# RPN-55 SD — Classic HP Calculator w/Timer

> Photo-realistic simulation of a classic HP programmable scientific calculator with timer. (iPhone/iPad app by CuVee Software.)

- Source: https://appshunter.io/ios/app/rpn-55-sd/id1469907917 (this page in markdown: same URL + `.md`)
- Developer: [CuVee Software](https://appshunter.io/developer/424873503)
- Category: Productivity, Utilities
- Price: $1.99
- Rating: 5.00/5 from 8 App Store ratings · 5 written reviews indexed
- Age rating: 4+
- Requires: iOS 12.0 · 17 MB
- Languages: American English
- Released: 2019-07-03
- Data updated: 2026-09-06
- Monetization: paid
- User reviews in markdown: https://appshunter.io/ios/app/rpn-55-sd/id1469907917/reviews.md

## What is RPN?

RPN-55 SD is the first photo-realistic simulation for the iPhone of Hewlett-Packard's low-cost alternative to the famous HP-65 programmable scientific calculator. Produced from 1975 through 1977, it was the "Merlin" member of the classic HP calculator series, priced at $395. HP claimed that it featured "(...) more pre-programmed functions than any other scientific pocket calculator in the world."

A main feature of the HP-55 was a crystal-controlled 100-hr digital timer with the ability to store times while the timer was running and later recall them. On RPN-55 SD the timer may also act as a 12-hr or 24-hr clock, including date and weekday display.

While the simulator works just like the original calculator – except being much faster and having twice the program memory – you can turn on extended features to make it even more enjoyable to use.

RPN-55 SD comes complete with original HP programs built in.


Features:

• Photo-realistic look-and-feel
• Vintage mode or Extended mode
• Programmable (99 steps), conditional branching
• Insert and delete steps with automatic address correction
• Delete range of steps
• Subroutines (4 levels)
• 20 registers, "continuous", with unrestricted store/recall arithmetic
• "Live" register view
• Program listing view
• Program library view
• Store and retrieve any number of programs
• 100-hr timer supporting 10 split times and 12/24-hr clock mode with date
• Swap unit conversions with additional functions, e.g. hyperbolics
• Sharing: e-mail, AirDrop, Dropbox, copy, paste, print
• 112 programs pre-installed

Calculator functions:

• Statistics (linear regression, mean, standard deviation, correlation coefficient, x- and y-estimates)
• Trigonometric functions (degrees, radians, grads)
• Polar/rectangular conversions
• Inverse, square root, logarithm, exponential, pi
• Storage arithmetic (store and recall)
• Percent, absolute, integer, fractional value, round
• Factorial
• Random number
• Unit and hours conversions

Details: cuveesoft.ch/rpn55

## Key features

- Photo-realistic look-and-feel
- Programmable with conditional branching
- 100-hr timer and 12/24-hr clock
- Original HP programs built-in
- Extended features for enhanced usability
- Sharing via email, AirDrop, and more

## Chart rankings

- #192 in Top Paid · Productivity (US App Store)

## Recent user reviews (5 of 5)

All indexed reviews: https://appshunter.io/ios/app/rpn-55-sd/id1469907917/reviews.md

### 5/5 — Love the newest version

*2025-09-07*

I've been using this app for several years, it's my favorite calculator app by far.  My only gripe was minor - when launching the app the image of the calculator would appear immediately but it took a few seconds before it would accept keystrokes.  So I would hit buttons but they wouldn't register, a bit frustrating.

If you just leave the app running in the background that's not an issue, the calculator is instantly ready to go when you bring it to the foreground.  But I'm in the (bad?) habit of closing apps after I'm done with them so I have to relaunch it every time I need it.  Leftover habit from the 1970s when you had to live with 4K of RAM in your 8080 micro computer...

The new version is much much quicker to launch and completely solves the lost keystrokes problem.  A great improvement for old geezers like me.  Thank you!

### 5/5 — Hyperbolic functions

*2023-06-29*

In the next update for this calculator please add the six hyperbolic functions.  HSIN, HCOS, HTAN, ARCHSIN ARCHCOS, ARCHTAN.
This is a five star appreciation.

