RPN-55 SD — The first photo-realistic HP-55 simulator for the iPhone

Version 1.2

RPN-55 SD is a photo-realistic simulation of Hewlett-Packard's classic scientific calculator HP-55.

Released in early 1975, the HP-55 was the lower-cost alternative ($395) to the famous HP-65 ($795). Its main feature was a quartz timer. The HP-55 was discontinued in 1977.

RPN-55 SD is a fully compatible version of the HP-55 with twice the program memory.
By setting a simple switch, you can turn on a number of useful extensions.

"The HP-55 features more pre-programmed functions than any other scientific pocket calculator in the world." (Owner's Handbook, 1975)



Tap the wrench symbol or double-tap the display to access the
settings screen:

iPhone X Screenshot:
(extended mode)

Register View:




HP-55 Brochure (in German)
Published in March 1975 by Hewlett-Packard, Switzerland. Download

Price List – Note that 1,195 Swiss francs in 1975 ≈ $2,300 in 2019.

HP-55 in HP-65 User's News
Hewlett-Packard introducing the HP-55 in the first issue of "HP-65 User's News", mid-1975: Download



Version History RPN-55 SD

1.2 – Aug 14, 2019
  • New:
  • • Improved LNx! function:
      - Higher accuracy
      - Now returns y and z such that x! = y * 10^z (where z is an integer)
  • • Added program "Exponential & Log Curve Fit"

  • Bug Fix:
  • • Sharing could cause a crash

1.1 – Aug 12, 2019
  • New:
  • • Optionally replace unit conversions with additional functions (in Settings)
  • • Added "x-estimate" for a given y to linear regression functions. Note that the key for correlation coefficient (r) has moved one row up.
  • • Programs saved in vintage mode are now marked with a "V" in the listing view
  • • Added programs "Hyperbolic Functions", "Gamma Function (Toth)", "Solve f(x)=0", "TVM with Unknown Interest" as well as all test statistics programs
    from the book "HP-55 Statistics Programs"
  • • Tight program loops with many repetitions will no longer cause the display to stutter

  • Bug Fixes:
  • • Pressing EEX when a number showing separators was displayed would not remove the separators
  • • Touching a main register value would not dismiss the register view
  • • The % function showed as 1/x in program listings
  • • Deleting a range of steps using g CLP in RUN mode could modify unaffected branch commands

1.0 – July 2, 2019
  • First public version.


RPN-55 SD is available here:



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