HP21 scientific RPN calculator
As with most HP calculators then and now, the HP-21 used RPN entry logic, with a four-level stack. It also had a single user-accessible memory register. A shift key provided access to functions whose legends were printed on the fronts of the tall trapezoidal keys.
To enhance practical usability, display was modified to show both X and Y register. Also I have eliminated mechanical DEG/RAD switch and moved the function under DSP button. HP21 has few big enough buttons to be used comfortably on mobile devices.
Category : Tools
Reviews (30)
Perfect calculator app. I feel transported back to the 70s, when calculator technology reached perfection. Everything later became a bloated mess. Great app if you loved those early HPs.
Excellent HP calculator SIM. It has the best tactile feedback simulation. Beautifully photo-captured skin too. True to the original. Thank you for sharing this with others.
I love this app. When I did 3 years of engineering at Ryerson Uni 71-74 I did all the math with a slide rule. In 72 one of my (rich dad) classmate bought an HP-35.... we used to line up to get him to do calls. for us. When the HP-21 came out I had a job and could afford it. Great calculator... love RPN. Used it for years till the battery died.... and a new calculator was cheaper than a battery. Tossed it out years later, wish I had kept it. This app is an exact replica and NO ADDS. Thanks!
I love using the RPN logic since buying the HP21 as my first calculator at university. Exchanged it later for the HP15C - of which I also bought the Android version. As some other reviewers mentioned, since the software upgrade of my Samsung Note10 earlier this week (One UI for Android Pie), not all the buttons are active anymore - on both the Android HP21 and 15C (will add a similar review remark with the latter app).
Was my go to calculator app until the latest software upgrade on my S9. Sadly the app does not work properly anymore. Hope they fix it soon or I'll be deleting it. April 2 Update: Back to 5 stars as app is working again. Thanks!
I have been making mistakes using a conventional calculator I have use an hp15c for almost half a century. some habits are hard to break. RPN is one for sure. Thanks for the great app. Jan Lucasiewicz and Paul Olinski
this was a great calculator, until it was "fixed", now the bottom two rows of keys do not work. I'm using an HTC-12+ phone.
you can change sign. just press the minus button. download it just for nostalgia. works just like the original.
Very impressive, and enormously useful! I use it at least daily. Thank you.
not well written. App can not be moved to external SD memory
Great app. I love RPN in a calculator. Just wish you could change value +/-
bottom row doesn't work....a pity because otherwise this would be my favorite calculator...
nice try, but the display does not look like a real hp21.
Feels, looks and behaves like the real thing.
Really useful and fun to use!
Excellent implementation, love it.
Best ever RPN calculator app.
Look and feel of the original!!
Best RPN calculator app!!!
Loved the reverse Polish implementation of UP calculator.
The keypad is very responsive, the two levels of the stack that are always visible are very helpful. The implementation of this venerable HP design on Android rocks, because the functionality is basic enough for quick occasional calculations, there is nothing more but an efficient and very handy calculator machine at hand all the time with you. A pure delight. In its last update the display gets larger numbers. They look even better than before.
Love the look of this but one problem totally ruins this: idiotic vibration on every key press, and NO way to turn off the key-press vibration. Does not respect settings screen switch to turn off the ridiculous honking. Fix that, 5 stars. Until then, ugh.
Had one of these in high school. Your nearly perfect rendition brings back fond memories. I'd nearly forgotten how much I prefer RPN calculators, having owned several HPs. This is my preferred calculator app now. Many thanks.
Excellent emulation, keys work great with good feedback feel. As good as the real thing. Highly recommend if you like RPN.
An exquisite rendition of the best RPN calculator ever made. I learned how to program starting with this machine and it was a life changer. Warms me up to see it again in all of its glory.
I used one of these in the 1970s. Great emulation, right down to the red LED and front HP label. Well done.
I have tried 8 RPN calculators and this is now my favorite. Simple and to the point. I no longer have a need for inverse hyperbolic arc tangents.
A good calculator, well done. But please allow us to turn of the key click and vibration! Thanks for removing unneeded permissions. it no longer works on the new version of android though, a recompile is probably in order.
Absolutely takes me back to the good old days of calculating. Marvellous even with the negatives reported by other reviewers
Edit: latest update fixes alignment issue. Brings back fond 1975 memories of my first full-function calculator. RPN (v. algebraic notation) is much more functional. NOTE: "Temporarily only," I hope, to 1 star. No longer works with Android 9 update, April 2019; touch screen is misaligned.