A notepad that does the math.
Type text and numbers naturally, and see the answers line by line.

Open Reckon →

Docs · Syntax · Glossary · Units · For LLMs

Reckon is a free, open-source take on notepad calculators like Numi and Soulver. It runs in any browser, installs as an app, works offline, and keeps your notes on your own device.

Features

Available now

• Answers as you type, aligned beside each line. Click an answer, or press ⌘/Ctrl ⇧ C, to copy it.
• Natural syntax: 20% of 50, 10% off 80, 5 as % of 20, 3 apples + 2 apples
• Variables and references: hourly rate = 85, prev, line3, sum, avg, count, min, max
• Units: 5 km in miles, 60 mph in km/h, 6 ft 2 in in cm, 72 °F in °C, 1 GiB in MB, 2 cups in ml, CSS 12pt in px
• Currencies and crypto: $30 in EUR, €20 + $5, 0.01 BTC in USD, with rates cached for offline use
• Units that cancel: hourly rate = $85/h, then 12.5 h × hourly rate is $1,062.50
• Dates and time zones: today + 2 weeks, days until Dec 25, next friday at 3pm, 3pm PST in London, time in Tokyo
• Conditions and comparisons: if total > $100 then 10% off total else total, 5 km > 3 miles, and/or/not, bitwise & | xor << >>
• Your own functions and units: tip(bill, rate) = bill × rate, 1 sprint = 2 weeks, even a metric 1 cup = 250 ml
• Precise decimals: 0.1 + 0.2 is 0.3, not 0.30000000000000004
• Many notes with search, pinning, trash and undo
• Share links: the note is compressed into the URL itself, so no server ever sees it
• Export with answers as aligned text, Markdown tables or a printable web page
• Backup and restore as JSON, plus import of .txt and .md files
• Syncs between open tabs and works offline once loaded
• Command palette (⌘/Ctrl K), autocomplete for variables, units and functions, and a row of math keys above the phone keyboard
• Settings: theme, font size, number format, decimal places, angle unit, and a switch that turns off all network requests
• Light and dark themes, a phone layout, and screen reader support

For developers and AI (see the docs)

• Links that open a note: …/reckon/#/new?text=<percent-encoded note>, easy for any LLM or script to write
• engine.js: the calculation engine as one ES module, evaluate(note) in Node, Deno or a browser
• llms.txt, a prompt that makes any chat assistant answer with a clickable Reckon link, and an agent skill

Syntax at a glance

What	Examples
Numbers	1,000 · 1_000 · 1.5e3 · 1.5k · 2M · 3bn · 2 million · 0xFF · 0b1010 · ½
Operators	+ - * / ^ mod ! · × ÷ − · plus, times, divided by · 3 x 4 · 2(3 + 4) · 2pi
Percentages	20% of 50 · 50 + 10% · 10% off 50 · 10% on 50 · 5 as % of 20 · 5 is what % of 20 · 20% of what is 5
Variables	rent = 1,200 · multi-word names: monthly rent * 12
References	prev / ans (last answer) · line3 (answer on line 3)
Totals	sum / total · avg · count · min · max (each covers the lines since the last heading or blank line)
Functions	sqrt · cbrt · root(x, n) · round(x, 2) · floor · ceil · abs · sin / cos / tan (degrees) · log · ln · exp · fact
Constants	pi / π · e · tau · phi
Output formats	255 in hex · 10 in binary · 8 as oct · 1500 in sci
Units	5 km in miles · 1 m + 20 cm · 60 km/h in m/s · 3 m × 4 m · sqrt(16 m²) · 1/2 cup · 5' 10" · 1 h 30 min · 100 °C in °F · 1 GB in MiB · 24px in pt
Dates	today · tomorrow at 9am · next friday · Dec 25 · 2026-07-04 · today + 2 weeks · Jan 31 + 1 month · Dec 25 - today · days until Dec 25 · 3 days ago · in 45 min
Times and zones	3pm · 15:45 · noon · 3pm + 90 min · 5pm - 3pm · now in Tokyo · time in New York · 3pm PST in London · 9:00 EST to CET · 3pm in UTC+5:30
Currency	$30 · €20 + $5 · 100 GBP to yen · 20 canadian dollars in USD · 0.5 BTC in USD · 100k sats in USD · $85/h × 37.5 h
Structure	# Heading · // comment · label: 42 (text before a colon is ignored)

A line Reckon can't make sense of shows no answer, never a wrong one.

