Ohm's Law Calculator

What is Ohm's law and how do you calculate it?

Ohm's law and power formulas

The engine supports all six unordered input pairs. It first converts selected prefixes to SI base units, uses V = IR and P = VI, and checks that all four results are finite and strictly positive.

V = I × R
I = V / R
R = V / I
P = V × I = I² × R = V² / R
I = √(P / R)
V = √(P × R)

How to use the Ohm's law calculator

1. 1

   Choose the two values you know

   Select a pair such as voltage and resistance, voltage and current, or power and resistance.

2. 2

   Enter the numbers and units

   Use V, mV, or kV; A, mA, or µA; Ω, kΩ, or MΩ; and W, mW, or kW.

3. 3

   Read volts, amps, ohms, and watts

   The result card shows all four quantities, the formula with your numbers, and the Ohm's law triangle.

4. 4

   Check the resistor wattage note

   When power is solved, the page suggests the next common rating at or above twice that power. That note is a rule of thumb, not a thermal design.

Ohm's law worked examples

V = 12 V; R = 6 Ω

I = 2 A; P = 24 W

V = 5 V; I = 20 mA

R = 250 Ω; P = 0.1 W

V = 120 V; P = 60 W

I = 0.5 A; R = 240 Ω

I = 2 A; R = 10 Ω

V = 20 V; P = 40 W

I = 2 A; P = 100 W

V = 50 V; R = 25 Ω

R = 20 Ω; P = 80 W

I = 2 A; V = 40 V

Frequently Asked Questions

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Educational DC/purely resistive calculations, not installation, AC motor, breaker, or conductor safety advice.