Ohm's Law Calculator

How to use this calculator

1. Enter any two of the four values: voltage, current, resistance or power.
2. Leave the other two blank.
3. Click Calculate — the remaining two are worked out instantly.
4. Use Clear to reset all fields.

The Ohm's law formulas

Ohm's law links voltage (V, volts), current (I, amps) and resistance (R, ohms):

Electrical power (P, watts) adds three more relationships:

With any two known quantities, the remaining two can always be found. This calculator picks the right formula automatically based on which fields you fill in.

Symbols and units

Example

A 12 V supply drives a 100 Ω resistor. Enter V = 12 and R = 100. The calculator returns I = 0.12 A and P = 1.44 W.

Frequently asked questions

What is Ohm's law?

Ohm's law states that voltage equals current times resistance: V = I × R. Combined with P = V × I, you can find any one of voltage, current, resistance or power from two known values.

How do I calculate power from voltage and current?

Multiply them: P = V × I. For example, 12 V × 2 A = 24 W. You can also use P = I² × R or P = V² / R.

Does this work for AC circuits?

It applies to DC and to purely resistive AC loads. For reactive AC loads you also need to account for impedance and power factor.

How many values do I need?

Enter exactly two of V, I, R and P.