Ohm's Law Calculator

An Ohm's Law calculator finds resistance and power from a circuit's voltage and current. It divides voltage by current for resistance (R = V ÷ I) and multiplies voltage by current for power (P = V × I). A circuit reading 120 V at 10 A has 12 Ω of resistance and draws 1,200 W.

Last reviewed September 2026Method & sources Report an error

Resistance

12.000 Ω

Breakdown
Resistance	12.000 Ω
Power	1,200.00 W

How to Calculate Ohm's Law

Ohm's Law relates voltage, current, and resistance with `V = I × R`. Rearranged here as `R = V ÷ I`, it solves for resistance from the voltage and current entered. Power follows a separate but related formula, `P = V × I`. Together, voltage and current are enough to find all four of the common electrical quantities (voltage, current, resistance, power) that make up the "power wheel" used throughout basic electrical work.

Example: Circuit reading 120 V at 10 A

A circuit reading 120 V at 10 A: 1. Resistance: R = V ÷ I = 120 ÷ 10 = 12 Ω 2. Power: P = V × I = 120 × 10 = 1,200 W That circuit is drawing 1,200 W (1.2 kW) through a 12 Ω load.

What Is Ohm's Law?

An Ohm's Law calculator finds a circuit's resistance and power from its voltage and current. Voltage and current are the two most commonly known values on a nameplate or a meter reading. It's the starting formula behind most basic electrical troubleshooting and sizing, from checking whether a component's resistance looks right to estimating how much power a circuit is drawing.

Ohm's Law: Pros and Cons

Pros

Uses the exact, textbook Ohm's Law and power formulas. No approximation is involved
Gives both resistance and power from the same two inputs, covering all four common electrical quantities at once
Works for any DC circuit, or an AC circuit at a stated power factor of 1 (a purely resistive load)

Cons

Assumes a simple, linear resistive load. It doesn't account for AC power factor, reactance, or non-linear loads (motors, electronics with switching power supplies), which change the real relationship between voltage, current, and power
Doesn't measure voltage or current for you. Accuracy depends entirely on how those two readings were taken
For a circuit's real (usable) power under AC with a power factor other than 1, see the Watts, Amps & kVA calculator instead

Ohm's Law: What a Pro Checks

This assumes a simple, linear resistive load. For a motor or electronics with a power factor other than 1, check the Watts, Amps & kVA calculator instead for the circuit's real usable power.
Your result is only as accurate as the voltage and current readings you enter, so take those measurements carefully before relying on the calculated resistance or power.
Use this as a quick sanity check. Compare a component's calculated resistance against its expected nameplate or spec-sheet value to catch a wiring or measurement error early.

Frequently Asked Questions

What is Ohm's Law?

Ohm's Law states that voltage equals current times resistance: V = I × R. Rearranged, it also gives resistance from voltage and current (R = V ÷ I) or current from voltage and resistance (I = V ÷ R).

How do you calculate resistance from voltage and current?

Divide voltage by current: R = V ÷ I. A circuit reading 120 V at 10 A has a resistance of 120 ÷ 10 = 12 Ω.

How do you calculate power from voltage and current?

Multiply voltage by current: P = V × I. A circuit reading 120 V at 10 A is drawing 120 × 10 = 1,200 W.

What is the Ohm's Law power wheel?

It's a reference diagram showing all the ways to find voltage, current, resistance, and power from any two of the other three quantities. This calculator covers the voltage-and-current starting point, finding resistance and power from it.

Does Ohm's Law apply to AC circuits?

It applies directly to purely resistive AC loads (like an incandescent bulb or a resistive heater) at a power factor of 1. Motors, electronics, and other reactive loads need power factor accounted for separately. See the Watts, Amps & kVA calculator for that case.

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