RP42 · for iPhone and Mac
A modern calculator, powered by RPN.
The discipline of the stack, the finesse of SwiftUI.
New
RP42 comes to the Mac.
Same engine, same cards, at desk‑scale. Three columns, keyboard‑driven, ⌘‑shortcuts for everything. Read the Mac guide →
Why RP42
Built for the way you actually think.
RP42 is an advanced calculator with a visible stack of type-aware cards, units that compose, and color and matrix types as first-class citizens. 290+ built-in functions — from finance and statistics to timecode, color spaces, and linear algebra — sit one tap away. Under the hood it's pure RPN — a discipline as old as the HP-35 and still the fastest way to compute. The surface is modern SwiftUI; the engine is rpn-lang, open source. New to RPN? You'll be fluent in five minutes. Already fluent? You're home.
Feature 01 · Stack
The stack, made visible.
Every value on the stack is a card — what's on the card depends on what's in your hand. Numbers render in real LaTeX for publication-quality look and feel. Booleans get a colored pill. Integers wear a radix badge. Colors show their swatch. Vectors and matrices lay out in grids. Quantities carry their unit. Timecode keeps its frames. Open any card for a typed detail sheet: chromaticity diagrams, matrix grids, property breakdowns. Calculator stacks have always known their types. RP42 shows you.
Feature 02 · Card calcs
Whole problems, one card.
Swipe past the keypad and the word categories to reach a grid of tiles for common calculating problems. Tap TVM for time-value-of-money, RunPace for the pace-time-distance triangle, RunPlan for Daniels VDOT predictions, Pizza Dough for a hydration-and-yeast recipe. Each opens a card of named rows you fill in by tapping — pick which variable to solve for, edit any input, watch the rest recompute. Quantity rows carry per-row unit chips; times read as clock; RunPlan's tables let you tap a predicted race time to lift it onto the stack.
Feature 03 · Equations
Equations on the stack.
Push an equation the way you'd push a number. QUAD-EQ gives you
a x² + b x + c = 0, rendered in publication-quality LaTeX. Tap it and a
live solver opens: pick which variable to solve for, type the
other values in a numeric-keypad field, and the answer updates as you scrub —
no separate "compute" step. Twelve canned formulas ship built in — Ohm's law,
kinematics, Pythagoras, circle area, sphere volume, wave phase velocity, kinetic
energy — plus ->EXPR to lift your own from a string. Push the
answer back to the stack as a bare value or as a self-documenting
x = 108.333 equation. It's a calculator that finally understands what
an equation is.
Feature 04 · Units
Units that just work.
Tap a value, pick a unit. RP42 carries dimensional analysis through every
operation: add 0.45_m to 12_in and you get a Quantity,
not a stale double. CONVERT goes between any two compatible units;
SIMPLIFY normalizes; UNIT-DEF lets you name your own.
Length, mass, time, current, temperature, amount, luminous, angle — all 8 SI
dimensions, all the time.
Feature 05 · Color & Linear algebra
Color and matrices, first-class.
RP42 speaks color and linear algebra natively. Push an RGB value and you can convert to XYZ, Lab, ICtCp, or YCbCr; the EOTF and color space ride along as proper stack types. Matrices are first-class too — 3×3 multiply, dot, cross, Cramer's rule for systems of three. The chromaticity diagram and the matrix detail sheet both live one tap away.
Everything else
And 290+ functions of toolkit, all told.
REPL + slab keypad
Type RPN directly or tap glass keys.
Tooltips on every word
Long-press any function for stack effects and a one-liner.
Finance
HP-style TVM with P/Y, C/Y, and continuous compounding. Cashflows, NPV/IRR. 38 financial functions.
Statistics
Mean, median, regression, percentiles.
Timecode math
SMPTE drop and non-drop. 24, 23.976, 29.97, 30 fps.
Fractions
Exact arithmetic on rationals.
Geometry
Circles, triangles, distance, area.
User-defined words and units
Save your own; they persist across sessions.
Themes
Four themes, all built in.
Midnight is the default. Pick one in Settings; the choice persists.
Mac · Menus and shortcuts
Menu bar for everything.
Every word in every package is one menu pick away — the Words menu is built live from the engine. Tiles puts every card‑calc a click away. ⌘‑shortcuts for the essentials: ⌘, Settings, ⌘1 / ⌘2 / ⌘3 jump focus between REPL, stack, sidebar. ⌘= / ⌘‑ walk precision. ⌘C / ⌘⇧C copy the top‑of‑stack card's display or RPN deparse. Type RPN straight into the REPL and hit Return.
Under the hood
Built on rpn-lang.
RP42 is built on rpn-lang, an open-source RPN engine. The dictionary is real Forth-style: words combine, definitions stack, and you can teach it new ones. The Swift layer is purely UI; the math is C++. The engine is yours to read, fork, or embed.