Calculate Power Dissipation & Select Correct Resistor Rating
Select Calculation Mode
Voltage (V)
Resistance (Ω)
Current (A)
Resistance (Ω)
Voltage (V)
Current (A)
Result
Power Dissipation
Recommended Rating
Step-by-Step Calculation
Power Formulas
P = V × I (given voltage and current)
P = I² × R (given current and resistance)
P = V² / R (given voltage and resistance)
Recommended rating: ≥ 2 × calculated power
Power dissipation causes resistor heating. Always select a resistor with power rating at least double the calculated value for safety and longevity. Common resistor power ratings: 1/8W (0.125), 1/4W (0.25), 1/2W (0.5), 1W, 2W, 5W, 10W.
⚠Derating is critical! Running a resistor at its maximum rating reduces lifespan. Use 50% derating (2× safety margin). For high-temperature environments, use even more margin.
What is Resistor Wattage?
Resistor wattage (power rating) indicates the maximum power a resistor can safely dissipate as heat. Exceeding this rating causes overheating and failure. Power depends on both voltage across and current through the resistor, related by Ohm's law formulas.
P = V²/R
Use when you know voltage and resistance. