Ohm's Law Calculator

Solve for voltage, current, resistance or power from any two known values.

I know	Voltage and resistance
Voltage	12 V
Resistance	24 Ω

I know

Voltage

Resistance

Result

12 V · 500 mA · 24 Ω · 6 W

V = 12 V, I = 500 mA, R = 24 Ω, P = 6 W.

Voltage: 12 V
Current: 500 mA
Resistance: 24 Ω
Power: 6 W

Current against voltage for 24 Ω (amps up, volts across)

12 V, 500 mAat (12, 0.5)Point at the chart, or Tab to it and use ← →

12 V, 500 mA: (12, 0.5)

About the Ohm's Law Calculator

Choose which two values you know (any pair of voltage, current, resistance and power) and enter them. The calculator gives all four, with sensible units such as mA and kΩ.

Ohm's law says the current through a resistor is proportional to the voltage across it: V = I R. Combined with power P = V I, any two of voltage, current, resistance and power give the other two.

The formula

V = I R; P = V I = I² R = V² ÷ R

Worked example

12 V across a 100 Ω resistor

I = 12 ÷ 100 = 0.12 A = 120 mA
P = 12 × 0.12 = 1.44 W
So use a resistor rated at least 2 W, ideally 3 W for headroom, not a common ¼ W one

Questions people ask

Does Ohm's law apply to LEDs?

Not directly; LEDs are non-linear. Use the LED resistor calculator.

What resistor wattage do I need?

At least the computed power, ideally 2× for headroom.

Does it work for AC?

For purely resistive loads such as heaters, yes, with RMS values; motors and capacitors need impedance.