Free calculator

Ohm's Law Calculator

Ohm’s law says voltage equals current times resistance. Type any two of voltage, current, resistance and power, leave the other two boxes empty, and the calculator works out the rest.

Result

Voltageyou entered this: 12 V
Currentcalculated: 500 mA
Resistanceyou entered this: 24 Ω
Powercalculated: 6 W

Current = voltage ÷ resistance, and power = voltage² ÷ resistance.

The formula

Ohm’s law

\htmlData{sym=U}{U} = \htmlData{sym=I}{I}\,\htmlData{sym=R}{R}

Power, three ways

\htmlData{sym=P}{P} = \htmlData{sym=U}{U}\,\htmlData{sym=I}{I} = \htmlData{sym=I}{I}^2 \htmlData{sym=R}{R} = \dfrac{\htmlData{sym=U}{U}^2}{\htmlData{sym=R}{R}}

What each letter means

$U$: Voltage (V · volt) : The electrical push between two points.
$I$: Current (A · ampere) : How much charge flows each second.
$R$: Resistance (Ω · ohm) : How strongly something opposes current.
$P$: Power (W · watt) : How fast energy is used or turned into heat.

How it works

Picture water in a pipe. Voltage is the pressure that pushes, current is how much water flows past every second, and resistance is how narrow the pipe is. Push twice as hard through the same pipe and twice as much flows. Power is how fast energy is turned into heat or light: multiply the voltage by the current.

Four quantities are tied together by two rules (U = I × R and P = U × I), so knowing any two lets you find the other two. The six pairs you can enter each use a rearranged form of those rules, and the note under the result shows the one that was used. The calculator needs positive numbers and describes steady DC values.

Worked example

A 12 V supply across a 24 Ω resistor. You know the voltage and the resistance.
Current: I = U ÷ R = 12 V ÷ 24 Ω = 500 mA.
Power: P = U × I = 12 V × 500 mA = 6 W. Check it another way: I² × R = (500 mA)² × 24 Ω = 6 W.

Common questions

Why do I have to fill in exactly two boxes?

Voltage, current, resistance and power are linked by two rules, so two of them fix the other two. With only one value there are many answers, and with three or four values they could contradict each other.

What is the difference between volts, amps and watts?

Volts measure the push, amps measure how much charge flows each second, and watts measure how fast energy is used. A kettle and a phone charger can both run from the same wall voltage, but the kettle draws far more current and so uses far more watts.

Does Ohm’s law work for AC?

For plain resistors it does, if you use RMS values (the steady value that would give the same heating). Capacitors and coils also change the timing of the current, so they need impedance instead of simple resistance.

Try it live

More calculators

All calculators · All lessons · How components work