I have always liked the HP calculators and have had a couple of different models including the very bulky HP 48 GX machine. Now when I started my new job I wanted something that would fit a shirt pocket but still be powerful enough and allow me to do more or less advanced equation solving without using Matlab or Excel or something similar.

And I found the answer to my prayers, the HP 35s which is a really cool ”old skool” calculator but with many useful functions such as equation solving and substitution. And above all it speaks RPN which is something I demand of my calculator which rules out Texas and Casio and many other brands.

RPN is the ability to quickly calculate complex series of operations without having to use parenthesis and other awkward functions. Using RPN to calculate 5 +8 * 9 means the folloing keypresses:

8 <enter> 9 <multiply> 5 <plus>

It may seem backwards and complex but it means you always get things in the right order and priority and that without using parenthesis. To calculate something more complex 10 * log (k * T * 1000):

1000 <enter> T <mul> k <mul> <log> 10 <mul>