Try 1-Calc Lite for free from here:
http://forum.xda-developers.com/showthread.php?t=324483
Omega One 1 Calc Lite for Windows Mobile based Pocket PCs. This calculator includes all common functions, full memory functionality, 10 short cut keys to perform quick everyday calculations, tip calculator, and unit converter.
Features include:
-Basic Functions: The most common calculator functions at your fingertips: +, -, x, ?, +/-, 1/x, %, ?, ??, x?, x?, full memory functions, x^y, !, Exp, 10^x, e^x. Backspace to edit mistakes, brackets to frame your expressions correctly. Also supports Algebraic, RPN & DAL calculator modes.
-Unit Converter: A full unit converter that not only helps for the everyday conversions (e.g. km/miles, Fahrenheit/Celsius), but also for the unexpected (what’s my shoe size in Europe?!?)
-Memory Banks: 1-Calc has 5 different memory banks to store your last calculation,
-Tip Calculator: Quickly work out who should pay when you’re out with friends!