# GO-21 SciRPN

> Retro scientific calculator simulating the HP-21 with RPN and extensive functions. (iPhone/iPad app by Steve Lidie.)

- Source: https://appshunter.io/ios/app/go-21-scirpn/id327651563 (this page in markdown: same URL + `.md`)
- Developer: [Steve Lidie](https://appshunter.io/developer/1869780159)
- Category: Utilities, Productivity
- Price: $2.99
- Rating: 4.90/5 from 13 App Store ratings · 19 written reviews indexed
- Age rating: 4+
- Requires: iOS 16.0 · 15 MB
- Languages: American English
- Released: 2009-08-28
- Data updated: 2026-07-23
- Monetization: paid
- User reviews in markdown: https://appshunter.io/ios/app/go-21-scirpn/id327651563/reviews.md

## What is GO?

GO-21 SciRPN simulates the venerable HP-21 Pocket Scientific Calculator from 1975.  Virtually indistinguishable from the real item in operation, GO-21 is a powerful yet simple retro calculator. 

• RPN (Reverse Polish Notation)
• 10-digit LED calculator display, 10-99 to 10+99
• Four stack registers with roll down
• One storage register with arithmetic memory operators
• Math, trigonometric, polar/rectangular, logarithmic, power and other miscellaneous functions
• One additional window with views of:
	◦ stack/storage registers
	◦ keystroke log
• Copy-from and Paste-to the calculator display
• Simple Paper Tape annotations

## Key features

- RPN (Reverse Polish Notation)
- 10-digit LED calculator display
- Four stack registers with roll down
- One storage register
- Math, trigonometric, logarithmic functions
- Keystroke log
- Copy/Paste display functionality

## Recent user reviews (10 of 19)

All indexed reviews: https://appshunter.io/ios/app/go-21-scirpn/id327651563/reviews.md

### 5/5 — Hp-21 a constant companion !

*2022-06-22*

I bought my HP-21 shortly after i saw it in Machine Design magazine in 1976. From then on it became my constant companion until appx 1995 or so when it gave up the ghost and  would no longer illuminate. Much later on I was overjoyed to find an e-version replica of my faithful pocket resource, though he was no longer needed after I retired from field work. Today I have switched to Android apps and regret that I cannot find a replica of this trusty device,,,,, and would like to ask IF THERE IS ONE (?) or one under development?  All iOS products havre become too expensive and unreliable !

### 5/5 — 50 years later - Thank you

*2022-06-15*

Many thanks for giving me the opportunity to use this RPN calculator. I bought my very first HP 21 C  in 1972 when studying at Polytechnic School of Cameroon. 50 years later, I am still very happy and proud using RPN devices and programming Common LISP.

### 4/5 — Hate that high visibility setting

*2020-01-11*

To me, the value of this app (and all the other RPN apps that I have) is the authenticity of them. Otherwise, there are a gazillion calculators with different sized displays out there with RPN.

Instead of making the X view high visibility, why not enlarge the calculator itself to be full screen (even getting rid of the stack view optionally)

Then I’ll go back to 5 stars again which otherwise this app absolutely deserves.

**Developer response:** For some thoughts on sizing, see:

     https://www.bigcatos.com/BigCatOs/go-2x-scirpn/go-21sizehistory.html

The App seems to be sized as authentically as possible. The stack view is and always has been optional, simply hide it forever, as described in the documentation.

The High Visibility Display setting is optional and disabled by default so you never have to see it, thus the calculator remains in its pristine state.  This option has been requested by two groups of users: as an Accessibility assist for those with diminished vision, and folks who use the App outdoors as part of their work.

### 5/5 — Works fine

*2018-09-20*

Wonderfull app that I use daily... A recent display bug was corrected and now everything is OK. Just one regret : why the go-29 doesn’t exist !!!

**Developer response:** I am starting on HP-29c, but it will be a while before completion!

### 5/5 — Lean & Mean

*2018-09-07*

I’ve been using this app for a number of years and have been very satisfied and figured it was time I gave it a review.  As an engineer, I loved my HP-21 and was sad when it finally died many moons ago. I searched and searched for a simple reverse Polish calculator app, but could never find a good one. This is a good one.  A clean but elegant emulation of a classic design.  Highly recommended.

### 5/5 — My thoughts on the HP 21 app

*2016-03-25*

I have an hp 21 that we purchased new and I've always loved it, not only for the RPN feature but for the simplicity. This is such a nice app for anyone familiar with the HP line plus it shows the stack. Highly recommend.

### 5/5 — Return with us now to the days of yesteryear

*2015-08-15*

I bought an HP-21 during my third semester of college in 1976.  At that time they were still teaching students how to use a slide rule.

The app is a very good reproduction of the  HP-21's look and functions/functionality.  I has the functions I want and doesn't have a lot of functions that I don't use.  It has a useful Help file.

