From iFD Software Consult:
iFD Formula is a compact powerful utility for Pocket PC that allows you to work with a sufficient set of formulas (mathematical, physical, and electrical). You can also add your own formulas where you can add new formula categories, new formulas and their parameter description. The calculation is easy and fast - just enter the parameter values and iFD Formula will calculate the result value. You can change any component of the formula and calculate it again.