I know that your iPhone already has a calculator in it, but here’s a specialty app for people who suffer from the curse of Man Thumbs.
SwipyCalc is a basic calculator that gives all the screen space to the numbers. Only the numbers. You enter your basic functions — adding, subtracting, multiplying, and dividing — with swipes in one of four directions. It’s fast and easy, especially once you realize that the comma in the lower left corner is what you use for a decimal point.
Unless you’re in one of the 60+ countries that uses the decimal comma. That won’t slow you down at all.