Electronics CalculatorsResistor power rating
A resistor turns electrical energy into heat. If it gets more power than its rating, it gets hot and may burn.
Calculate the power
P = V² / R = I² × R = V × I. Use whichever pair of values you know.
| Case | Calculation | Power | Choose a rating of |
|---|---|---|---|
| 5 V across 100 Ω | V² / R = 25 / 100 | 250 mW | at least 500 mW |
| 12 V across 470 Ω | V² / R = 144 / 470 | 306.4 mW | at least 612.8 mW |
| 20 mA through 220 Ω | I² × R | 88 mW | at least 176 mW |
Choosing the rating
A common rule of thumb is to choose a resistor rated for at least twice the calculated power, which leaves room for heat, for higher temperatures in a box and for a voltage that is a little higher than planned. Common ratings of small resistors are 1/8 W, 1/4 W, 1/2 W and 1 W; larger ones are wire-wound or chassis types.
Watch the heat
A resistor running near its rating gets hot to the touch, which is normal for its design but can discolour a circuit board or damage nearby parts. Leave space around power resistors and do not enclose them tightly.
Data sheets
The rating is given at a stated ambient temperature, and drops at higher temperatures (a derating curve). Read the data sheet of the part for high-power or hot designs.
Does the resistor value change when it gets hot?
Yes, by its temperature coefficient (the sixth colour band, where present).
Can I use two resistors to share the power?
Yes: two equal resistors in series or in parallel share the power equally.
This is an independent guide. It is not connected with any standards body or company named here. The formulas are the usual textbook ones; the colour code, E-series and AWG values are those printed in common references, because the standards IEC 60062, IEC 60063 and ASTM B258 are paid documents that we did not read. Tables and examples were calculated with Python on 14 November 2026. Check data sheets and local safety rules before you build.