Resistor Power Dissipation Calculator
Calculate how much heat a resistor dissipates from any two of voltage, current, and resistance. Returns recommended standard wattage rating with 2× derating for safety and lifetime.
How to Use
- Enter resistance R with standard suffixes (k, M).
- Enter the voltage across the resistor or the current through it (or both).
- The calculator uses whichever form of P = V²/R, I²R, or V·I is possible and returns the answer.
- Pick a resistor whose rated wattage is at least 2× the calculated dissipation.
- Hover the standard package suggestions on the right to see what body size you need.
Show Work
Formulas
Standard Package Wattages
History of Resistor Power Dissipation
The physics comes from James Prescott Joule's 1841 experiments showing that heat generated in a conductor equals I² × R × t — now known as Joule heating. Joule's measurements were precise enough to establish the mechanical equivalent of heat (4.186 J/calorie) and effectively discovered the first law of thermodynamics. Every resistor wattage rating in every datasheet traces back to Joule's paddle-wheel experiments.
The standard wattage ladder (1/16, 1/10, 1/8, 1/4, 1/2, 1, 2, 5, 10, 25, 50 W) was codified by the Electronic Industries Alliance in the late 1940s, covering carbon-composition, metal-film, and wirewound resistor families. The 2× derating rule ("Military Standard 975") emerged from US Navy reliability studies in the 1960s showing that operating resistors at half their rated power doubles the mean-time-between-failures — a guideline still cited in every MIL-HDBK-217 reliability document and most industrial design handbooks.
About This Calculator
Enter resistance plus any one (or both) of voltage and current, and this tool picks the right form of Joule's law to compute dissipated power, then recommends a standard wattage rating with 2× derating. It also shows which SMD or axial package can handle that power.
For pulse applications or unusual thermal environments (hot enclosures, conductive potting, heatsinked parts), the steady-state rating alone isn't enough — check the datasheet's pulse-energy SOA curve and apply the appropriate temperature derating. Everything runs client-side.
Frequently Asked Questions
Why derate by 2×?
A resistor rated at 1/4 W can dissipate 1/4 W continuously and survive — barely. Running at rated power means running hot (often 70–100 °C case) and drifting out of tolerance within months. Industry practice is to use a resistor at half its rated power for long-term reliability; every 10 °C below rated junction temperature roughly doubles the life.
What about pulses?
Resistors tolerate brief overloads because the thermal mass of the body takes time to heat up. Typical guideline: a thick-film SMD can handle 5–10× its continuous rating for milliseconds. Datasheets provide pulse-energy graphs showing the safe operating area as a function of pulse width.
Why does temperature matter?
The rated wattage assumes a specific ambient (usually 70 °C). Above that, linear derating kicks in — at 100 °C ambient you can only dissipate maybe 60% of rated. In a hot enclosure, step up two wattage sizes.
Do low-ohm resistors have the same rules?
Current-sense resistors (milliohm range) are usually specified by maximum current (peak and continuous) rather than pure wattage, because at such low resistance the I²R heat spreads through the package and PCB pads differently than a typical higher-value resistor. Check the sense-resistor datasheet for its ampacity curves.
What's the difference between carbon, metal-film, and wirewound?
Carbon-composition: cheap, noisy, loose tolerance, good pulse handling — legacy part. Metal-film: most common modern resistor, low noise, tight tolerance (1% standard), limited pulse tolerance. Wirewound: high power (5–100 W), slight inductance, used for load banks and dummy loads.
Common Use Cases
Current-Sense Shunt
A 0.01 Ω shunt measuring 10 A drops 0.1 V and dissipates 1 W — needs a 2 W rated part. Four-terminal Kelvin resistors recommended for accuracy.
Bleeder Resistor
A 100 kΩ bleeder across a 400 V capacitor bank discharges it safely but continuously dissipates 1.6 W — pick a 5 W wirewound.
Snubber Network
Pulse energy in a MOSFET snubber can briefly exceed the resistor's continuous rating 10×. Size for pulse energy, not average dissipation.
Dummy RF Load
A 50 Ω dummy load for a 100 W transmitter must dissipate the full 100 W continuously — needs a finned wirewound or oil-cooled professional load.
Brake Resistor for Motor Drive
VFD dumps regenerated energy into a resistor during deceleration. Sized for the duty cycle of the regenerative pulses, not steady-state.
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