I’ve looked at a number of calculators for iOS that run on the iPhone, iPod Touch and iPad and have a few that I really like. In my list I will also include a few others that you may not use all of the time but when you need them they are great as well.
Calculator
The calculator application that comes on the iPhone, iPod Touch or the iPad is actually quite good. In vertical mode it is a nice basic calculator and in horizontal mode it becomes a great scientific calculator. You can copy and paste the answer if you need it to another application. For a while this was the calculator I used all of the time.
Universal App that came with your iPhone or iPad (Free)
Tom's Tech Tips 