Ohm's Law Calculator

Enter any two of voltage, current, resistance and power (leave the unknowns at 0) and the solver returns the remaining two. It combines Ohm's law, V = I × R, with the power equation, P = V × I, so it works from whichever pair you know.

Results are estimates for planning and education, based on your inputs and standard engineering values (AWG resistance, NEC ampacity, resistivity). Electrical work can be dangerous and is governed by the NEC and your local code — verify all sizing with a licensed electrician and your authority having jurisdiction (AHJ). Not a substitute for professional design.

Calculator

Voltage	20.00 V
Current	10.00 A
Resistance	2.00 Ω
Power	200.00 W

Formula

Ohm's law links voltage, current and resistance as V = I × R, and power adds P = V × I. From any two quantities the others follow: I = V / R, R = V / I, P = I² × R, P = V² / R, V = √(P × R), and I = √(P / R). Enter the two you know and leave the rest at 0.

Worked example

With a current of 10 A through a 2 Ω resistance: V = I × R = 10 × 2 = 20 V, and P = V × I = 20 × 10 = 200 W (the same as I² × R = 100 × 2 = 200 W). Knowing instead 120 V across 60 Ω gives I = 120 / 60 = 2 A and P = 120 × 2 = 240 W.

Ohm's law, V = I × R, is the single relationship underneath almost every other calculation on this site, and this tool solves for whichever of voltage, current or resistance you are missing from the other two. It is the fastest way to sanity-check a circuit: if you know the supply voltage and the load's resistance, you know the current it will draw and therefore the wire and breaker it needs. Pair it with the power form, P = V × I, to move between watts, amps and volts. Real components drift with temperature and are never perfectly linear, so treat the result as the ideal figure a design starts from rather than an exact measurement.

Frequently asked questions

What is Ohm's law?

Ohm's law states that the current through a conductor is proportional to the voltage across it and inversely proportional to its resistance: I = V / R, usually written V = I × R. It is the foundation of almost every DC circuit calculation.

How do I find power from voltage and current?

Multiply them: P = V × I. If you know current and resistance instead, use P = I² × R; if you know voltage and resistance, use P = V² / R. This solver picks the right form automatically from the two values you enter.

Which two values do I need to enter?

Any two of voltage, current, resistance and power. Leave the two unknowns at 0 and the calculator derives them. For example, voltage and power, or current and resistance, are both enough.

Does this work for AC circuits?

It applies directly to DC and to purely resistive AC loads. For reactive AC loads you also need the power factor, which the power converter and three-phase tools handle.

Source: Ohm's law (V = I × R) and the electrical power equation (P = V × I) · All sources