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

What’s New

Version 5.7

- International hardware keyboard support for iOS and macOS.
- Similar to some CuVee Software products and with permission, add ability to annotate, clear, copy, email and print the Paper Tape.

Ratings and Reviews

4.9 out of 5
7 Ratings

7 Ratings

NanoRon ,

Lean & Mean

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.

Dhjdhj ,

Hate that high visibility setting

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:

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.

joemeyer ,


I was 18 when the HP-21 was released but did not buy one until years later when I was in College (I was in the US Army until age 21). This iPhone version is delightful to use and it works very well on my iTouch with the latest OS and indeed fast app switching works well. The display and buttons are easy to see and use. Swiping to see the entire stack is useful. Thanks for writing this app.


Stephen Lidie
9 MB

Requires iOS 9.0 or later. Compatible with iPhone, iPad, and iPod touch.



Age Rating


  • Family Sharing

    With Family Sharing set up, up to six family members can use this app.

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