### 5/5 — Perfect

*2023-03-15*

Very realistic. A delightful calculator app especially if one doesn’t have a real HP-55 handy.

### 5/5 — My Calculator of Choice

*2022-07-22*

The HP 55 was my first scientific calculator. It got me through high school physics and an electrical engineering degree at UC Berkeley. (I used a Pickett N-500-ES slide rule for high school chemistry.) These days, the RPN-55 app is my calculator of choice. It provides all the functions I tend to need, packaged in a way that brings back pleasing memories from decades ago. In terms of function, the standard HP 55 H.MS± functions are surprisingly useful (on occasion), and I don't "Replace Unit Conversions" because I find the unit conversions more nostalgic and useful.

In a future release, it would great to have the "high contrast mode" feature implemented (such as in RPN-25 CE). You might imagine that my eyes have aged somewhat since my time with the physical HP 55, and any help would be appreciated! Even better would be an HD edition which can display the stack and registers (as in RPN-45 HD).

But in its current incarnation, RPN-55 is eminently functional and fun to use.

### 5/5 — Perfekte Umsetzung

*2019-07-04*

technisch und optisch eine perfekte, fast traumhafte Umsetzung. Sehr gut sind auch die optionalen Erweiterungen für die Programmierung. Man kann aber auch ganz „Old School“ bleiben, wie mit dem Original.

## Frequently asked questions about RPN

### What is RPN-55 SD?

RPN-55 SD is a photo-realistic simulation of the classic Hewlett-Packard HP-55 programmable scientific calculator. It replicates the original functionality and appearance for modern devices.

### Does RPN-55 SD have ads?

No, RPN-55 SD does not contain advertisements. It is a paid application designed for an uninterrupted user experience.

### What are the main features of RPN-55 SD?

Key features include a photo-realistic interface, programmability with 99 steps, a 100-hour timer with split times, a 12/24-hour clock with date, and over 100 pre-installed HP programs.

### What devices does RPN-55 SD support?

RPN-55 SD is supported on iPhone and iPod devices. It was last updated on October 17, 2025, indicating ongoing support.

### How much does RPN-55 SD cost?

RPN-55 SD is available for a one-time purchase of $1.99. There are no in-app purchases or subscriptions associated with this application.

### Can I share calculations or programs from RPN-55 SD?

Yes, RPN-55 SD allows you to share programs and data via email, AirDrop, Dropbox, copy/paste, and printing. This facilitates collaboration and backup.

### What is the age rating for RPN-55 SD?

RPN-55 SD has an age rating of 4+, making it suitable for a wide range of users. It offers a straightforward simulation of a historical calculator.

### How often is RPN-55 SD updated?

The application was last updated on October 17, 2025, with version 3.3. This suggests that the developer actively maintains and improves the app.

## Version history (last 5 releases)

### 3.3 — 2025-10-17

• Improved compatibility with iPhone Air and iOS 26.

### 3.2 — 2025-07-28

Bug fix: If non-volatile memory is enabled (Calculator Mode = on), reloading the app does not clear the stack register X anymore.

### 3.1 — 2025-05-08

• Faster startup: no delay until the first button press is recognized after switching on.

### 3.0 — 2024-10-22

• Added warning when attempting to replace a Sample Program.

• Visual adjustments to iPhone 16 Pro and iPhone 16 Pro Max.

### 2.9 — 2024-01-13

• Internal optimizations and cleanups.

• Modified Info screen to avoid clipping by the Dynamic Island.

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## More apps by CuVee Software

- [RPN-45 HD](https://appshunter.io/ios/app/rpn-45-hd/id424873500)
- [RPN-70 HD](https://appshunter.io/ios/app/rpn-70-hd/id436127144)
- [Smart Sudoku](https://appshunter.io/ios/app/smart-sudoku/id457564906)
- [Vintage-45](https://appshunter.io/ios/app/vintage-45/id506402272)
- [RPN-45 SD](https://appshunter.io/ios/app/rpn-45-sd/id509923051)
- [RPN-67 NP](https://appshunter.io/ios/app/rpn-67-np/id603100249)

All apps by CuVee Software: https://appshunter.io/developer/424873503

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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 2026-09-06.*