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Ohm's law is the single most-used relationship in electronics: the voltage across a resistor equals the current through it times its resistance, V = I × R. Rearranged, the current is I = V / R and the resistance is R = V / I. Power — the rate at which the component turns electrical energy into heat or work — adds a second relationship, P = V × I, which combines with Ohm's law to give the two equivalents P = I² × R and P = V² / R. Together these four quantities form the V–I–R–P 'Ohm's law wheel', and knowing any two of them fixes the other two.

This calculator lets you pick which quantity to solve for and enter any two of the others, then returns the full wheel plus the three equivalent power formulas side by side. For example, a 12 V supply driving a 6 Ω resistor gives I = 12 / 6 = 2 A and P = 12 × 2 = 24 W; check it with P = I² × R = 4 × 6 = 24 W and P = V² / R = 144 / 6 = 24 W, all agreeing. Because the tool back-fills every missing value, entering power and current instead is just as valid — 24 W at 2 A returns 12 V and 6 Ω.

Ohm's law holds exactly for any DC resistive circuit and for the resistive part of an AC circuit, which makes it the everyday tool for sizing current-limiting resistors, choosing LED droppers, working out fuse and wire ratings, reading how much a heater or motor will draw, and predicting the heat a resistor must dissipate. It does not by itself describe capacitors, inductors or non-ohmic parts like diodes, where the voltage–current relationship is no longer a straight line.