Ohm's Law Calculator

Enter any two of voltage, current, resistance and power, and this calculator works out the other two using Ohm's law (V = I × R) and the power formula (P = V × I).

Fill exactly two fields.

How It Works

The two base relations

Ohm's law links voltage, current and resistance; the power formula links voltage, current and power:

  • V = I × R
  • P = V × I

From these, any quantity can be found from any two others (for example R = V² / P, or I = √(P / R)).

Worked example

With V = 12 V and I = 2 A: resistance R = V / I = 6 Ω, and power P = V × I = 24 W.

Ohm's Law Formula Wheel

Voltage (V)

V = I × R
V = P / I
V = √(P × R)

Current (A)

I = V / R
I = P / V
I = √(P / R)

Resistance (Ω)

R = V / I
R = V² / P
R = P / I²

Power (W)

P = V × I
P = I² × R
P = V² / R

Frequently Asked Questions

What is Ohm's law?

Ohm's law states that the current through a conductor is proportional to the voltage across it and inversely proportional to its resistance: V = I times R.

How do I calculate power from voltage and current?

Power is voltage multiplied by current: P = V times I. For example 12 V at 2 A is 24 W.

How do I find resistance from power and current?

Resistance equals power divided by current squared: R = P / I squared.

Does Ohm's law work for AC circuits?

For purely resistive AC circuits, yes. With reactive loads you use impedance and power factor instead, which the watts-amps-volts calculator covers.

Why do I enter exactly two values?

Any two of the four quantities fully define a resistive circuit, so the calculator derives the remaining two from the two you provide.

Related Calculators

Continue with these related electrical tools

Watts Amps Volts Calculator

Convert between watts, volts and amps (DC, 1-phase, 3-phase)

LED Resistor Calculator

Series resistor for an LED: ohms, power and nearest standard value

Voltage Drop Calculator

Voltage drop over a run by length, current and conductor (AWG/mm2, 1ph/3ph/DC)

Embed This Calculator

Add this free calculator to your website. Copy the code below and paste it into your page.

Electrical Calculators Electrical Calculators

Fast, free calculators for electricians and engineers

More Calculators

© 2026 Electrical Calculators. All rights reserved.

Privacy Policy Terms of Service Cookie Policy