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In-Depth Review

HP-15C Calculator Implementation on iPhone is Solid

Hewlett-Packard is famous and revered for its line of scientific and financial calculators that launched with the HP-35 in February, 1972. After that, for over 35 years, HP has been producing great calculators. Now, the famous HP-15C hand held calculator has been reincarnated, exactly, for the Apple iPhone, complete with a PDF of the original paper manual.

HP-15C app

It's identical to the original calculator (iPod touch screen shot)

To say that there is extensive lore and literature surrounding HP calculators is an understatement. Also, to say that a generation of Baby Boomers grew up with a slide rule, then voraciously embraced the HP-35, 45, and 67 in the mid 1970s would also be an understatement. For those who want to find out more or acquire one of these calculators, I've included some references at the end of this review.

The HP-15C is a scientific calculator that uses the Reverse Polish Notation (RPN) that HP made famous. RPN logic uses a stack, like the Forth programming language, and one of its principal advantages is that every intermediate calculation is shown in the display, providing a visual sanity check on each mathematical operation. Those who grew up with this logic find it very hard to enter a long string of numbers and functions in the more conventional algebraic calculator, then trust the final answer when the "=" is struck.

Hewlett Packard has taken the ROM code in the HP-15C and built it into an iPhone or iPod touch app that duplicates the function of the original calculator, even implementing the original Binary Coded Decimal (BCD) encoding. BCD assigns each decimal digit four bits. It's more complex to implement, but has greater accuracy than the typical binary implementation used in computers and calculators.

App and calc, side by side

App on iPod touch and original calculator, side by side

As you can see in the photo above, the iPhone/iPod touch implementation fills the 75 mm wide display while the actual HP-15C calculator is more like 126 mm wide. (Yes, I still have mine 20 years later, and it still works perfectly.)

It's not until you use the actual calculator app that you realize how cramped the display is on the iPhone, and with some kinds of protective cases, it can be difficult to get to some of the virtual buttons along the edge.

Instead of an "ON" button, the iPhone version has a MENU button (horizontal mode) that brings up several handy buttons: Reset, HP Calculators, Read Manual, and Swap Digit Separator (for European use.) The HP Calculators button launches Safari and takes the user to HP's calculator store. The Read Manual button brings up what seems to be a PDF of the original paper manual. That's it for the settings, and I noted that there is no option to turn key clicker off. HP should fix that in the next release.

The app launches in a simple RPN calculator mode, vertically, suitable for very basic calculations. It's not until you turn the iPhone horizontally that you'll see the full scientific display of the 15C.

The Manual

You'll have to have good eyesight to read that manual, and there's no way to print it or move it to a Mac if needed. However, paper manuals for many HP calculators are available on the Internet. Also, the page number prompt at the top is out of sync with the actual page numbers. However, the full 64 pages are included. Finally, swiping the screen is a bit tedious for a manual with that many pages. Patience is mandatory to find the right entry in the Table of Contents and then swipe, continuously, to the correct page -- shown in blue in the PDF itself. It would be nice to have a "Go to page #" function. The text can be increased in size with a two-finger pinch, but there's a limit to how big the text can be.

Bad manual numbering

Bad Manual numbering in on screen prompt - there are 64 pages

Touching the HP-15C logo, in horizontal mode, brings up shortcuts and notes that were originally printed on the back of the calculator, so, in effect, there has been an attempt to preserve the experience of using the original, physical calculator.


Notes from back

Back of calculator, notes, (partial)

BCD Math

One could suspect that Hewlett Packard might use the binary math libraries in the iPhone, as other native iPhone calculators do, but HP did something much smarter. The developers implemented all the original code, in an emulator, using BCD math, so that the results provided on the virtual calculator will always match the results on the real, physical calculator. Is it indeed possible to determine, from the keyboard, whether a calculator is using BCD or binary math, and I verified BCD. I'd like to thank Mr. Joseph K. Horn, an old friend from our days of publishing in Richard J. Nelson's PPC (Personal Programmable Calculator) Journal, for showing me how to do that.

Given that implementation, I didn't presume to do any accuracy tests to compare the real and virtual calculators. I will note however that in the late 1980s, my wife was doing orbital calculations for Lockheed Martin, and a bug in her Fortran Compiler prevented her from getting the accuracy she needed, even with double precision binary math in Fortran. Her HP-67 using BCD math saved the day, so my regard for BCD math has always remained high. That said, most users will never see the difference in every day calculations to 7 decimal places.


There was a noticeable difference in the speed of the iPhone implementation.  While early HP calculators operated at a few MHz, the iPhone, depending on model, runs from 412 to 600 MHz. As a result the sine of pi on my 15C takes visibly about a second to complete. On the iPhone, the answer comes up so fast, it's instantaneous for all practical purposes.


This is a very nice implementation of the original HP-15C. I appreciated the inclusion of the manual, even though the print is small, and the preservation of BCD operation. However, the screen is a bit cramped, and the keys are small, easy to fumble finger, and the key clicker can't be turned off.

Those who grew up with Algebraic calculators may wonder what all the fuss is about, look for the "=" key which is missing in RPN logic, and perhaps revert to to their handy desktop calculator. However, note that HP sells both Algebraic and RPN calculators, and some can be used both ways, so check the specs if you intend to purchase one. This implementation is, of course, RPN only.

On the other hand, for those who yearn for the time when they had an HP-15C in college, and it's long lost or sold, or those who want to explore the world of RPN scientific calculators, this app is nice. A little expensive, but very nice, and reflects, I think, the caliber of iPhone app that both Apple and HP can be proud of.

Will I use it routinely? Probably not. I am hooked right now on PCalc for the iPhone, it does what I need, and it's a lot more customizable. Besides, my wife wanted the 15C app for her iPod touch, and I'm not going to pay twice. That is, unless HP sends me a redemption code....

[Update: Ted Landau and Jeff Gamet reminded me that I can cross-authorize Macs with multiple iTunes accounts, then copy iPhone apps in/out of iTunes. So the HP-15C is now on my iPhone as well.  We'll be posting a QuickTip/HOWTO on this soon.]


1. Museum of HP calculators

2. Calculator Information for Educators

3. Wikipedia entry for HP-35

4. Wikipedia entry on History of HP Calculators

5. Database of HP Calculators

6. Hewlett-Packard's Calculator Store

7. HP Calculator FAQ for owners

8. HP Article celebrating 35 years of calculators

Just The Facts


Accurate implementation of original code, including BCD, electronic version of original paper manual, good keyboard response.


No setting for key clicker, no way to move manual to Mac for larger view.

1 comments from the community.

You can post your own below.

kimhill said:

The 15C emulation is for wimps!  wink

Anyone who used the programmable HP-41 series calculator will be impressed by the i41cx apps:

I have the i41cx+—it’s amazing.


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