How ohm's law calculator works
Ohm's law says that the voltage V across a resistor equals the current I through it times its resistance R: V = I × R. Electric power closes the loop with P = V × I, which also equals I² × R and V² / R. That makes four quantities — voltage, current, resistance and power — bound together by two equations, so any two of them pin down the other two exactly. This calculator is that solver: fill in any two fields, leave the other two blank, and every remaining value appears with the formulas that produced it.
The arithmetic is elementary, and showing it is the point. A 12 V supply driving a 4 Ω load draws I = 12 / 4 = 3 A and dissipates P = 12 × 3 = 36 W. Knowing only that a motor draws 5 A at 230 V, the resistance is 230 / 5 = 46 Ω and the power 11.5 kW. When all you have is a resistor's power rating and its resistance — say 100 W and 25 Ω — the solver works backwards through V = √(P × R) = 50 V and I = √(P / R) = 2 A. Every pair has a closed-form answer, so nothing is ever estimated.
Enter values in volts, amps, ohms and watts (not milli- or kilo- multiples — convert first if needed) and give exactly two, because a third value would over-determine the system and could disagree with the other two after rounding. All four quantities must be greater than zero: a zero resistance or zero current is a short or open circuit that the algebra cannot resolve into finite answers. To sanity-check a power bill or an appliance's draw, pair this with the Electricity Cost Calculator; to convert between W, kW and horsepower, use the Power Converter.