Ohm's Law Calculator
Enter any two of voltage, current, resistance or power and get the other two instantly, with the formula used shown.
How to use it
- Fill in any two of the four boxes.
- The remaining two are calculated and highlighted.
- Clear a box to change which pair you are solving from.
Examples
| 12 V, 3 A | R = 4 Ω, P = 36 W |
Voltage, current, resistance and power are linked by two formulas — V = I × R and P = V × I — so knowing any two of the four values is enough to work out the other two. Enter whichever pair you have and the rest fill in automatically, with the formula used shown.
Every combination this covers
| You know | You get | Formula used |
|---|---|---|
| Voltage + Current | Resistance, Power | R = V ÷ I, P = V × I |
| Voltage + Resistance | Current, Power | I = V ÷ R, P = V × I |
| Current + Resistance | Voltage, Power | V = I × R, P = V × I |
| Power + Voltage | Current, Resistance | I = P ÷ V, R = V ÷ I |
| Power + Current | Voltage, Resistance | V = P ÷ I, R = V ÷ I |
| Power + Resistance | Voltage, Current | V = √(P × R), I = √(P ÷ R) |
Reading the relationship, not just the numbers
Voltage and current are directly proportional at fixed resistance — double the voltage across a resistor and the current through it doubles. Current and resistance are inversely proportional at fixed voltage — double the resistance and the current halves. This matters for practical sizing: a 12V circuit through a 4Ω load draws 3A and dissipates 36W, and knowing which of those numbers moves and which stays fixed is what lets you pick the right resistor, fuse or power supply.
Questions people ask
What is Ohm's law in simple terms?
Current through a conductor is proportional to the voltage across it and inversely proportional to its resistance: V = I × R. Double the voltage and the current doubles; double the resistance and it halves.
Last updated 17 August 2026