Engineering & PLMFree calculatorPDF, Excel, Word exportElectronics Manufacturing

Resistor Power Derating Calculator

A resistor's wattage rating applies at a stated ambient, usually 70 C. As the board gets hotter the allowed power falls along a derating curve. This calculator checks the dissipation V squared over R against the derated limit and shows how much margin you have left.

Your numbers

V
ohm
85 C
C

Taken from the manufacturer's datasheet curve.

C

Temperature where the curve reaches zero watts.

50 %

Fraction of the allowed power you accept in service.

Your results

Load against design limit
635.8%
Above 100 means the part exceeds your derating target.
Power margin to design limit
-0.5516
Actual dissipation
0.6545
P = V squared divided by R, in watts.
Allowed power at ambient
0.2059
Design limit with derating target
0.1029
Current through resistor
0.0545

Linear derating model using values you enter. Always confirm with the manufacturer's datasheet, pulse ratings and board thermal design.

Save results:

Allowed and design limit power vs ambient temperature

Sweeps Ambient temperature from -40 to 155, holding your other inputs fixed.

Parts rules in your system

Optional: a short review of how engineering rules could be checked against your item master.

Optional. The tool and every export work without this. No spam.

How to use it

Enter the voltage across the resistor and its value. Pick the rated power at 70 C, then set the ambient temperature around the part. Take the derating start and zero power temperatures from the datasheet curve; many thick film chip resistors derate linearly from 70 C to 155 C. Set your design derating target, for example 50 percent. The result shows the load as a percentage of your design limit, and the margin in watts.

The formula

Dissipation is voltage squared divided by resistance. The allowed power at ambient is the rated power multiplied by the fraction of the curve remaining, which is the zero power temperature minus ambient, divided by the zero power temperature minus the derating start. That fraction is clamped between zero and one. The design limit multiplies that by your derating target. Load is dissipation divided by the design limit.

Reading the result

A load under 100 percent means the part meets your derating target at that temperature. Between 100 and 200 percent it still operates within its rating but breaks your derating policy, which matters for reliability programs. Beyond the allowed power the part is outside its rating. The chart shows how the margin collapses as ambient temperature rises, which is useful when choosing between a 0603 and an 0805.

Common mistakes

Using ambient room temperature instead of the local board temperature next to hot parts underestimates the stress. Another mistake is ignoring the maximum working voltage, which can limit small packages before the power does. Also remember that pulsed loads have different limits, and that a larger footprint, copper area and airflow change the real temperature rise.

Frequently Asked Questions

Is 50 percent derating standard?

It is a common design rule for commercial and industrial work, while high reliability programs often specify their own tables by part class. Use the policy that applies to your product or customer.

Why does the curve start at 70 C?

Many datasheets rate chip resistors at 70 C ambient and derate linearly above that. Some families use different points, so always read the curve for the exact series.

Can I use this for power resistors?

You can if you enter the datasheet values, but power resistors rated on a heatsink or case temperature need that mounting condition modeled separately.

Does it cover voltage derating?

No. It covers power only. Check the working voltage and pulse limits in the datasheet as a separate step.

If you maintain approved resistor families in your ERP or PLM, Netray can help connect derating rules to parts so substitutions are checked automatically.

Talk to an ExpertExplore More Free Tools

Related Tools

Electronics Manufacturing

PCB Panel Utilization and Boards per Panel Calculator

See how many boards fit on a fabrication panel after rails and spacing, with utilization and cost per board.

Electronics Manufacturing

FIT Failure Rate to MTBF Conversion Calculator

Convert a FIT rate to MTBF, expected failures per year across a fleet and the reliability over a mission.

Electronics Manufacturing

Electronics BOM Cost Calculator

Estimate loaded BOM cost per board and annual material spend, including volume pricing tiers, procurement overhead, and attrition.

Electronics Manufacturing

Component Obsolescence Risk Assessment

Answer 10 questions to score how exposed your BOMs are to component EOL, DMSMS, and lifecycle blind spots, with graded recommendations.

More Engineering & PLM tools

See all
Back to All Tools