Surprisingly enough, the iPad does not come with a calculator app installed by default and I'm sure there are times when we need to do a bit of adding and subtracting. But, thankfully, the App Store has a lot of them. Some of them are free but most cost anywhere from 99¢ to $25.
I've been using PCalc Lite Calculator which is free. It has very clean design and comes with the RPN mode and unit conversions. Most of my calculator use is just basic math, nothing too complicated so if you are looking for a scientific calculator that you might use in a math or statistics course then prepare to shed some cash but in the mean time give PCalc Lite Calculator a shot!