I used Mathpad for longer calculations, a bit of a niche application by Mark Widholm, which exists since System 7 or 8. It's a bit like Matlab, and also a bit like those apps where you write small steps and can track the computations (Soulver is one of those), but simpler and more direct than the former and much more capable than the latter.
For simple calculations, I used the standard calculator in "normal" mode. I've also always got one or two calculators on my desk (currently a NumWorks and an HP Prime). And since a few months, I also have Free42 on laptop and iPhone. If you like RPN, check it out: https://github.com/thomasokken/free42
For simple calculations, I used the standard calculator in "normal" mode. I've also always got one or two calculators on my desk (currently a NumWorks and an HP Prime). And since a few months, I also have Free42 on laptop and iPhone. If you like RPN, check it out: https://github.com/thomasokken/free42