RPN Calc vs 3D Stress
Price, ratings, monetisation and update history for both apps, side by side — with what reviewers say about each.
RPN Calc
This app provides a straightforward reverse Polish notation (RPN) calculator experience, inspired by classic HP models. It offers a clean interface with large buttons for quick calculations on iPhone, iPad, and Apple Watch, along with a convenient Today Extension for immediate access.
- Reverse Polish Notation (RPN) input
- Clean, simple interface
- Large, easy-to-press buttons
- iPhone, iPad, and Apple Watch support
- Today Extension for quick access
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A simple reverse polish notation calculator loosely based on my trusty HP-32SII that runs on iPhone, iPad, and Apple Watch! If you are familiar with HP RPN calculators, the operation of this calculator should seem very straightforward - albeit simplified. It can't be programmed, and it doesn't handle binary, or hexadecimal numbers. But, it has a clean, simple interface with large, easy to press buttons. It has all of the functions required for quick calculations when your iPhone is handier than your computer or calculator. And, remember those cool calculator watches from the 1980s? Unfortunately, none of them were RPN. RPN Calc brings a reverse polish notation watch calculator to the Apple Watch! In addition to the standard RPN Calculator app that runs on iPhone, iPad, and Apple Watch, RPN Calc includes a handy micro RPN Calc Today Extension available for convenient access to quick calculations from the pull down Today screen.
3D Stress
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Explore and understand 3D material stress through Mohr's circle. Calculates principal stresses, maximum shear stress and Von Mises stress
Screenshots
Verdict
The clearest difference is update cadence: 3D Stress at every 2 weeks against RPN Calc's every 3 months. RPN Calc's advantage is price ($0.99 vs $2.99). On ads, in-app purchases, device support and iOS requirement there is nothing between them.
Scored on Price · Rating · Positive reviews · Number of ratings · Update frequency · Ads · In-app purchases · Monetization · Best chart rank · Devices · Requires iOS
RPN Calc costs $0.99 and 3D Stress $2.99 up front. Neither carries in-app purchases, so what you see is what you pay.
RPN Calc ships an update every 3 months, 3D Stress every 2 weeks. The most recent releases landed on November 10, 2026 and November 4, 2026 respectively.
| Parameter | RPN Calc | 3D Stress |
|---|---|---|
| Price | $0.99 — better | $2.99 |
| Rating | 4.0 (6 ratings) — better | — |
| Positive reviews | 34.3% of reviews | — |
| Number of ratings | 6 — better | — |
| Update frequency | Every 3 months | Every 2 weeks — better |
| Ads | No | No |
| In-app purchases | No | No |
| Monetization | Free | — |
| Devices | iPhone, iPad, Apple Watch | iPhone, iPad, iPod |
| Requires iOS | 16.0 | 15.0 — better |
| Further details — not scored | ||
| Size | 3 MB | 4 MB |
| Age rating | 4+ | 4+ |
| Developer | James Mattis | Konstantinos Ladiellis |
In-app purchases
RPN Calc
No in-app purchases
3D Stress
No in-app purchases
