Ohm's law is one of the most useful electrical calculations you can ever know. The law states basically that the current through a conductor between two points is directly proportional to the potential difference (voltage drop) across the two points, and inversely proportional to the resistance between them.
The mathematical equation is I = E / R
Where I = Current in amps, E = Voltage in volts and R = Resistance in ohms
It is just a matter of transposing this equation to calculate the value of the other two known values.
I = E / R
E = I x R
R = E / I