Willis15C · RPN scientific calculator
Trust every digit,
in Reverse Polish Notation.
A programmable RPN scientific calculator for people who think in a stack. It computes in true 10-digit decimal arithmetic, not binary floating point, so 0.1 + 0.2 is 0.3, and rounds the way a classic scientific calculator rounds, from 1e-99 to 9.999999999e99.
A classic faceplate, drawn in code
A brushed-metal panel, bevelled keys, gold and blue shifted functions and a seven-segment display. It stays sharp at any size, and on iPhone it fills the screen in landscape.
A complete scientific calculator
The math engine has been checked against more than 20,000 independently computed cases.
Programming
- Keystroke programs with labels and subroutines
- Loops, tests and flags
- SOLVE finds the roots of your own functions
- Numerical integration of your own functions
Complex & matrices
- Complex numbers, with functions that work on both parts
- Up to five matrices
- Inverse, determinant and linear systems
Statistics & more
- Statistics and linear regression
- Permutations, combinations and random numbers
- Hyperbolic functions, polar and rectangular conversion
- Time and angle conversion
See the whole stack
- Slide-out panel with X, Y, Z, T and LAST X
- Storage registers update as you work
- The imaginary stack in complex mode
Built for the keyboard
- On Mac, type digits and operators directly
- Return for ENTER
- An on-screen key map any time
- Copy and paste the X value
Always where you left it
- Stack, registers, programs and settings saved automatically
- Built-in help with worked examples
- No account, no ads, no tracking
- No internet connection needed — Privacy Policy
$3.99. Once.
No subscription, no in-app purchases, no account.
Willis15C runs on iPhone and, once it clears App Store review, on Mac (macOS 26 or later). It is not available for iPad, Windows, Android or Linux. Privacy Policy
We’re happy to help
Something not adding up, or a feature you’d like to see? Reach out through our support center.
Willis15C is original software. It contains no third-party firmware or ROM, and Lacy Digital Labs, LLC is not affiliated with or endorsed by any calculator manufacturer.