Ohm's law formulas
Ohm's law says the current through a conductor equals the voltage across it divided by its resistance. Combined with the power formula P = V × I, any two values give you the other two.
V = I × R I = V ÷ R R = V ÷ I
P = V × I P = I² × R P = V² ÷ R
I = √(P ÷ R) V = √(P × R)
Examples
- 12 V across a 6 Ω resistor: I = 12 ÷ 6 = 2 A, P = 24 W
- A 60 W bulb on 120 V draws 0.5 A and has 240 Ω hot resistance
- An LED needs 20 mA from a 5 V supply with a 2 V drop: R = (5 − 2) ÷ 0.02 = 150 Ω
Where Ohm's law applies
It applies exactly to resistive loads such as heaters, incandescent bulbs and resistors. Motors, LEDs and power supplies are non-linear, so use their rated current rather than calculating it from resistance.
Frequently asked questions
What are the units in Ohm's law?
Voltage in volts (V), current in amperes (A), resistance in ohms (Ω) and power in watts (W).
How do I calculate a resistor for an LED?
R = (Supply voltage − LED forward voltage) ÷ LED current. Use the next standard resistor value above the result.
Does Ohm's law work for AC?
Yes for purely resistive loads. With motors or capacitors, use impedance (Z) instead of resistance and include power factor.