Keyboard shortcuts

Shortcut	Action
⌘/Ctrl K	Command palette: every action, plus jump to any note
⌘/Ctrl ⇧ C	Copy the answer on the current line
⌘/Ctrl /	Comment or uncomment lines
⌘/Ctrl F	Find and replace
Tab	Accept an autocomplete suggestion (Ctrl Space to ask for one)
?	Show all shortcuts (also the keyboard button in the notes list)

Your data

• Notes are stored in your browser (IndexedDB). There are no accounts, no servers and no analytics.
• The only network requests fetch exchange rates (open.er-api.com, falling back to Frankfurter; CoinGecko only if a note mentions crypto). They never include your notes, and you can turn them off in Settings.
• Clearing site data deletes your notes, so use Export all notes (sidebar footer) for a backup.
• A share link holds the whole note in the part of the URL after #, which browsers never send to a server.

Development

Requires Node 22+.

npm install
npm run dev          # http://localhost:5173/reckon/
npm test             # unit and golden tests (Vitest)
npm run test:e2e     # browser tests (Playwright, desktop and mobile)
npm run lint && npm run typecheck
npm run screenshots  # regenerate the README screenshots
node scripts/smoke-engine.mjs  # check dist/engine.js after a build

Every push to main runs CI and deploys to GitHub Pages.

• CHANGELOG.md lists what changed in each version.
• PLAN.md has the product spec, full syntax reference and roadmap.
• AGENTS.md covers project structure, conventions and how-to recipes (adding a unit, a function, or new syntax).

Built with TypeScript, Vite, CodeMirror 6, decimal.js, Temporal (with temporal-polyfill), IndexedDB (via idb) and vite-plugin-pwa. Exchange rates come from ExchangeRate-API, Frankfurter and CoinGecko.

How Reckon compares

Reckon builds on ideas from these notepad calculators. Details come from each project's site or repo as of October 2026. "See site" means we didn't confirm the detail.

App	Platforms	Open source	Price	Ideas Reckon borrows
Reckon	Any browser; installable PWA	✅ MIT	Free	Works offline, stores everything locally, shares notes by link
Numi	macOS, Windows, Linux CLI, Alfred	CLI only (repo)	See site	Natural phrases ($20 in euro - 5% discount), # headers, prev/sum/avg, CSS units, JS extensions
numbr	Web, Chrome extension	✅ (repo)	Free	TypeScript parser/evaluator split, share by URL, k/M suffixes, ignores surrounding text
Soulver	macOS, iOS, iPadOS	❌	Paid	Live line references, subtotals and totals, multi-word variables, percentage phrasing, calendar math
Parsify	macOS, Windows, web	❌	Free (5 lines) or €30 one-time	Over 200 currencies with hourly rates, time zones, plugins, theming
Notes Calculator	Mac, Windows, Linux, iOS, Android, web	❌	One free note; lifetime purchase for more	Large-number shorthand, hex/binary, conditionals, offline-first
Antinote	macOS	❌	See site	Scratchpad feel, sum/average/count, reactive variables, local-only privacy
Calcator	macOS, Windows, Linux	See site	Free	Variable autocomplete, multi-cursor editing, number-format settings, tabs
Ganaka	macOS 14+	✅	Free	$1/$last line refs, min/max/count, Unicode × ÷ −, tabbed workspaces
Figr	See site	See site	See site	The minimal notepad-calculator framing

Thanks to all of them for the inspiration. The project plan has a longer breakdown, including what Reckon avoids.

License

Reckon is released under the MIT License. © 2026 Sam Parmar.