Resistor Power Rating Calculator

How it works

A resistor turns all the electrical power it carries into heat. That power follows from any two of the current through it, the voltage across it and its resistance: P = I² × R, P = V² ÷ R or P = V × I, which are the same law (Ohm's law, V = I × R) written three ways.

A resistor's power rating is the heat it can shed continuously under the manufacturer's stated conditions. The rating margin multiplies the calculated power to give a minimum rating; the calculator then shows the smallest of the common ratings 1/8, 1/4, 1/2, 1, 2, 5 and 10 W that is at least that large. Electronics Tutorials describes picking a resistor able to dissipate two or more times the calculated power as the better choice, which is why the margin defaults to 2.

The third quantity (voltage, current or resistance) is shown as well, so the operating point is complete.

Formula

P = I² × R = V² ÷ R = V × I
minimum rating = P × margin            (margin ≥ 1; default 2)
rating = smallest of 1/8, 1/4, 1/2, 1, 2, 5, 10 W that is ≥ the minimum rating

Example

12 V across a 470 Ω resistor dissipates 12² ÷ 470 = 306.383 mW and carries 12 ÷ 470 = 25.5319 mA. With a margin of 2 the minimum rating is 612.766 mW, so the smallest common rating that covers it is 1 W; a 1/2 W part would run above half its rating.

A resistor with 12 V across it and 50 mA through it dissipates 12 × 0.05 = 600 mW (0.6 W) and is 12 ÷ 0.05 = 240 Ω. Twice 0.6 W is 1.2 W, so the smallest common rating that covers it is 2 W.

Assumptions and limitations

• DC, or RMS values for AC. Peak or peak-to-peak readings overstate the power.
• The resistance is constant; the heating of the resistor does not change it.
• A power rating applies at the ambient temperature the manufacturer states; above it the allowed power falls (derating), and mounting, airflow and nearby parts change how hot the part runs. The manufacturer's data sheet governs.
• The listed ratings are common values, not every rating made: 1/8 to 2 W as listed by Electronics Tutorials (2026) for carbon resistors, and 5 W and 10 W as ratings of Vishay's FVTL/FVTS/FVWL and FSTL/FSTS/FSWL tubular wirewound series (Vishay wirewound product table, retrieved 2026-10-01).
• The margin is an input; 2× is a common rule of thumb, not a standard.
• This is an estimate for circuit design and learning, not a substitute for the component's data sheet or a qualified engineer's review.

Frequently asked questions