Resistor Power Rating Calculator
Calculate resistor power dissipation and find the correct wattage rating.
Component Values
Results
P = V²/R = I²R = V×I
Power Derating vs Temperature
Resistor power derating vs temperature
Resistor Power Dissipation
Every resistor dissipates power as heat. The fundamental formulas are P = V²/R, P = I²×R, and P = V×I — all equivalent for a resistive load. The key question is: which wattage resistor should you buy? Always use a resistor rated for at least twice the calculated dissipation. Running a resistor at 100% rating significantly shortens its life and causes dangerous heating.
Standard power ratings are 0.1W, 0.125W (1/8W), 0.25W (1/4W), 0.5W (1/2W), 1W, 2W, 5W, 10W. The most common is 0.25W (1/4W) for signal circuits. If your calculation yields more than 125 mW, move to 0.5W. Above 500 mW, use metal film or wirewound resistors.
Temperature derating is critical in hot environments. Carbon film resistors are rated for 100% power up to 70°C, then derate linearly to 0W at 155°C. Metal film resistors handle heat better. At 100°C ambient, a 0.25W carbon film can only dissipate about 0.125W safely.
Power Formulas
P = V² / RP = I² × RP = V × IKey Points
- Always use a resistor rated for ≥ 2× the calculated power
- Standard ratings: 1/8W, 1/4W, 1/2W, 1W, 2W, 5W, 10W
- Carbon film: derate above 70°C, fails at 155°C
- Metal film: lower temperature coefficient, better stability
- Wirewound: highest power, lowest inductance if non-inductive type
- SMD resistors: 0402 = 0.062W, 0603 = 0.1W, 0805 = 0.125W typical
Applications
- Current limiting resistors for LEDs and transistor bases
- Pull-up and pull-down resistors in digital circuits
- Power supply bleeder and dummy load resistors
- Voltage divider design for ADC reference
- Gate resistors for MOSFET switching circuits
FAQ
How do I know what wattage resistor to use?
Calculate dissipation with P = V²/R, I²R, or V×I. Multiply by 2 for safety. Select the next standard rating above that value: 0.125W → 0.25W → 0.5W → 1W → 2W → 5W.
What happens if a resistor is undersized?
An undersized resistor overheats, drifts in value, emits smoke, or burns out. In worst case it can cause a fire. A resistor running at 100% of rating can reach 100–155°C surface temperature.
How to calculate resistor power dissipation?
Use P = V²/R (given V and R), P = I²×R (given I and R), or P = V×I (given V and I). All formulas give the same result for a resistive load.
What is resistor derating?
Derating reduces maximum allowed power at elevated temperature. A 0.25W carbon film resistor can handle only 0.125W at 70°C and 0W at 155°C. Check the datasheet derating curve.
What is the difference between 1/4W and 1/2W resistors?
A 1/2W resistor is physically larger, runs cooler for the same application, and can handle twice the power. Use 1/2W when dissipation exceeds 125 mW.
How hot does a resistor get?
A resistor at 100% of its rating can reach 100–155°C. At 50% rating, surface temperature is roughly 50–70°C above ambient. If you cannot touch it for 3 seconds, it needs a higher-rated part.
Did you know? Resistors are rated at 70 °C ambient for their full power dissipation. Above 70 °C, you must derate — a 0.25 W resistor at 100 °C ambient can only safely dissipate about 0.13 W. For high-temperature applications (engine control units, industrial gear), 125–155 °C rated components are essential.