Key takeaways
- Calculate dissipation with P = I²R or P = V²/R.
- Choose a part rated at least twice the calculated figure.
- Manufacturer ratings assume free air at 70 °C — derate above that.
What the rating actually means
A resistor's wattage rating is the power it can dissipate continuously in free air at a stated ambient temperature — usually 70 °C — without exceeding its maximum body temperature. It is a thermal limit, not an electrical one.
Exceed it and the resistance drifts first, then the part discolours, then it opens or, with enough energy behind it, chars the board.
Calculating the dissipation
Use whichever form matches what you know:
- P = I² × R — current and resistance
- P = V² ÷ R — voltage across it and resistance
- P = V × I — both measured
Worked example
A 100 Ω resistor drops 5 V.
- P = 25 ÷ 100 = 0.25 W
- Doubled for margin: 0.5 W
- Fit a half-watt part — a quarter-watt would sit at 100% of rating and run far too hot to touch.
Standard sizes
| Rating | Typical body | Design max (50%) | Usual application |
|---|---|---|---|
| 1/8 W | 3.2 × 1.8 mm | 62 mW | Dense signal circuits |
| 1/4 W | 6.3 × 2.3 mm | 125 mW | General purpose |
| 1/2 W | 9.0 × 3.2 mm | 250 mW | LED strings, dividers |
| 1 W | 11.5 × 4.5 mm | 500 mW | Small power supplies |
| 2 W | 15 × 5 mm | 1 W | Bleeders, snubbers |
| 5 W | Ceramic wirewound | 2.5 W | Loads, current sensing |
LED Resistor CalculatorSizes the series resistor and reports the dissipation plus a suggested standard power rating.
Why double the calculated value?
The margin covers four things at once:
- Ambient temperature. Inside a sealed enclosure the air is far hotter than the room.
- Restricted airflow. Ratings assume free convection, which a crowded board does not provide.
- Supply tolerance. A 10% high supply raises dissipation by 21%, because power follows voltage squared.
- Component tolerance. A 5% low resistance draws 5% more current.
Running at half the rating also keeps the body cool enough that nearby capacitors and the board itself are not cooked.
Derating with temperature
Above the rated ambient, capability falls in a straight line to zero at the maximum body temperature. A 1 W part rated at 70 °C, with a 155 °C limit, loses roughly 1.2% of its rating per degree above 70 °C — at 100 °C it can only handle about 0.65 W.
Pulse loads
Short pulses are limited by the resistor's energy absorption, not its continuous rating. A part can survive brief peaks many times its wattage if the average stays low — but check the manufacturer's pulse curve rather than guessing.
When no single part is big enough
- Series: two 50 Ω half-watt parts give 100 Ω at 1 W, splitting the voltage.
- Parallel: two 200 Ω half-watt parts give 100 Ω at 1 W, splitting the current.
- Use identical values so the sharing is even — the series and parallel calculators confirm the totals.
Recognising a thermal problem
A resistor rarely fails without warning. The signs appear well before it opens, and all of them are visible or measurable.
- Discolouration. A beige body turning brown or the paint blistering means the part has been running far too hot.
- Board staining. A dark halo on the laminate underneath is a clear sign of sustained overheating.
- Drifting value. A resistor measuring 10-20% high after use has been thermally stressed and will keep drifting.
- Smell. Hot phenolic and scorched lacquer are distinctive, and usually noticed before anything is visible.
Surface-mount ratings
| Package | Size (mm) | Typical rating |
|---|---|---|
| 0402 | 1.0 × 0.5 | 1/16 W |
| 0603 | 1.6 × 0.8 | 1/10 W |
| 0805 | 2.0 × 1.25 | 1/8 W |
| 1206 | 3.2 × 1.6 | 1/4 W |
Surface-mount parts rely on the board copper to shed heat, so the pad and track area around them is part of the thermal design.
A quick sizing routine
- Find the current or voltage with the V-I-R-P solver.
- Calculate dissipation.
- Double it.
- Pick the next standard rating at or above that figure.
- If it is inside an enclosure, go one size further still.
Frequently asked questions
What power rating resistor do I need?
Calculate the dissipation with P = I²R, double it for margin, then choose the next standard rating at or above that number. In a sealed enclosure, go one size larger again.
What happens if a resistor exceeds its power rating?
Its resistance drifts first, then the body discolours and the part eventually goes open circuit. With enough energy behind it, it can char the board or start a fire.
Can I use two resistors to get a higher power rating?
Yes. Two identical resistors in series or in parallel share the dissipation, doubling the effective rating. Use matched values so the sharing stays even.