Resistor Wattage Calculator
Power dissipated in a resistor and the minimum safe power rating.
What it does
Works out how much power a resistor turns into heat and the rating you should buy, including a safety factor and derating for hot surroundings.
Where it's used
LED and load resistors, dummy loads, current-sense resistors and anything that feels warm to the touch.
How to use it
- Pick which two values you know.
- Enter them — prefixes such as
50mwork. - Set a safety factor (2× is a common rule).
- Enter the ambient temperature; resistors must be derated when it is hot.
The formula explained
- P
- V²/R = I²R = V·I — heat in the resistor
- Safety factor
- extra margin so the part runs cool
- Derating
- resistors lose rating above a set temperature (often about 70 °C)
Worked example
100 mA through a 47 Ω resistor inside a warm (50 °C) enclosure.
Inputs:
- Known values: Current & Resistance
- Current: 100m A
- Resistance: 47 Ω
- Safety factor: 2 ×
- Ambient temperature: 50 °C
The calculator gives:
- Minimum rating: 1 W
- Dissipated power: 470 mW
- Voltage: 4.7 V
Tips and common mistakes
- A resistor at its full rating can exceed 100 °C on its surface.
- Air inside a closed box is much hotter than the room — use the enclosure temperature.
- Short pulses can still destroy small resistors even when the average power is low; check the pulse rating.
Questions people ask
What power ratings can I buy?
Through-hole resistors commonly come in 1/8, 1/4, 1/2, 1, 2, 3 and 5 W. Surface-mount ratings follow the package size: roughly 1/16 W for 0402, 1/10 W for 0603, 1/8 W for 0805 and 1/4 W for 1206.
What does derating mean?
Above a certain ambient temperature — often around 70 °C — a resistor may only dissipate a reduced share of its rated power, falling to zero at its maximum temperature. The datasheet’s derating curve gives the exact figures.
Why does the example need a 1 W part for 470 mW?
With a 2× safety factor the resistor should be able to handle 0.94 W, and the next standard rating above that is 1 W. Running parts well below their rating keeps them cooler and makes them last longer.
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Enter your values to get the result with the formula, substituted values and an explanation of each step.