Resistor power rating and derating: how much heat, and which resistor to buy
Every resistor turns the energy it drops into heat, P = V × I = V²/R = I²R, and its power rating is how much of that it can shed without its element getting too hot. The catch is that the rating is given for a certain ambient temperature (70 °C for film resistors, 25 °C for wirewound) and shrinks above it, in a straight line to zero at the element's limit. This calculator works out the dissipation from the voltage across the resistor or the current through it, how much of a chosen rating is left at your ambient, whether the part is comfortable, hot or overloaded, and the rating to buy for a safe half-load.
How to use the resistor power calculator
- Type the resistance and either the voltage across it or the current through it (in mA); the last one typed is used and the other is filled in.
- Set the ambient temperature, the air at the resistor: 25 °C on the bench, 50–70 °C inside an enclosure, more near a heatsink.
- Pick a rating in the menu row: ⅛ W to 2 W film, 3 to 10 W wirewound, 25 and 50 W housed. The drawing shows the resistor with its readings and a load bar, the panel the derating curve with your point on it. The Ratings tab compares every rating at once, and the recommendation button sets the one that runs at half-load.
The formulas
P = V × I = V² / R = I² R
Pick whichever two quantities you know. The square matters: 10 % more current is 21 % more heat, twice the voltage is four times the power. A 100 Ω resistor with 12 V across it carries 120 mA and dissipates 1.44 W, which is far too much for the quarter-watt part most people reach for first. For AC use RMS values; for pulses the average over a few seconds is what heats the part.
Derating with temperature
capacity = rating × (Tzero − Tambient) / (Tzero − Tstart) above Tstart
A resistor's rating assumes it can get rid of its heat into the surrounding air. The hotter that air, the less heat the part can shed before its element reaches its limit, so data sheets give a derating curve: full power up to a start temperature, then a straight line down to zero. For metal-film and carbon-film parts that is 70 °C to 155 °C, so at 100 °C only 65% of the rating is left. Wirewound parts are rated at 25 °C and run to 275 °C or more. The ambient to use is the temperature at the resistor, inside the box, next to whatever else is warm.
| Rating | 25 °C | 50 °C | 70 °C | 100 °C | 125 °C |
|---|---|---|---|---|---|
| ⅛ W film (0805) | 125 mW | 125 mW | 125 mW | 80.9 mW | 44.1 mW |
| ¼ W film (axial, 1206) | 250 mW | 250 mW | 250 mW | 162 mW | 88.2 mW |
| ½ W film | 500 mW | 500 mW | 500 mW | 324 mW | 176 mW |
| 1 W film | 1 W | 1 W | 1 W | 647 mW | 353 mW |
| 2 W film | 2 W | 2 W | 2 W | 1.29 W | 706 mW |
| 3 W wirewound | 3 W | 2.7 W | 2.46 W | 2.1 W | 1.8 W |
| 5 W wirewound (cement) | 5 W | 4.5 W | 4.1 W | 3.5 W | 3 W |
| 10 W wirewound | 10 W | 9 W | 8.2 W | 7 W | 6 W |
The half rule
The rating is the limit at which the element reaches its maximum temperature, not a comfortable operating point. At full rating a film resistor's surface runs about 100 °C above ambient: it discolours, its value drifts, the solder joints age and the board scorches. The usual rule is to load a resistor to no more than half its derated rating, which keeps the rise to roughly 50 °C and the part stable for years. This calculator's "comfortable" verdict and its recommended rating both use that half.
Resistor types and their ratings
Chip resistors: 0402 ⅟₁₆ W, 0603 ⅒ W, 0805 ⅛ W, 1206 ¼ W, with the board's copper doing most of the cooling. Axial film: ¼ W (the common 6 mm body), ½ W, 1 W, 2 W, larger bodies for more. Wirewound and cement: 3, 5, 10, 20 W, running hot by design. Aluminium-housed: 25 to 250 W, rated only when bolted to a heatsink. For precision circuits the self-heating also matters: a resistor's value changes with temperature (50–100 ppm/°C), so a part run at half rating holds its value far better than one at full.
Your resistor, step by step
- Dissipation: 12 V × 120 mA = 1.44 W.
- Rating left at 50 °C: 2 W × 100% = 2 W → load 72%: Within the rating, but hot: aim for half.
- To buy: for half-load you need ≥ 2.88 W; this rating takes at most 14.1 V or 141 mA here.
Worked example: 100 Ω with 12 V across it in a 50 °C box
The current is 12 V / 100 Ω = 120 mA and the power 12 V × 120 mA = 1.44 W. A 2 W film resistor is rated up to 70 °C, so at 50 °C its full 2 W is available and the load is 72%: within the rating but well over half, so it will run hot, around 122 °C. For half-load you need 2.88 W or more: the 5 W wirewound (cement) (the 3 W wirewound has only 2.7 W left at 50 °C, a shade short of half-load), or two 200 Ω 2 W film resistors in parallel sharing 720 mW each at 36%. If the same box ran at 100 °C the 2 W part would have only 1.29 W left and would be overloaded at 111%.
Questions
Can I use a resistor right at its rating?
For a while. It will run about 100 °C above ambient, drift and discolour the board. For anything meant to last, stay at half.
Why did my resistor burn with the "right" rating?
Usually the ambient: a ¼ W part in a hot enclosure at 100 °C is a 0.16 W part. Or the power was computed from the nominal voltage while the real voltage, and its square, were higher.
Does a bigger resistor value run cooler?
Only if the voltage is fixed (P = V²/R). With a fixed current the opposite (P = I²R). Work out which the circuit holds constant.
What about surface-mount resistors?
Their ratings assume a specified copper pad area on the board; with less copper they manage less. 0805 is ⅛ W, 1206 ¼ W, and the derating curve is the same 70 °C to 155 °C.