One calculator.
Six modes.
One editable tape.
Six modes share one engine and one editable paper tape. Correct any earlier step and every result that depended on it recomputes on its own.
Simple and School modes are free on all platforms. Linux version free. Windows is available now — macOS, Linux, Android, iOS, and Samsung Watch are on the way.
Start simple. Go as deep as you need.
Every mode shares one engine and one paper tape. Switch between them from the mode strip at the top of the window, and your tape and settings follow you.
Simple
The everyday four-function calculator — memory keys, a running grand total, and a docked tape. Free to use, with no time limit.
School
A full scientific calculator for study — natural textbook entry, graphing, 3D surfaces, statistics, equations, and a spreadsheet — following the conventions of graphing calculators like the Casio fx-CG100. Free, with no time limit.
Engineer
Everything in School, plus a computer-algebra notebook, numerical solvers, matrices and vectors, and a units-and-constants library. Exact and decimal answers sit side by side.
Financial
Time value of money, cash flow with NPV and IRR, amortization, bonds, and depreciation, worked the way the TI BA II Plus Professional does, on a two-colour printing tape.
Programming
Turn a calculation you keep repeating into a saved sequence in the Catena language. Record it by pressing keys, snap it together as blocks, or type it out.
Programming+
Everything in Programming, plus an AI assistant that drafts Catena sequences from a plain-language description — using your own provider key.
One engine. One tape. Every mode.
Every mode computes on the same engine and writes to the same editable paper tape: a sum from Simple, a step from School, a cash flow from Financial all land on one shared log. Correct a number partway up the list and every row that depended on it recomputes automatically, showing the old value struck through against the new. It is the check-and-correct idea that makes the tape worth learning even if you only ever add up a shopping list.
Free on Linux. Free to start everywhere else.
The Linux build is the complete app, free. On every platform, Simple and School are also free forever, with no timer — School alone is a complete scientific calculator, enough for most everyday and study work. You only pay if you need unlimited time in Financial, Engineer, or Programming mode. No ads and no account to create — your work stays on your own device.
Teach it once with Catena.
Catena is Castiel's small language for calculation sequences: prompt for values, do the arithmetic, make a decision, show the result. Build a sequence three ways — record it by pressing keys, assemble it from visual blocks, or type it as text — then step through it line by line in the debugger. A sequence can compute and ask you questions, but on its own it cannot reach your files or the network, so one a colleague sends you is safe to open and run.
A manual written to be read.
Every mode, every surface, and the mathematics behind the functions — explained plainly, then shown key-by-key in Castiel.
Desk, pocket, and wrist.
Desktop
The full app on Windows today, with macOS and Linux ready, release pending.
Mobile
Android and iOS are developed and ready, release pending. Sharing the same engine and paper tape as the desktop app.
Watch
A Samsung Watch is in final stage of release process, with Simple mode only.
Put a serious calculator on your desk.
Free to try — Simple and School have no limits at all. A small payment removes the session limit on the advanced modes; Linux ships the full app for free.