Ohm's law, enter any two.

Short answer

Ohm's law says voltage = current x resistance. With 12 V across a resistance carrying 2 A, resistance is 6 ohms and power is 24 W.

Worked example

Known: 12 V and 2 A. Resistance = 12 / 2 = 6 ohms; power = 12 x 2 = 24 W. Known power and resistance: 100 W through 4 ohms gives voltage = square root of (100 x 4) = 20 V and current = square root of (100 / 4) = 5 A.

Formula, assumptions and sources

HyperPhysics states that for many conductors the current is directly proportional to the applied voltage, and defines resistance as the ratio of voltage to current: V = IR. Power is P = VI, and convenient expressions follow by substitution: P = I squared x R and P = V squared / R. These hold for AC when voltage and current are RMS values and the load is a resistor. Inductive and capacitive AC loads need a power factor and reactance, which this tool does not handle, and some devices (diodes, filaments as they heat up) are not ohmic at all.

Source: HyperPhysics (Georgia State University), Ohm's Law (opened 2026-10-08)

Source: HyperPhysics (Georgia State University), Electric Power (opened 2026-10-08)

Safety and limits

This is a planning aid, not professional advice. Electrical and HVAC work can cause fire, shock and carbon monoxide hazards; have a licensed professional size, install and inspect equipment.

FAQ

What is Ohm's law?

Voltage equals current times resistance (V = I x R). HyperPhysics describes resistance as the ratio of voltage to current for materials where that ratio is constant.

How do I find current from power and resistance?

I = square root of (P / R). For 100 W and 4 ohms, I = 5 A.

Does it work for AC?

For resistive loads at RMS values, yes. For motors and other reactive loads you also need power factor and reactance.