BASICS · CALCULATOR

Ohm’s law calculator

Free Ohm's law calculator: get current, power, voltage and resistance from voltage, resistance, current and power. The formula is shown; verify against the NEC.

Written by Chameera Sampath · Not yet reviewed by a licensed electrician. See the methodology.

US (NEC) only · NFPA 70 / NEC — locally adopted editionDefaults are US values. Verify the locally adopted NEC edition and project data.Official reference ↗

Project inputs

Measurements

Known quantitiesVoltage

Resistance

Updates as inputs change. Unit changes preserve the same physical values.

THE MATH BEHIND THE ANSWER

Method & assumptions

V = IR; P = VI = I²R = V²/R — solve from any two known values

DC or purely resistive loads. For reactive AC loads use impedance.

Results do not check every aspect of an electrical installation. The calculators follow US (NEC) practice only; a regional preset changes defaults such as voltage and units, and no result certifies NEC compliance.

Sources and validation →

Worked example

Known quantities: Voltage + resistance · Voltage: 120 V · Resistance: 24 Ω

Current: 5 A · Power: 600 W · Voltage: 120 V · Resistance: 24 Ω

This example uses the stated inputs; results for the entered values appear above.

Explore the basics branch →See the related electrical concepts →

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