Resistor Power Rating Calculator

Check whether a 1/4 W resistor is enough, or whether your circuit needs a bigger one.

Power dissipated
With 2× margin
Standard rating to use

Runs entirely in your browser. Nothing you enter is sent anywhere.

How it works

A resistor turns electrical power into heat. If it dissipates more than its rating it overheats, drifts in value and can fail or burn.

Common practice is to choose a rating at least twice the calculated dissipation, which keeps the resistor cooler and longer-lived, especially in enclosures without airflow.

P = V² / R    or    P = I² × R
Rating ≥ 2 × P

Worked example

12 V across a 470 Ω resistor dissipates 12² / 470 = 0.306 W. With a 2× margin you need at least 0.61 W, so choose a 1 W resistor; a 1/4 W part would overheat.

Frequently asked questions

Why use a 2× safety margin?

Resistor ratings assume good airflow at 25 °C ambient. Inside a warm enclosure the safe power drops, so a margin keeps the part from running hot.

What happens if a resistor is underrated?

It runs hot, its resistance drifts, and it can discolour the PCB, crack or burn open. Very hot resistors can also damage nearby parts.

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