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Every resistor turns some electrical energy into heat. If that heat exceeds the resistor's power rating (wattage), it overheats, drifts in value, and can burn out. So you must pick a resistor rated comfortably above the power it will actually dissipate — a safety factor of about 2× is common practice.
| Quantity | Formula |
|---|---|
| Power (from R & I) | P = I² × R |
| Power (from R & V) | P = V² / R |
| Power (from V & I) | P = V × I |
| Max current for a rating | Imax = √(Prated/R) |
| Max voltage for a rating | Vmax = √(Prated×R) |
Standard through-hole ratings are 1/8, 1/4, 1/2, 1, 2, and 5 W. Remember that a resistor's rating assumes free air at 25 °C — in a hot or enclosed space it must be derated.
The maximum power (in watts) a resistor can safely dissipate as heat without overheating or drifting. Common values are 1/8, 1/4, 1/2, 1, 2, and 5 W.
Use P = I²R if you know current and resistance, P = V²/R for voltage and resistance, or P = V×I for voltage and current. All three give the heat generated.
A factor of about 2 is standard — pick a rating at least twice the calculated power so the resistor runs cool and lasts. Use more for pulsed or high-reliability designs.
It gets hot, its value drifts, its lifetime drops, and it can char, crack, or go open-circuit — sometimes damaging nearby components.
Reducing the allowed power at higher ambient temperature. Ratings assume 25 °C in free air; above about 70 °C the safe power falls linearly to zero near the maximum temperature.
Not necessarily — power depends on the current and voltage in your circuit, not just resistance. For a fixed voltage, a larger resistance dissipates less; for a fixed current, more.
It depends on the resistance: Imax = √(0.25/R). For 1 kΩ that is about 15.8 mA; for 100 Ω about 50 mA.
Yes — by package size: 0402 ≈ 1/16 W, 0603 ≈ 1/10 W, 0805 ≈ 1/8 W, 1206 ≈ 1/4 W, depending on the PCB copper for heat spreading.
Use a higher-wattage resistor, a heat-sinked or aluminium-clad resistor, or split the power across several resistors in series or parallel.
No — tolerance is about value accuracy, power rating is about heat. They are independent specifications.
Yes. Two equal resistors in parallel share the current, so the pair can dissipate roughly double the power of one (each still limited to its own rating).
Even within rating a resistor runs warm — a 1/4 W part at full rating can reach 100 °C+. If that's a problem, use a bigger rating so it runs cooler.
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