# RP42 — A modern RPN calculator

> A modern RPN calculator (iPhone/iPad app by Q, Inc Video.)

- Source: https://appshunter.io/ios/app/rp42/id6479565118 (this page in markdown: same URL + `.md`)
- Developer: [Q, Inc Video](https://appshunter.io/developer/1736047301)
- Category: Utilities
- Price: $19.99
- Age rating: 4+
- Requires: iOS 17.0 · 20 MB
- Languages: American English
- Released: 2026-09-13
- Data updated: 2026-10-02

## What is RP42?

RP42 is an advanced calculator with a visible stack of type-aware cards, units that compose, and colors and matrices 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: the discipline as old as the HP-35 and still the fastest way to compute.  The surface is modern SwiftUI.

THE STACK, MADE VISIBLE
Every value on the stack is a card, and what's on the card depends on what's in your hand.  Numbers render in real LaTeX for publication-quality look.  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.  Tap any card for a typed detail sheet: chromaticity diagrams, matrix grids, property breakdowns.

CARD CALCS — WHOLE PROBLEMS, ONE CARD
Swipe past the keypad and word categories to reach a grid of tiles for common problems.  Tap TVM for time-value-of-money.  RunPace for the pace-time-distance triangle.  RunPlan for Daniels VDOT race predictions.  Pizza Dough for a hydration-and-yeast recipe.  Each opens a card of named rows you fill 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.

EQUATIONS ON THE STACK
Push an equation the way you'd push a number.  QUAD-EQ gives you ax² + bx + c = 0, rendered in publication-quality LaTeX.  Tap it and a live solver opens: pick which variable to solve for, type the other values, 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.

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.  All eight SI dimensions, all the time.

COLOR AND LINEAR ALGEBRA, FIRST-CLASS
Push an RGB value and convert to XYZ, Lab, ICtCp, or YCbCr.  EOTF and color space ride along as proper stack types.  20+ RGB working spaces — every SMPTE and camera-log space you'd want, plus ACES.  Matrices are first-class: 3×3 multiply, dot, cross, Cramer's rule for systems of three.  The chromaticity diagram and the matrix detail sheet live one tap away.

AND 290+ FUNCTIONS ALL TOLD
REPL plus slab keypad.  Tooltips on every word with 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: SMPTE drop and non-drop; 24, 23.976, 29.97, 30 fps.  Fractions: exact arithmetic on rationals.  Geometry: circles, triangles, distance, area.

FOUR THEMES
Midnight, Obsidian, Dusk, Arctic.  Pick one in Settings; the choice persists.

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.

New to RPN?  You'll be fluent in five minutes.  Already fluent?  You're home.

## Is RP42 safe?

Not enough ratings yet to assess RP42.

## Is RP42 legit?

RP42 appears legitimate, with a few gaps. The AppsHunter Legitimacy Score for RP42 is 63 out of 100 points across 7 verification signals from App Store and developer data.


## Version history (last 1 release)

### 1.0 — 2026-09-13

No release notes.

## More apps by Q, Inc Video

- [Fan Tagger](https://appshunter.io/ios/app/fan-tagger/id6670758810)

All apps by Q, Inc Video: https://appshunter.io/developer/1736047301

---

*Data collected daily from the US App Store and indexed by [AppsHunter](https://appshunter.io/). User reviews are verbatim App Store reviews. Ratings, prices and chart positions refresh continuously; this snapshot is from 2026-10-02.*