How to use Ohm’s Law Calculator

Choose which two circuit quantities you know.
Enter their values in the labelled base units.
Calculate and review the equations and remaining quantities.

Example: Ohm’s Law Calculator

Ohm’s Law Calculator: Voltage 12 V and resistance 6 ohm: current 2 A, power 24 W.

You add

Which two values do you know?: vr Voltage (V): 12 Current (A): (blank) Resistance (ohm): 6 Power (W): (blank)

You get

Voltage 12 V and resistance 6 ohm: current 2 A, power 24 W. Known: voltage 12 V and resistance 6 ohm Voltage = 12 V Current = 2 A Resistance = 6 ohm Power = 24 W Working: Current = voltage / resistance = 12 / 6 = 2 A Power = voltage x current = 12 x 2 = 24 W Model: an ideal resistor supplied by DC, with P = VI. Known values | Voltage and resistance Voltage | 12 V Current | 2 A

Options

Known quantities: Select the calculation and enter its required electrical quantities using the units named beside the controls.
Power result: Check whether the result uses voltage, current or resistance before comparing it with a component rating.

Supported inputs and limits

A single ideal resistive DC model with nonnegative magnitudes and positive resistance. It does not model AC impedance, reactive components, nonlinear devices or safe component ratings. Underdetermined pairs refuse. Entered quantities range from zero to 10¹², subject to the selected pair’s positive-value requirements. Enter plain decimals with at most 60 characters, 40 digits and 20 fractional places per number; exponent notation is not accepted.

Where your input is processed

This tool processes your input in this browser. Your text and files are not uploaded to UseFreeTools. Check this tool's limits for anything it may save on your device.

Use the resistance model within its limits

Ohm-law arithmetic assumes the entered relationship applies to the device and operating conditions. Many components are nonlinear, and an AC impedance model can differ from a resistance-only calculation. A numerical result does not establish that a circuit is safe to build or connect.

Questions about Ohm’s Law Calculator

Can I use this for an AC circuit?

This model does not include phase, reactance or frequency. Use it only when the simple resistive assumptions describe your calculation.

Why can some zero inputs be undefined?

Dividing by zero cannot determine a finite result, and two zero readings may leave another quantity unknown. The tool explains rather than inventing a value.

Does the calculated power give a safe resistor rating?

No. Choosing a component also requires operating conditions, margins and its datasheet. The output is model arithmetic.

Project manager: Tony Hines · Content updated 4 October 2026 · Report a problem

Ohm’s Law Calculator

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