Reverse Polish Notation (RPN) is a little weird to look at if you're not familiar with it, but it still has a diehard fanbase to this day.
Al Williams breaks down the RPN calculators you can still buy today.
@hackaday I showed my HP 11c to all my students for 35 years. Sometimes I just let them try to use it.
Oct 19, 2026, 3:13 AM
@cmflyer @hackaday I used to have an 11C with me in the classroom. Always fun when a student asked to borrow it, only to return a couple of minutes later with a wtf look on their face 😄
The 11C has been mostly replaced by @jamesthomson’s PCalc (because it also has an RPN mode!), but I still have it in my desk drawer. Aside from the joys of RPN, it's still running on batteries that I haven't replaced for decades.
@hackaday I haven't touched my HP45 in at least 20 years. I bought a HP-19c? after I couldn't get batteries for the 45 any more. Haven't bothered using either in at least 20 years.
And I forgot how to use the Picket Slide Rule Dad gave me for my High School graduation. Maybe I could multiply or divide something on it but I do recall doing the mental math to get a ballpark value in my head along with where to put the decimal place.
@hackaday When I was in science, I loved using it. It makes a lot of sense once you wrap your mind around it. :)
Oct 19, 2026, 5:32 AM@hackaday i highly recommend swissmicro. Very happy with mine (even though I have an entire stable of old ones)
If someone doesn’t want to put up big bucks, I think HP 35s is a perfect balance
Took me a few days getting used to RPN, but have used it ever since. RPN is a much more efficient use of muscle memory 💪
@hackaday To this day I can vividly recall the lecture where RPN was introduced; how we did exercises converting conventional notation to RPN; how the relationship with a stack was our lightbulb moment and then, magically, how all came alive as we saw how to computationally process any arithmetic expression in a compiler to produce machine code with correct operator precedence. It was a good lesson in how something that’s weird at first sight transforms into something vital.
@hackaday I love m'y HP 35s and feel clumsy wirh a regular calculator...
@hackaday RPN FTW. My HP48G is still going strong and is used almost every day.
@hackaday I still use my HP-15C for doing math that I can't do in my head. The battery lasts forever and is the same one used in the cat's laser toy. On my phone I use a calculator app that uses RPN