### 4/5 — HP 21 workhorse of the 70's

*2014-07-01, version 4.1.2*

This calculator was the best around in 1975 and saw me through college in electronic engineering. It is a RPN (reverse polar notation) unit that's is a powerful method of doing calculations without the need for brackets and "equal" buttons. Once one get used to entering the numbers on a stack and then choosing an operation, it makes for fast and logical processing.
In 1992 I up graded to a HP 48S which can do much more mathematics than I am able to understand. A truly wonderful machine, but with this comes complexity and is not as easy to use as the 21 which does almost everything I need. I love the HP 21 and glad I can enjoy it on my IPad.

### 5/5 — İt's like the good old days

*2013-04-06*

Great calculator, I use it everyday in my engineering office work. Money well spent.

### 5/5 — Nice

*2011-08-14*

Loved my 21; spent many hours with it in college.  Doesn't charge anymore, but I still keep it around.  This is an excellent reproduction.  And then some.

## Frequently asked questions about GO

### What is GO-21 SciRPN?

GO-21 SciRPN is a mobile application that simulates the operation of the classic HP-21 pocket scientific calculator from 1975. It offers a faithful recreation of its interface and functionality, including Reverse Polish Notation (RPN).

### What functions does GO-21 SciRPN offer?

GO-21 SciRPN provides a comprehensive set of functions including math, trigonometric, polar/rectangular conversions, logarithmic, and power functions. It also features four stack registers and one storage register for calculations.

### Does GO-21 SciRPN have ads?

No, GO-21 SciRPN is ad-free. The application is a paid utility designed for a focused user experience without advertisements.

### What devices does GO-21 SciRPN support?

GO-21 SciRPN is available on a wide range of Apple devices, including iPhone, iPad, iPod, Mac, and Apple TV. This broad compatibility ensures users can access it across their ecosystem.

### How often is GO-21 SciRPN updated?

The latest version, 7.1, was released on September 17, 2024. Updates are provided periodically to maintain compatibility and performance on supported devices.

### What is the price of GO-21 SciRPN?

GO-21 SciRPN is a paid application, available for $2.99. This one-time purchase grants access to all its features without further in-app purchases or subscriptions.

### What is Reverse Polish Notation (RPN)?

Reverse Polish Notation (RPN) is a mathematical notation where every operator follows all of its operands. GO-21 SciRPN utilizes this method, which is a key feature of the HP-21 calculator it emulates.

### What is the age rating for GO-21 SciRPN?

GO-21 SciRPN has an age rating of 4+. This indicates it is suitable for users of all ages, with no mature content.

## Version history (last 5 releases)

### 26.5.1 — 2026-06-11

- Repair broken icon and update documentation.
- Update to Apple OS versions 26.5.1.

### 26.5 — 2026-05-31

- ENG display format & other bug fixes and enhancements.
- Provide a complete PDF hardcopy of the HTML user manual, including an index.
- Minimum macOS version now 13.0 (Ventura).
- Minimum iOS / iPadOS / tvOS versions now 16.0.
- Update to Apple OS versions 26.5.

### 7.1 — 2024-09-17

- Update for iOS 18.0, iPadOS 18.0, tvOS 18.0, macOS 15.0.

### 7.0 — 2024-05-15

- New Settings option, Auxiliary View Assist, enables draggable buttons for quickly opening / closing the stack window.
- New Settings collection, Accessibility, groups High Visibility Display, Magnify Calculator and Auxiliary View Assist.
- The Settings option Auto-On now defaults to YES.
- Battery power level reporting has changed in iOS, iPadOS and macOS: adjust power level monitoring algorithm to compensate.
- Bug fixes, miscellaneous improvements and internal modernization.
- Refresh documentation.
- Update for iOS 17.5, iPadOS 17.5. tvOS 17.5, macOS 14.5.

### 6.6.3 — 2024-03-09

- Miscellaneous improvements recommended for everyone.
- Update for iOS 17.4, iPadOS 17.4.

## Apps similar to GO

| App | Rating | Price | Category |
| --- | --- | --- | --- |
| [RPN-25 CE](https://appshunter.io/ios/app/rpn-25-ce/id1529311398) | 4.6 (9) | $2.99 | Utilities |
| [RPN-97 Pro](https://appshunter.io/ios/app/rpn-97-pro/id816249055) | 4.9 (11) | $4.99 | Utilities |
| [HP-35](https://appshunter.io/ios/app/hp-35/id6738010641) | 5.0 (5) | Free | Utilities |
| [RPN-67 SD](https://appshunter.io/ios/app/rpn-67-sd/id1109722386) | 4.6 (16) | $3.99 | Utilities |
| [RCL-57](https://appshunter.io/ios/app/rcl-57/id1621959742) | — | Free | Utilities |
| [RPN Calc](https://appshunter.io/ios/app/rpn-calc/id285002799) | 3.3 (4) | $0.99 | Utilities |

## More apps by Steve Lidie

- [GO-25 SciRPN](https://appshunter.io/ios/app/go-25-scirpn/id408249517)
- [Photo Gopher](https://appshunter.io/ios/app/photo-gopher/id662735770)
- [Solar Coaster](https://appshunter.io/ios/app/solar-coaster/id1218422388)
- [GO-29 SciRPN](https://appshunter.io/ios/app/go-29-scirpn/id1472811056)

All apps by Steve Lidie: https://appshunter.io/developer/1869780159

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*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 2026-07-23.*