Ohm's Law Calculator

About this tool

Solve Ohm's Law (V = I × R) for whichever value you don't already know, and see the resulting power dissipation alongside it. Pick which two values you have — voltage & current, voltage & resistance, or current & resistance — and the third value and power are computed instantly.

Frequently asked questions

What units does this use?

Volts, amps, and ohms throughout. If your current is in milliamps, divide by 1000 first (e.g. 20mA → 0.02A).

How is power calculated?

Power is always P = V × I, using the value you entered directly alongside the one just solved for — equivalent to P = I²R or P = V²/R depending on which pair you supplied.

Why does it say "undefined" or show a hint for zero resistance?

Resistance of 0Ω with a nonzero current would mean an undefined (infinite) voltage-per-ohm relationship in some directions, so those combinations show a hint instead of a misleading number.