Resistor Power Dissipation Calculator (Watts)

Resistor power dissipation in watts

Push current through a resistor and some of that electrical energy turns into heat. That's the Joule effect. You can work out the dissipated power from any two of the three quantities at play: P = V·I = I²·R = V²/R, where V is the voltage across the resistor in volts, I is the current in amperes and R is the resistance in ohms. Take 0.5 A flowing through 100 Ω: that gives P = 0.25 · 100 = 25 W, enough to cook a standard 1/4 W resistor in a matter of seconds.

A good habit is to pick a resistor rated for at least 2× the dissipation you calculated, then check the derating curve in the datasheet. Most manufacturers quote the nominal wattage at 70 °C ambient and derate it linearly down to zero at 155 °C. Through-hole carbon-film parts usually come in 1/8, 1/4, 1/2, 1, 2 W. SMD packages 0402, 0603, 0805, 1206 handle roughly 1/16, 1/10, 1/8, 1/4 W respectively.

Applications

Getting the power right matters in a lot of places: current-sense shunts, LED ballast resistors, snubber networks across switching transistors, inrush-limiting NTCs, the dummy loads people use when testing audio amplifiers, and the bleeder resistors that drain high-voltage capacitors. The IEC 60115 series and the JEDEC standards spell out the electrical, mechanical and climatic test methods for fixed resistors.

FAQ

Which formula should I use? Whichever one lines up with the values you already have. With sense resistors you tend to know I and R, so P = I²·R is the easy pick.

What is derating? Once the ambient goes past 70 °C, the dissipation a part can take starts falling off in a straight line. By 100 °C a 1 W resistor might only manage 0.65 W, and at 155 °C it's down to 0 W.

Can I parallel resistors to share power? Yes. Put two equal resistors in parallel and each one carries half the current, so each dissipates a quarter of the total power. The effective rating doubles.