Resistor Power Calculator
Calculate power dissipation (P = V·I, I²R, V²/R) and find the safe resistor power rating.
How it works
Power is the rate at which electrical energy is transferred by an electric circuit. For resistors, electrical power is converted directly into heat.
The three equivalent formulas for electrical power in a resistive circuit are:
- P = V × I (Voltage times Current)
- P = I² × R (Current squared times Resistance)
- P = V² / R (Voltage squared divided by Resistance)
When choosing a physical resistor, a good engineering practice is to use a derating factor. We recommend picking a resistor rated for at least twice (2x) the calculated power dissipation to prevent overheating and ensure long-term reliability.
Worked example
You want to drop 12V across a 100 Ω resistor.
Using P = V² / R, we get P = 12² / 100 = 144 / 100 = 1.44 W.
A 1.44 W power dissipation requires a safety margin of at least 2.88 W, so a standard 3 W resistor is recommended.
Frequently Asked Questions
Why do I need a safety margin for power rating?
Resistors run hot when operated near their maximum rated power, which can melt nearby plastic, degrade the component, or shorten its lifespan. A 2x safety margin keeps the resistor at a reasonable temperature.
What are the common resistor power ratings?
Through-hole resistors are commonly found in 1/8W, 1/4W, 1/2W, 1W, and 2W sizes. SMD resistors have typical ratings tied to their package size (e.g., 0402 is typically 1/16W, 0805 is 1/8W, 1206 is 1/4W).
Sources
- Joule's first law of heating.
- Standard resistor derating guidelines (IPC-2221 / Military derating standards).
Results are for engineering assistance. Verify important calculations and check official datasheets. See the Engineering Disclaimer.