How to use this calculator
Choose which two values you know and enter them. Units like mA and kΩ are converted automatically.
Formulas
Ohm’s lawV = IR
PowerP = VI = I²R = V²R
Worked example
12 V across 220 Ω (the default)
- I = 12/220 = 54.5 mA.
- P = 12²/220 = 0.655 W, so a ¼ W resistor would overheat; use 1 W.
Frequently asked questions
Why does resistor power rating matter?
A resistor turns P into heat. Running it above its rating shortens its life or burns it; a common rule is to use a part rated at least twice the calculated power.
Does Ohm’s law work for LEDs and diodes?
Not directly. Their current is not proportional to voltage. Use Ohm’s law on the series resistor instead: R = (Vsupply − VLED)/I.