Enter any two of power, voltage, current, and resistance to solve for the other two automatically.

How to Use the Watts-Volts-Amps-Ohms Calculator

Fill in any two of the four fields — power, voltage, current, resistance — and leave the other two blank. As soon as exactly two hold valid numbers, the calculator solves for the remaining two automatically and highlights them as results. Enter zero, one, or more than two values and it just asks for exactly two instead of guessing.

Ohm's Law by itself only ever relates three quantities: voltage, current, and resistance. Add the power law — P = V × I — and a fourth quantity enters the picture, and with it, a lot more flexibility. Between the two relationships, any two of the four values are enough to pin down the other two, which is why this is sometimes called the "Ohm's Law wheel": twelve formulas arranged in a circle, one for solving each quantity from each possible pair of the other three.

V & I known:  P = V × I,        R = V ÷ I
V & R known:  I = V ÷ R,        P = V² ÷ R
V & P known:  I = P ÷ V,        R = V² ÷ P
I & R known:  V = I × R,        P = I² × R
I & P known:  V = P ÷ I,        R = P ÷ I²
R & P known:  I = √(P ÷ R),     V = √(P × R)

Twelve formulas sounds like a lot until you notice they're really just six relationships, each one solved for two different unknowns — this tool implements exactly those six pairs, and the other six formulas floating around on wheel diagrams are just the same equations rearranged the other way. You'll never need to know which formula applies; picking your two known values does that automatically.

This is probably the single most reused reference on this site's electrical section — sizing a resistor for an LED, checking a multimeter reading against a datasheet, or backing into a device's current draw from its wattage label all reduce to the same four-quantity relationship.