Ohm's Law Calculator

Enter any two of voltage, current, resistance, or power to calculate the other two.

Voltage (V)

Current (A)

Resistance (Ω)

Power (W)

Voltage (V) —

Current (A) —

Resistance (Ω) —

Power (W) —

V = I × R | P = V × I = I²R = V²/R. Enter any two known values; the other two will be calculated.

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Common Values

Component	Typical Voltage	Typical Current
USB port	5V	0.5–2A
AA battery	1.5V	0.5–3A max
Car battery	12.6V	200–600A (cranking)
Standard LED	1.8–3.3V	20mA
Arduino GPIO	5V / 3.3V	20–40mA max
US wall outlet	120V AC	15–20A circuit

Understanding Ohm's Law

Ohm's law is the foundation of electrical engineering. It describes the relationship between voltage (V), current (I), and resistance (R): V = I × R.

Power can be derived from any two of these values: P = V × I = I²R = V²/R. This calculator lets you enter any two known values and computes the remaining two.

This relationship holds for resistive (ohmic) loads. For reactive components like capacitors and inductors in AC circuits, impedance replaces resistance.

To use this calculator, enter any two of the four values. The most common starting point is knowing voltage and resistance: a 12V battery supplying a 100Ω resistor draws 12/100 = 0.12A (120mA) and dissipates 12 × 0.12 = 1.44W.

Unit check: current is often given in milliamps (mA). Convert to amps before entering (divide by 1000). A 20mA LED current is 0.020A. Resistance is in ohms (Ω); kilohms (kΩ) must be multiplied by 1000.

Power dissipation determines whether a component gets hot. A resistor rated 1/4W (0.25W) will overheat if the calculated P exceeds that rating — choose a higher-rated resistor or increase resistance to reduce power. The formula P = I²R shows that doubling current quadruples dissipation.

Ohm's law applies to resistive (DC or AC-resistive) loads. For motors, capacitors, and inductors in AC circuits, use impedance Z in place of R; the basic relationships still hold with complex numbers.