Ohm's Law Calculator

Enter any two of voltage, current, resistance and power to get the other two, with the formulas used and answers in units like mA, kΩ and kW.

Ohm's law links voltage, current and resistance: V = I × R. Together with the power law, P = V × I, any two of the four values give the other two. 12 V across a 4 Ω resistor drives 3 A and turns 36 W into heat. A resistor that drops 3 V at 20 mA is 150 Ω and dissipates 60 mW. Enter any two of voltage, current, resistance and power, and this calculator works out the other two, shows the formulas it used, and gives answers in convenient units such as mA, kΩ and kW. If you fill in more than two boxes, it uses the last two you typed.

Enter any two values. The other two are worked out.

V
A
Ω
W
Voltage
12 V

given

Current
3 A
Resistance
4 Ω

given

Power
36 W
How it was worked out
  1. I = V ÷ R = 3 A
  2. P = V² ÷ R = 36 W

Common use cases

  • Choose a current-limiting resistor for an LED
  • Check the current and power in a heater element from its resistance
  • Work out a resistor's power rating before buying it
  • Check answers for electronics and physics homework

How to use

  1. Enter any two of voltage, current, resistance and power.
  2. Read the other two values in the results.
  3. Check the formulas in the working.
  4. Clear a box, or press Clear all, to work from a different pair.

FAQ

What are the Ohm's law formulas?

V = I × R, I = V ÷ R and R = V ÷ I. With power: P = V × I = I² × R = V² ÷ R. From those, V = P ÷ I = √(P × R), I = P ÷ V = √(P ÷ R) and R = V² ÷ P = P ÷ I².

How do I choose a resistor for an LED?

Subtract the LED's forward voltage from the supply, then divide by the current you want. With a 5 V supply, a 2 V LED and 20 mA: (5 − 2) ÷ 0.02 = 150 Ω. The resistor dissipates 3 V × 0.02 A = 0.06 W, so a common ¼ W resistor is plenty. Check the LED's data sheet for its forward voltage and current.

Does Ohm's law work for AC?

It works for resistive loads such as heaters and incandescent bulbs. Motors, transformers and electronics have reactance as well as resistance, so on AC you use impedance in place of resistance, and power also depends on the power factor.

Why does the calculator ignore one of my values?

Two values are enough to fix the other two, so a third could disagree with them. The calculator uses the last two boxes you typed in and works out the rest. Clear a box to choose a different pair.

Related tools

Results are worked out from the figures you enter, for information only. They are not electrical, engineering or professional advice. Mains electricity can kill and start fires. Check cable and circuit sizes against the wiring rules where you live (the NEC and local amendments in the US, BS 7671 in the UK, AS/NZS 3000 and 3008 in Australia and New Zealand), and use a licensed electrician where the law requires one.