Resistor Power Rating Calculator
Calculate power dissipation using P = V²/R, P = I²R, or P = VI.
Returns watts and minimum power rating from 1/4W to 2W with 50% derating for safety.
Resistor power dissipation is the amount of energy converted to heat as current flows through a resistor. Exceeding a resistor’s power rating causes overheating, damage, or fire.
Three equivalent formulas (derived from Ohm’s Law and the power equation):
- P = V x I: Power equals voltage times current
- P = V² / R: Power equals voltage squared divided by resistance
- P = I² x R: Power equals current squared times resistance
Where:
- P = Power in watts (W)
- V = Voltage across the resistor in volts (V)
- I = Current through the resistor in amperes (A)
- R = Resistance in ohms (ohm)
Standard resistor power ratings:
- 1/8 W (0.125 W): Tiny SMD components, low-power signal circuits
- 1/4 W (0.25 W): Most common through-hole resistor for hobbyist electronics
- 1/2 W (0.5 W): Medium power, common in audio and power supply circuits
- 1 W: Higher power applications
- 2 W: Power circuits, LED drivers
- 5 W and above: Power resistors, often wire-wound with heat sinks
Safety rule: Always choose a resistor rated for at least 2x the calculated power dissipation. This provides a safety margin for temperature variations, voltage spikes, and component aging. A resistor running at its maximum rating gets very hot and has a shortened lifespan.
Practical example: A 100 ohm resistor with 12V across it dissipates P = 12² / 100 = 1.44 W. You would need at least a 3W resistor (2x safety margin).
Temperature derating: Resistor power ratings are specified at 25 degrees C (77 degrees F). In hotter environments, derate by 50% for every 50 degrees C above ambient. A 1W resistor at 75 degrees C ambient can only safely handle 0.5W.
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This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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