Ohm’s law describes the relationship between voltage, current, and resistance: V = I × R. The related power identities are P = V × I, P = I²R, and P = V²/R.

Which two values can I enter?

You can enter any valid pair from voltage, current, resistance, and power. The calculator solves the remaining two values and keeps the units visible.

Does Ohm’s law work for AC circuits?

For purely resistive AC loads, RMS voltage and current can use the same relationship. Reactive AC circuits require complex impedance and phase, which this simple calculator does not model.

Why should I allow extra resistor wattage?

A resistor operated continuously at its nameplate limit runs hot. Select a rating above calculated dissipation and check the manufacturer derating curve, ambient temperature, enclosure, pulse energy, and tolerance.

Can I open the calculated circuit?

Yes. Use the editor action after calculating to build an editable source-and-resistor circuit and continue with supported-model analysis.

Ohm’s Law Calculator — Free Engineering Tool

Electrical lab bench with calculator, multimeter, oscilloscope, and low-voltage test circuit used to verify Ohm’s Law Calculator results. — Calculate the nominal result, then verify assumptions, limits, and real behavior with appropriate measurements.

Ohm’s Law Calculator formula, inputs, result, verification, or design-boundary visual 1 of 8 — Ohm’s Law Calculator input-to-result overview.

Ohm’s Law Calculator formula, inputs, result, verification, or design-boundary visual 2 of 8 — Formula and unit relationship.

Ohm’s Law Calculator formula, inputs, result, verification, or design-boundary visual 3 of 8 — Worked-example calculation path.

Ohm’s Law Calculator formula, inputs, result, verification, or design-boundary visual 4 of 8 — Input and assumption checklist.

Ohm’s Law Calculator formula, inputs, result, verification, or design-boundary visual 5 of 8 — Nominal result and interpretation.

Ohm’s Law Calculator formula, inputs, result, verification, or design-boundary visual 6 of 8 — Tolerance and worst-case verification.

Ohm’s Law Calculator formula, inputs, result, verification, or design-boundary visual 7 of 8 — Editable-circuit and related-tool next step.

Ohm’s Law Calculator formula, inputs, result, verification, or design-boundary visual 8 of 8 — Safety, model, and professional-review boundary.

Ohm’s Law Calculator formula and engineering interpretation

Ohm’s law connects voltage, current and resistance through V = I × R. With the power identity P = V × I, any two positive values determine the remaining two for a steady-state resistive circuit.

Formula

V = I × R; I = V/R; R = V/I; P = VI = I²R = V²/R.

Worked example

At 12 V across 1 kΩ, current is 12 mA and nominal resistor dissipation is 0.144 W. A 0.25 W part has limited thermal margin; a 0.5 W part runs cooler when ambient and enclosure conditions permit.

How to use the result in a real design

Use the result to check supply loading, resistor heating and measurement range. For AC networks with capacitance or inductance, replace simple resistance with complex impedance and account for phase.

What the result does not prove

The solver assumes positive finite values, an ideal source and a fixed lumped resistance. Temperature coefficient, component tolerance, wiring resistance, transient energy and nonlinear devices are outside the model.

Verification workflow

Confirm every input and unit from a source document or measurement.
Calculate the nominal result and record the assumptions.
Repeat with credible minimum and maximum input values.
Compare component ratings, temperature and abnormal conditions.
Open an editable circuit, run supported checks, and obtain professional review where safety depends on the answer.

Related design tools

Continue with the voltage drop calculator, wire size calculator, supported-model circuit simulator, or open the browser editor.

Review note: Equations and example arithmetic were reviewed on 2026-07-30. Results are planning aids, not certification or installation instructions.