Ohm’s law calculator
Voltage, current, resistance and power from any two of them.
About this calculator
This Ohm's law calculator finds voltage, current, resistance and power from any two of them.
It's handy for electronics projects, choosing resistors for LEDs, checking fuses and learning physics.
How it works
Ohm’s law says V = I × R: voltage equals current times resistance. Power is P = V × I, which also gives P = I² × R and P = V² ÷ R, so any two values give the other two.
Last reviewed:
Worked example
- I know: Voltage and resistance
- Voltage: 12 V
- Resistance: 6 Ω
Current: 2 A
Common questions
What is Ohm's law?
Voltage = current × resistance, V = I × R. Double the voltage across a resistor and the current doubles too.
How do I choose a resistor for an LED?
Subtract the LED's forward voltage from the supply voltage, then divide by the LED current. For a 5 V supply, a 2 V LED and 20 mA, that's 3 ÷ 0.02 = 150 Ω.
How many watts does a resistor need?
Work out the power with P = I² × R and choose a resistor rated at least twice that, so it doesn't overheat.
The story behind it
Where the idea came from: The German physicist Georg Ohm published his law in 1827. The unit of resistance, the ohm, is named after him.
Ohm's work was dismissed at first, but his simple rule turned out to describe almost every circuit. It's still the first equation taught in electronics.
Why you can trust this calculator
- It runs in your browser. Nothing you type into it is sent anywhere.
- The method is explained on this page, so you can check it yourself.
- Free to use, with no sign-up.
- Results are estimates, so please double-check anything important.
- Spotted a mistake? Tell us and we'll look into it.
After 15 years as a mathematics educator, I built CalculStore to give students, teachers, professionals and anyone working with numbers simple, reliable tools.
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