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Guido's RPN calculators

RPN calculators are not so common because it takes a bit of learning until you can use them. However once you have understood the logic and used them for a while you would never want a different calculator again.

The algebraic notation as used by most calculators LOOKS simpler. Telling a student "Just type it in as you find it in the textbook" is easier than explaining how RPN works. Have however a look at this interessting experiment that compares calculators with RPN and algebraic notation.

The gsrpn calculator

I have been using HP RPN calculators since the beginning of my university years. Now that PCs and Laptops are one every office table I have been looking for a good calculator that just runs independent of the operating system on any computer out there. I did not find one so I wrote one my self. Beginning of 1997 I started to write rpnjcalc. I have used it once in a while but it never really felt like a real HP pocket calculator. I always found my self looking for a real HP pocket calculator when I had to do some calculations. So finally in March 2008 I decided that it must be possible to write a more usable calculator in Javascript and I developed gsrpn

Later I discovered that Greg Hewgill has written a very authentic version of HP 15c which is available at [Local copy]. He did a very good job. If you prefer to install a hp15c on your PC then try

My collection of RPN pocket calculators

Originally I did not collect RPN calculators. I bought them because I used them. Later I bought some (like the hp12c and the hp-prime) just to have them an play with them and finally I bought some just to collect them.


I bought this hp15c in 1988 and it was my first RPN calculator. It has the right size and is robust. I am using it a lot but it is still as good as new. I really like this calculator. There are others calculators which are more powerful and faster but out of all the calculators I have I am only using this one. The HP15c is the best calculator. I just love it. You will never use another calculator once you get used to its logic and the HP beveled keys with this rotate-and-click effect.

It's interessting that this calculator is "1 * π inch" high and "Φ * π inch" wide (where Φ=1.61803...=(1+sqrt(5))/2 prounouce: phi, Φ is also known as the "golden ratio", π=3.141... prounouce: pi, Φ*π=5.083).

For several month a petition was circulating on the internet to bring back the 15c and it seems HP took notice and produced end of 2011 a "15c Limited Edition", the HP15c-LE. It does not only look similar to the original it is very much like the original with some small differences. On the down side are a number of firmware faults:, list of 15c LE bugs but on the up side is the advantage that the 15c-LE runs faster than the original 15c and it has the same good quality keyboard with an excellent "click".

The list of faults of the HP15c-LE might look long list but the only real problem is the fact that the PSE (pause) function can only be called once per program execution. The non working low battery indicator is pretty bad too but you can just use a voltmeter and test the batteries once in a while as described in my hp15c-le battery test article. There is no need to take the batteries out for testing.

All programs that I have ever written are however not affected by that PSE bug.

It's a bit unfortunate that the HP company of today is not any more the company that took pride in producing high quality measurement equipment and calculators. They have not provided a firmware update until a year later and I don't think they will.

I have investigated the HP15c-le power and battery problems and written about them at (previously known as produces a hp15c compatible calculator. It's emulating the original Voyager processor on new hardware and runs runs original HP15c firmware. The hardware is designed with firmware updates in mind. It has therefore as opposed to the HP15c-LE no software bugs. I like the fact that the color scheme follows the original hp15c. The keyboard could however be improved further. The pouch is made from real leather.

The DM-15 HP clone

My HP15c programs

The HP15c is very easy to use and program but the program code is hard to read (a bit like assembly language) without explanations. I include therefore the algorithm with examples.
  1. convert a floating point number into a fraction (numerator and denominator)
  2. quadratic equation solver, x2+p*x+q=0
  3. number conversion dec->binary
  4. Euclidean algorithm, Greatest Common Divisor (GCD)
  5. A dice
  6. Generic unit conversion

HP15c case/pouch

I have long been looking for a good replacement pouch for the 15c. Especially the HP15c-LE pouch is too hard and too sturdy my old one is on the other hand wearing out and getting too loose. After trying several options I made some pouches out of neoprene and they are really good. If you want one, you can order them here:


The HP-11c is basically the predecessor of the HP-15c. HP added, after the success of the HP-11c, a number of additional features to the HP-11c and released it 10 month later as the HP-15c. I bought this HP-11c on ebay just for nostalgic reasons.


This HP33c was my second RPN calculator and I got it second hand from a friend who did not want to use it anymore. Unfortunately I don't have the original battery pack.


I bought the HP42s in 1991. A very powerful calculator with a lot of functions.


I bought the HP48 in 1992 but I did not use it too much. It was good at the time but has too many function for which you would not use a pocket calculator but rather some Math-software on a PC. Over time I found that a pocket calculator has to be just handy and compact.


This is a 2008 hp-35s. For many years HP did not produce anymore any good scientific RPN calculators. They had only bulky graphing calculators in ugly metallic design. This is, after many years, the first scientific RPN calculator which comes close to the successful classic RPN calculators. I like it.

HP-12c platinum, special edition

This is a 2008 hp12c platinum. The HP12c financial calculator is one of the few calculators that changed very little since the introduction in 1981.


This is a hp12c which I bought on ebay. It's in pretty good condition and it must be from the 90's.

hp12c front hp12c back

The HP12c has no trigonometric functions but you could use the following approximations if you ever need those functions.
  1. HP12c sine, cosine, tangent approximations


This is a very interessting machine. It is based on the HP30b calculator with a keyboard overlay and a different firmware. There is a well an additional clock crystal installed. It's all open source apart from the hardware. It was designed by Walter Bonin and Paul Dale. You can order it at The manual and other documents are available at and there is a wiki at It is packed with functionallity and has 3 shift keys (f, g and h). I love the idea. It's like a hp42s++.


This is basically the successor of the HP-48sx but made in 2013. It's a very powerful graphing calculator. It has unfortunately a problem with the standby time on its rechargable litium battery (the same battery as used in a Galaxy S3 cell phone, 2100 mAh or 1500 mAh, note the bigger version with 3Ah and more don't fit). If you use it every day then the standby current that the processor draws is neglectable to its on-time power consumption. If you are on the other hand an occasional user like me who has the calculator on the shelf and uses it only once every one or two month then you will find that you can newer use it. The battery is always empty and you need to charge first. Oh, how I love my HP-15c: 3 LR44 battries last 5-10 years....


© Guido Socher