First, if you need it, open Rolf's Calculator. You will also need the following constants and conversions:

c = 3 x 10¹⁰ cm/s
h = 6.626 x 10^-27 erg-s
1 eV = 1.602 x 10^-12 ergs

(where c is the speed of light, h is Planck's constant, and the conversion is between electron-Volts (eV) and ergs for energy units), and you will need the formulas: