Ohm's Law Calculator

Georg Ohm published his law in 1827, and it remains the most important relationship in electrical engineering. The formula V = I × R states that voltage equals current multiplied by resistance. If you double the resistance while holding voltage constant, the current drops by half. If you double the voltage while keeping resistance constant, the current doubles. This proportional relationship means that any one change in a circuit ripples predictably through the others.

Electricians use this relationship every time they size wire gauges. Larger wire has lower resistance per foot, which reduces voltage drop over long cable runs. A 20-amp circuit run in 12 AWG wire over 100 feet drops significantly less voltage than the same circuit run in 14 AWG wire — a difference that matters for motor performance and lighting consistency at the far end. Hobbyists apply Ohm's Law when calculating the resistor value needed to limit current through an LED so it does not burn out immediately when connected to a 5-volt supply.

The power equation P = V × I extends Ohm's Law to energy consumption. Combined with V = IR, you can express power as P = I² × R or P = V² / R, which lets you calculate any of the four electrical quantities — voltage, current, resistance, or power — from any two known values. This is why the calculator accepts two of the four inputs and immediately solves for the remaining two simultaneously.

HVAC technicians diagnose faulty heating elements by measuring resistance with a multimeter and comparing the result to the nameplate power rating. A 240-volt, 5,000-watt element should measure approximately 240² ÷ 5,000 = 11.5 ohms. If the measured value deviates significantly, the element is damaged and needs replacement. This same technique applies to diagnosing electric water heaters, dryer heating elements, and electric baseboard heaters. No special test equipment is needed beyond a basic digital multimeter available at any hardware store.

Ohm's Law applies strictly to resistive (ohmic) loads — components where the voltage-to-current ratio remains constant regardless of the applied voltage. Most metal conductors and standard carbon or metal film resistors behave ohmically at normal operating temperatures. Components like diodes, transistors, and LEDs have non-linear voltage-current characteristics: a diode barely conducts below its forward voltage threshold, then passes current very rapidly once the threshold is exceeded. Ohm's Law alone cannot describe this non-linear behavior accurately.

For AC circuits, resistance is replaced by impedance (Z), which combines resistance and reactance from inductors and capacitors. The relationship V = I × Z still holds in form, but Z is a complex number that varies with signal frequency. A motor or transformer behaves very differently from a purely resistive load because the inductive reactance changes with frequency. This calculator focuses on DC circuits and AC circuits with purely resistive loads — for inductive or capacitive loads, consult an impedance calculator or an electrical engineering reference text.