Power Derating Calculator
Allowed power above the derating knee for resistors and semiconductors — with the 50 % conservative-design check.
Conservative design (NASA/military practice) loads parts to only 50 % of the DERATED limit — that is the second “verdict” threshold above. Remember Ta here is LOCAL ambient: a resistor next to a hot regulator sees that heat. SMD parts derate by land-pattern copper too — a 0805 on tiny pads can't reach its paper rating.
Power Derating Calculator computes the power a part may dissipate above its derating-knee temperature — free, instant and private in your browser. Designers of enclosed, hot or high-reliability hardware applying real margins use it to skip the datasheet algebra: type your numbers, read the answer with the substituted formula shown step by step, and share an exact permalink of the calculation.
About Power Derating Calculator
Power Derating Calculator computes the power a part may dissipate above its derating-knee temperature using the standard engineering relation: P = P(rated)·(T(max) − Ta)/(T(max) − T(knee)) above the knee. Worked live: a ¼ W resistor (knee 70 °C, zero at 155 °C) at 100 °C ambient is only good for ~160 mW. The result recalculates on every keystroke, the worked-example panel shows your numbers substituted into the formula, and the Copy permalink button encodes the inputs in the URL so a colleague opens exactly your calculation. Everything runs client-side — nothing you type leaves your device.
How to use Power Derating Calculator
- 1Enter your values — the tool starts with realistic defaults for this exact use case, so the worked example is meaningful immediately.
- 2Read the live result and the worked-example panel, which substitutes your numbers into the formula step by step.
- 3Adjust any input to compare scenarios, then use Copy result or Copy permalink to share the calculation.
Why use Power Derating Calculator?
- ✓Implements the real formula — P = P(rated)·(T(max) − Ta)/(T(max) − T(knee)) above the knee — with the substitution shown, not a black box
- ✓Built for designers of enclosed, hot or high-reliability hardware applying real margins
- ✓Copy result and permalink buttons — share the exact calculation in a README, forum answer or design review
- ✓100% free, no sign-up, runs entirely in your browser (works offline once loaded)
Frequently asked questions
How do you calculate power derating?+
The power a part may dissipate above its derating-knee temperature follows P = P(rated)·(T(max) − Ta)/(T(max) − T(knee)) above the knee. For example, a ¼ W resistor (knee 70 °C, zero at 155 °C) at 100 °C ambient is only good for ~160 mW. The calculator applies the same relation and shows the substituted arithmetic so you can verify every step.
Where do I find the derating curve for my part?+
Every resistor/semiconductor datasheet carries a power-vs-ambient plot — the knee (often 70 °C) and zero-power point (often 155 °C for film resistors) come straight off it. This tool linearises exactly that printed curve.
Why do reliability standards load parts to only 50 %?+
Two stacked margins: the derated limit handles temperature, and the 50 % loading (NASA/military practice) buys headroom against tolerance, transients and aging drift. A resistor at 25 % of nameplate in a hot box may already be at 80 % of its DERATED allowance.
Is the Power Derating Calculator free and private?+
Yes — completely free with no sign-up or usage limits, and it runs entirely in your browser: the values you enter are never uploaded or stored on a server.
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