsx rows.

## News
- [Your shoe size is counted in barleycorns](https://hopi.co.uk/news/why-shoe-sizes-are-counted-in-barleycorns/) (2026-11-04): Every UK shoe size is one barleycorn longer than the last, a third of an inch first defined by grains of barley. The royal decree said to be behind it is shaky, and the oldest written shoe sizes did not use barleycorns at all.
- [From April 2027 your pension counts towards inheritance tax. Is your estate over the line?](https://hopi.co.uk/news/pensions-inheritance-tax-april-2027/) (2026-10-29): Wednesday's Budget left the £325,000 and £175,000 tax-free bands frozen until 2031. The bigger change is already law: unused pensions join the estate from 6 April 2027, and HMRC expects 10,500 more estates to pay.
- [The full state pension is about to outgrow the tax-free allowance](https://hopi.co.uk/news/state-pension-tax-free-allowance-2027/) (2026-10-26): September's figures have settled next April's rises before Wednesday's Budget. Earnings growth of 3.9 per cent takes the full new state pension to about £13,036 a year, £466 above the frozen £12,570 allowance. Most benefits are on course for 3.5 per cent.
- [What a Bitcoin address actually is, and why a typo can't lose your coins](https://hopi.co.uk/news/what-a-bitcoin-address-actually-is-2026/) (2026-10-18): That jumble of letters and numbers is not random. It is built so a single wrong character is caught and refused before any money moves. Here is how the checksum works, why some addresses start with 1 and some with bc1, and the one thing it cannot catch.
- [There is no such thing as brown light](https://hopi.co.uk/news/there-is-no-brown-light-2026/) (2026-10-16): You can shine red, green or blue onto a wall, but never brown, because brown has no wavelength. Nor does magenta. Here is why the colours you are most sure of exist only in your head, and why that matters for anyone building a web page.
- [Why a computer can't actually pick a random number](https://hopi.co.uk/news/why-a-computer-cannot-be-random-2026/) (2026-10-14): Every dice roll, coin flip and password on this site comes from a machine that does the same thing every time. Here is how it fakes randomness, why a wall of lava lamps helps secure the web, and why you are worse at being random than a dice.
- [The invisible alphabet hiding in your inbox, your documents and your AI prompts](https://hopi.co.uk/news/invisible-characters-ascii-smuggling-2026/) (2026-10-11): Microsoft traced a 2.3 million-a-day phishing run to an unprintable character spliced into the word funding. The same invisible Unicode is used to smuggle instructions past AI assistants. Here is what it is, how to see it, and the honest answer on AI text watermarks.
- [Asked for your passport online? Redact it, stamp it, then send it](https://hopi.co.uk/news/redact-your-id-before-you-upload-it-2026/) (2026-10-10): A year after a Discord contractor leaked around 70,000 ID photos, age checks under the Online Safety Act ask for documents more often than ever. You cannot refuse, but you can hand over the minimum, in a form that is useless to anyone who steals it.
- [Three weeks to the Budget: what is decided, what is floated, and what is noise](https://hopi.co.uk/news/autumn-budget-2026-what-to-watch/) (2026-10-07): John Healey's first Budget is on 28 October. Some of it is already announced, some has a named source behind it, and the rest is kite-flying. Here is how to tell them apart, and how to work out what any line means for your own pay.
- [UK iPhones still can't switch on full iCloud encryption. So encrypt it yourself](https://hopi.co.uk/news/icloud-encryption-uk-encrypt-it-yourself-2026/) (2026-10-06): Apple pulled Advanced Data Protection from the UK twenty months ago over a secret Home Office order, and the tribunal fight is still going. The files that would hurt if they leaked do not need to wait: encrypt them before they go anywhere near the cloud.
- [Your electricity has had no VAT on it since 1 October. Here is what that is really worth](https://hopi.co.uk/news/electricity-vat-cut-october-2026/) (2026-10-05): The 5 per cent rate is zero until 31 March 2027, worth about £45 a year to a typical home and more to anyone on electric heating or charging a car. It also hid a near 6 per cent rise in the unit rate. The sums, and the two dates that matter next.
- [Your 'strong' password is probably weaker than three random words](https://hopi.co.uk/news/three-random-words-password-advice-2026/) (2026-10-01): It is Cyber Security Awareness Month, and the UK's own cyber agency has been saying the same unfashionable thing for years: forget the capital letter and the exclamation mark, and pick three random words instead. Here is the maths behind the advice, and the one account you must protect first.
- [Christmas is 85 days away. Here is the maths for not panicking in December](https://hopi.co.uk/news/85-days-to-christmas-weekly-savings-2026/) (2026-10-01): Twelve weeks, twelve Fridays, three paydays. The Bank of England says the average household spends about £713 more in December than in a normal month. Split from today, that is under £60 a week.
- [Freshers, your student loan is not the debt you think it is](https://hopi.co.uk/news/plan-5-student-loan-freshers-2026/) (2026-09-25): If you started a degree in England this autumn you could borrow more than £60,000 over three years. What you repay has almost nothing to do with that figure. Here is how a Plan 5 loan really works, with the 2026/27 numbers.
