Resistor Power Rating Calculator
Calculate power dissipation in a resistor from voltage, current, or resistance.
Find the right power rating.
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.