Resistor Power Dissipation Calculator (Watts)
Computes the power dissipated in a resistor in watts from current and resistance or from voltage and resistance.
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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.
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The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.