- [Hopi's calculators now work inside Claude, Cursor and any AI assistant](https://hopi.co.uk/news/hopi-tools-inside-ai-assistants-mcp/) (2026-09-20): Paste one address into your assistant and over two hundred UK calculators, converters and validators become tools it can call, with the right 2026/27 numbers and a link back to the page. No account, nothing stored.
- [Started a side hustle last year? You have until 5 October to tell HMRC](https://hopi.co.uk/news/register-self-assessment-5-october-2026/) (2026-09-19): Anyone who earned untaxed income for the first time in the 2025/26 tax year, from freelancing to renting out a room, has to register for Self Assessment by 5 October 2026. Here is who it catches, what happens if you miss it, and why £1,000 is the number to remember.
- [Inflation climbs to 3.1%, and you can blame the petrol pump](https://hopi.co.uk/news/uk-inflation-rises-august-2026/) (2026-09-16): UK inflation rose for a second month in August, and almost all of the jump came from motor fuel. Petrol added 9p a litre in a month and diesel 14p. Here is what the new figure does to your pay, your savings and Thursday's rate decision.
- [Waiting for mortgage rates to fall? Why 17 September probably won't be the day](https://hopi.co.uk/news/bank-of-england-rate-decision-september-2026/) (2026-09-06): The Bank of England sets rates again on Thursday, and the market expects a hold, not a cut. Some rate-setters even want a rise. Here is why, and what actually moves the rate on your mortgage.
- [The energy price cap is confirmed: £1,723 from October, up 4%](https://hopi.co.uk/news/energy-price-cap-confirmed-october-2026/) (2026-08-28): Ofgem has set the October to December cap at £1,723 a year, a rise of £60 on the typical bill. Gas is doing all the work, a VAT cut is quietly holding electricity down, and the cap still is not a limit on what you pay.
- [How much of your salary do you actually keep? From £15k to £250k](https://hopi.co.uk/news/take-home-pay-15k-to-250k-2026/) (2026-08-21): We put every salary from £15,000 to £250,000 through our take-home pay calculator. The results show four tax bands, two sweet spots, and one strange zone where a pay rise is worth barely a third of itself.
- [The October energy price cap lands next week: what to expect](https://hopi.co.uk/news/october-energy-price-cap-2026/) (2026-08-20): Ofgem confirms the October to December price cap next Tuesday, and forecasts point to a rise of about 4%. Here is what the cap actually controls, why it is not a cap on your bill, and how to work out your own.
- [Inflation is back up to 2.9%. Here is what it does to your money.](https://hopi.co.uk/news/uk-inflation-rises-july-2026/) (2026-08-19): UK inflation rose to 2.9% in July, its first climb in months, pushed up by energy bills. It nudges above the Bank of England's target and quietly shrinks both pay and savings. Here is what the figures mean for you.
- [How much water do you actually need in hot weather?](https://hopi.co.uk/news/how-much-water-in-hot-weather-2026/) (2026-08-19): The UK has just been through its fifth-hottest day on record. As the fiercest heat passes and hosepipe bans linger, here is how much water you really need, and the tired myths worth dropping.
- [GCSE results tomorrow: what a 9, a 5 and a 4 actually mean](https://hopi.co.uk/news/gcse-9-to-1-grades-explained-2026/) (2026-08-19): GCSE results land on Thursday. If you last sat exams when grades were letters, here is what England's 9 to 1 numbers really mean, the two passes that confuse everyone, and why a 9 beats an old A*.
- [Your pay rose 3.5%. After inflation, almost none of it stuck.](https://hopi.co.uk/news/pay-rise-real-terms-2026/) (2026-08-18): A 3.5% pay rise sounds like a win. Once inflation and tax take their share, and with the job market quietly cooling, most of it disappears. Here is what today's figures say, and how to check your own.
- [Home repossessions are falling, and mostly on old mortgages](https://hopi.co.uk/news/home-repossessions-falling-2026/) (2026-08-17): The latest UK Finance figures show mortgage arrears and repossessions falling for the first time since 2023. The detail behind the numbers is more reassuring than the headline, and it points to one useful move if money ever gets tight.
- [Fiscal drag: how frozen tax thresholds quietly raise your bill](https://hopi.co.uk/news/fiscal-drag-frozen-thresholds-2026/) (2026-08-16): You got a pay rise and kept less of it than you expected. The thresholds that decide how much tax you pay have not moved since 2021, and that stealthy freeze is doing a lot of work.
- [A-level results: the percentage-point trap in the headlines](https://hopi.co.uk/news/a-level-results-percentage-point-trap/) (2026-08-16): Top grades edged up again this year, to 28.5 per cent of entries. Call that a rise of 0.2 per cent and you have made the classic results-day error. Here is what the numbers really say.
- [How many calories do you actually burn in a day?](https://hopi.co.uk/news/how-many-calories-do-you-burn-a-day/) (2026-08-15): The famous 2,000 a day figure is an average dressed up as a rule. Your real number depends on your size, your age and how much you move, and the maths behind it is refreshingly simple.
- [Why Britain still weighs itself in stones](https://hopi.co.uk/news/why-britain-still-weighs-in-stones/) (2026-08-12): The country buys petrol in litres and butter in grams, yet steps on the bathroom scales and reads off a number in stones and pounds. Here is how one stubborn unit survived metrication.