From MetroAmp Wiki
Combining Watt's and Ohm's laws give us a set of formulas that apply to electronics and allow us to determine any variable with any two known values. These are displayed on the Electrical formula wheel below.
to determine voltage (V) in a circuit with known current (I) and resistance (R), the formula is:
V = I * R
to determine power in watts (P) in a circuit with known voltage (V) and resistance (R), the formula is:
P = V2 / R