Ohm's Law Calculator: Voltage, Current, Resistance

Solve any Ohm's law or electrical power problem by entering two known values: the calculator works out the other two.

Voltage
Current
Resistance
Power

Runs entirely in your browser. Nothing you enter is sent anywhere.

How it works

Ohm's law says the voltage across a resistor equals the current through it times its resistance. Combined with the power equation P = V × I, any two of the four quantities determine the other two.

Pick which two values you know, type them in (prefixes such as k, m and µ are understood), and the results update as you type.

V = I × R      I = V / R      R = V / I
P = V × I = I² × R = V² / R

Worked example

A 12 V supply drives 250 mA through a load. Resistance is 12 / 0.25 = 48 Ω and power is 12 × 0.25 = 3 W.

Frequently asked questions

Does Ohm's law work for LEDs and diodes?

Not directly. LEDs and diodes are non-linear: their current is not proportional to voltage. Use the LED resistor calculator, which accounts for the LED's forward voltage.

Can I use Ohm's law with AC?

For purely resistive AC loads such as heaters, yes, using RMS voltage and current. For motors, capacitors and inductors you need impedance and power factor instead.

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