Resistor Power Dissipation Calculator
Choose which two values you know, and see the watts turned into heat, the 2x recommended rating and the next standard resistor wattage. For learning and design checks, not safety certification.
Updated
Report a problem
Formula
Electrical power turned into heat in a resistor:
P = V × IP = I² × RP = V² ÷ R
Recommended rating: P_rated = 2 × P, then the smallest standard step at or above it (1/8, 1/4, 1/2, 1, 2, 3, 5, 10, 20, 25, 50, 100 W and up).
Worked example
12 V across a 100 ohm resistor: P = 12² ÷ 100 = 144 ÷ 100 = 1.44 W. With a 2x margin that is 2.88 W, so choose a 3 W resistor. The current is 12 ÷ 100 = 0.12 A = 120 mA.
Frequently asked questions
How do I calculate the power a resistor dissipates?
Use P = V x I, P = I² × R or P = V² / R, whichever matches the two values you know. For 12 V across 100 ohm, P = 144 / 100 = 1.44 W.
Why use a 2x safety margin?
A resistor run at its full rating gets very hot and drifts in value. Designers commonly choose a rating of at least twice the dissipation, so 1.44 W calls for 2.88 W or more.
What are the standard resistor power ratings?
Common steps are 1/8, 1/4, 1/2, 1, 2, 3, 5, 10, 25 and 50 W and up. This calculator picks the smallest step that is at least twice the dissipation, so 2.88 W becomes a 3 W part.
Is this safe to rely on for a real product?
It is for learning and design checks only, not safety certification. Real parts also need derating for ambient temperature, mounting, pulses and the maker's datasheet.