Ohm’s Law and Power Calculator
Enter any two of voltage, current, resistance and power. The other two are calculated, with a suggested power rating for a resistor.
How it works
Ohm’s law links voltage, current and resistance: V = I × R. The power turned into heat in a resistor is P = V × I, which with Ohm’s law is also P = I² × R and P = V² ÷ R. Any two of the four quantities fix the other two; the calculator uses whichever form fits the pair you choose. The suggested resistor rating is twice the dissipated power, rounded up to a standard size, which keeps the part cool and reliable. The ideas are explained in Circuit Basics.
Worked example
A 12 V supply and 20 mA of current: R = 12 ÷ 0.020 = 600 Ω and P = 12 × 0.020 = 0.24 W, so use at least a 1/2 W resistor. A 470 Ω resistor on 5 V: I = 5 ÷ 470 = 10.6 mA, P = 5² ÷ 470 = 53 mW, so even a 1/8 W part meets the 2 times rule, and a 1/4 W part is a comfortable choice. A load that draws 2 A and dissipates 10 W: V = 10 ÷ 2 = 5 V and R = 10 ÷ 2² = 2.5 Ω.
Things to know
This is for direct current and for resistive loads. For AC circuits with inductors and capacitors, impedance and power factor also matter.
Real resistors have a tolerance (often 1 to 5 percent) and change slightly with temperature, so the result is nominal.
Power ratings assume a free flow of air at normal room temperature. Resistors run hot long before they fail: keep to about half the rating where you can.
LEDs, diodes and batteries do not follow Ohm’s law as a fixed resistance: see the LED resistor calculator for those.
Common questions
Which two values should I give?
Any two you know. A supply voltage and a load current are the usual pair, or a resistor value and the voltage across it.
What does the power rating mean?
It is how much heat the resistor can dissipate continuously. A 1/4 W resistor carrying 0.2 W is running at 80 percent of its rating and will be hot: choose a larger one.
Why is current in mA and resistance in kΩ in some answers?
The result uses the most readable prefix: for example 0.0106 A is shown as 10.6 mA.
Sources
- Horowitz, P. and Hill, W., The Art of Electronics, 3rd ed., Cambridge University Press.
- IEC 60063, Preferred number series for resistors and capacitors.
Last reviewed: October 22, 2026