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Calculator

Ohm's Law Calculator

Fill in any two of voltage, current, resistance or power — get the rest.

V
mA
Ω
mW

Clear any two fields you want calculated, or fill any two and leave the rest blank.

Result

Voltage
5 V
Current
22.73 mA
Resistance
220 Ω
Power
113.6 mW
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How it works

Ohm's law links three quantities in every circuit: voltage (V), current (I) and resistance (R). Power (P) follows from them. Enter any two values below and the other two are calculated.

Use milliamps for current and milliwatts for power here — they are the units you will meet most in microcontroller work. Leave the two fields you want to calculate blank.

Formula

Ohm's lawV = I × R
PowerP = V × I = I² × R = V² / R

Worked example. 5 V across 220 Ω gives I = 5 / 220 = 22.7 mA and P = 5 × 0.0227 = 113.6 mW — fine for a 1/4 W resistor.

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Frequently asked questions

What is Ohm's law in one sentence?

Current through a resistor is proportional to the voltage across it: I = V / R.

Do LEDs obey Ohm's law?

Not directly. An LED has a nearly fixed forward voltage, so the resistor in series does the limiting — see the LED resistor calculator.

How do I know what power rating a resistor needs?

Calculate P = I² × R and choose a resistor rated at least twice that. Common ratings are 1/8 W, 1/4 W, 1/2 W and 1 W.

Why do I get a different current than expected in a real circuit?

Real sources have internal resistance and real parts have tolerance. A 9 V battery sags under load, and a 5% resistor can be 5% off, so expect results within roughly ±10%.

Boards this helps with

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