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V = IR · P = VI

Ohm's law calculator

Enter exactly two of voltage, current, resistance or power. The other two are computed from V = IR and P = VI.

Short answer: V = IR and P = VI. 12 V across 6 Ω draws 2 A and dissipates 24 W — which is also why a resistor needs a power rating above the wattage it will actually see.

The four equations it uses

V = IR, I = V ÷ R, R = V ÷ I, P = VI, plus P = I²R and P = V² ÷ R for the cases where the missing quantity is a current or a resistance. Each is the same pair of laws rearranged, so the answers always agree with each other.

Worked example

A 60 W bulb on 230 V: I = 60 ÷ 230 = 0.261 A, R = 230 ÷ 0.261 = 881 Ω. Choose a resistor for the same current and it must be rated above 60 W — a quarter-watt part would fail immediately.

Ohm's law questions

What is Ohm's law?

V = I × R: the voltage across a resistor equals the current through it multiplied by its resistance. Combined with the power equation P = V × I, any two known values determine the rest, which is what this calculator does.

Which pair should I enter?

Whatever you can measure or read from a datasheet. A 12 V supply across a 6 Ω resistor solves for 2 A and 24 W. A 60 W bulb on 230 V solves for 0.26 A and 881 Ω.

Does this apply to AC circuits?

Only where the load is purely resistive, or where you use RMS values with the impedance in place of resistance. With motors, capacitors or transformers, current and voltage are out of phase and the arithmetic here understates the current.

Why does it need exactly two values?

One value cannot determine a circuit: 12 V could be a 6 Ω resistor at 2 A or a 24 Ω resistor at 0.5 A. Three values would over-determine it and may contradict each other, so the page asks for exactly two rather than silently ignoring one.

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