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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.

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. While my calculators work with any old browser (pre 2010) you can find now calculators written for more modern web browsers and they look and work like authentic HP calculators.

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.


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 calculator is roughly the size of a credit card. The keyboard could however be improved a lot. You have to really watch what you type or you have to use a pencil with a soft rubber eraser and type with the rubber end of the pencil. Those rubber eraser tips that you can put onto the end of a pencil give the best results.

The DM-15 HP clone produces since Oct 2015 a new model called DM-15L. That calculator is really the best HP-15C clone that I have seen to this date. The keyboard is much better than the one on their small credit card sized editions. This is a very well designed calculator. The case is super strong, has the same dimensions as the original and weighs approximately the same as the original. The keyboard had initially some issues but they fixed them and it works now very well. The keys are flat and not beveled but they do the job. The original HP keyboard is better but the DM-15L keyboard is totally usable altought it is possible to presss a key without it registering which does not happen on original HP-15c calculators. This is a good and usable calculator but you should watch the display while you type.

The Swissmicros DM-15L

The DM-15L has a serial number on the case. I have three DM-15L with the S/N 430, 554 and 532.

The Swissmicros calculators have a few key combinations that are specific to Swissmicros. The most important ones are:
CHS and then ON: change LCD contrast with + or - keys
1/x and then ON: show firmware version and battery voltage
 Note: 1/X is the key left of CHS and is called E on the DM-16
7   and then ON: change the font
9   and then ON: change clock speed, 48MHz should be the normal speed

How to hold the HP15c in your hand

A lot of calculators are "portrait" shaped and you hold them with one hand while you type with the index finger of the other hand. This is not how the HP15c is supposed to be used. You hold the HP15c in both hands and you type with both of your thumbs. It's a much faster then just typing with one finger.

The HP15c has a very ergonomic shape allowing for fast typing.

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
  7. number conversion binary->dec
  8. Calculate Canadian sales tax (GST/PST)
  9. Small program to calculate the modulus: Y mod X

HP15c RPN tutorials

HP15c and the CHS stack lift with zero

There is an inconsistent behavior in the HP15c and other HP calculators with regards to the behavior of the CHS key. Consider this example:
3 Enter                   resulting stack: 3
2 Enter                                    2
CHS                                       -2
1                                          1
This results in the stack as shown on the right. The -2 is considered "computed" and we get an automatic stack lift when the next number is entered, 1 in this case. If we enter however "0" instead of the two then we get a different behavior.
3 Enter                   resulting stack: 
0 Enter                                    3
CHS                                        0
1                                          1
This is the only known bug in the original HP15c and you can work around it by always pressing an extra enter if the number you want to CHS on is already entered or computed. This would keep at least the X and Y registers (bottom two) the same even if there is a zero.

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.

swissmicros DM42

I bought this calculator from in 2018. This is a good calculator. It has a good weight of 175g. It is mechanically very solid and the keyboard is good. The original HP keyboard is better but the DM42 keyboard is something you can get used to. It is an improvment over the DM15L keyboard but occasionally it is still possible to presss a key without it registering. The DM42 is based on the code of Free42 ( by Thomas Okken). In other words it is a complete rewrite, not using any HP code. It does not have any of the bugs that the original HP42s had. I can recommend this calculator. It's very good. The display is very interesting. It is a digital paper display that can display images even when the calculator is off. The DM42 comes with a collection of "off-images". Those images are displayed when you turn the calculator off. Every time a different image. The display is quite large and the calculator can therefore show the entire stack and any software menus. The original HP42s had only two display lines. The DM42 calculator is about the same size as the original HP42s (a bit smaller) despite the larger display and an extra row of function keys.


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.

There is a HP48 emulator for linux but it is quite old and you may encounter problems with regards to keyboard mapping and usability.


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