Electronics Calculations Using Ohm’s Law and Joule’s Law
These equations, using Ohm’s Law and Joule’s Law, are the most useful you’ll find in electronics. They come in handy all the time for checking what’s going on in your circuits.
Unknown Value  Formula 

Voltage  V = I x R 
Current  I = V/R 
Resistance  R = V/I 
Power  P = V x I or P = V^{2}/R or P = I^{2}R 
where:

V = voltage (in volts)

I = current (in amps)

R = resistance (in ohms)

P = power (in watts)