Resistor Power (Wattage) Calculator
Calculate resistor power dissipation and get a suggested safe wattage rating.
Power dissipated
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Suggested resistor rating
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For reference and prototyping. Verify any safety- or design-critical value against the component datasheet or a second method before relying on it. Full disclaimer.
About this tool
Enter any two of voltage, current, resistance, or power and this works out how much power the resistor turns into heat, then suggests a standard wattage rating with safety headroom so you don't fit a part that runs hot and drifts or fails.
The formulas. Power dissipated in a resistor is P = V × I, and via Ohm's law that's equal to P = I² × R or P = V² ÷ R. Whichever pair you supply, the tool fills in the missing values and computes P.
- Know the current through it and its value → I²R is the direct route.
- Know the voltage across it and its value → V²/R.
- Note the I² and V² — power rises with the square of current or voltage, so doubling either quadruples the heat.
Worked example. An LED series resistor: 3.0 V across it at 20 mA. P = 3.0 × 0.02 = 0.06 W. A 1/8 W (0.125 W) part has plenty of margin. A different case: dropping 9 V at 100 mA through a resistor dissipates 0.9 W — you need a 2 W part, and it will still get warm.
Why the suggested rating has headroom. Running a resistor at its rated wattage continuously shortens its life, raises its temperature, and shifts its value (temperature coefficient). The common rule is to pick a rating at least 2× the calculated dissipation — more in a hot enclosure or with no airflow. This tool suggests the next standard size (1/8, 1/4, 1/2, 1, 2, 5 W…) above roughly double P.
Physical size follows wattage. Higher-power resistors are physically bigger to shed heat. A 1/4 W axial resistor is ~6 mm long; a 2 W one is ~15 mm; beyond that you're into wirewound or aluminium-clad parts on a heatsink. If a calculation calls for several watts, consider whether a different circuit approach (a switching regulator instead of a linear drop, for example) avoids the heat entirely.
Pulsed loads. The rating assumes continuous dissipation. A resistor can briefly handle far more than its average rating; check the datasheet's pulse-power curve for anything that isn't steady-state.
This shares its maths with the Ohm's law calculator. To read a resistor's value, use the resistor color code; for resistors in series or parallel, the series/parallel calculator; for wire heating, the wire gauge calculator.
Frequently asked questions
- Why does the suggested rating include headroom?
- Running a resistor at its rated wattage shortens its life and causes thermal drift. The tool suggests the next standard rating above about double the calculated dissipation — a common reliability margin.
- What formula does this use?
- P = V × I, equivalently P = I²R or P = V²/R. It applies Ohm's law first to fill in whichever values you didn't enter.
- Is this the same as the Ohm's law calculator?
- Same underlying equations, but this one focuses on power dissipation and choosing a safe wattage — including the case where you already know P and R.
- My resistor still gets warm at the suggested rating — is that normal?
- Yes. "Warm" is expected near half the rated power. "Too hot to touch" or discoloured means it's underrated or poorly ventilated — go up a size.
- Does resistor size matter beyond the wattage number?
- Yes — higher-wattage resistors are physically larger to dissipate heat. A design needing several watts may be better served by a different circuit topology.
- Does this apply to a short pulse rather than continuous current?
- No. The rating is for continuous dissipation. Resistors tolerate much higher brief pulses — check the datasheet's pulse curve.