Electronics · Free Calculator
Resistor Power Rating Calculator
Find how much power a resistor dissipates and which standard wattage rating to buy with a safety margin.
P = V²/RP = I²RSafe ratingDerating margin
V
× power
Power dissipated654.5 mW
Minimum rating with margin1.309 W
Suggested standard rating2 W
Current54.55 mA
How this result was calculated
- P = V² ÷ R = 12² ÷ 220 Ω = 654.5 mW
- With 2 × margin: 1.309 W
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Resistor Power Formula
P = V² ÷ R = I² × R = V × I
Standard ratings: 1/8, 1/4, 1/2, 1, 2, 3, 5 and 10 W. Resistors are rated at 70 °C ambient and must be derated above that.
Worked example
12 V across 220 Ω dissipates 144 ÷ 220 = 0.65 W. With a 2 × margin you need 1.3 W, so fit a 2 W resistor.
Key insight: a resistor running at its full rating gets hot enough to discolour a PCB, so a 2 × margin keeps the surface temperature and drift reasonable.
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Frequently Asked Questions
How do I choose a resistor wattage?
Calculate P = V²/R or I²R and choose a rating at least twice that value.
What happens if a resistor is under rated?
It overheats, drifts in value, can burn the PCB and eventually opens.
What power can an 0805 SMD resistor take?
Typically 1/8 W, and 0603 parts 1/10 W. Check the data sheet.
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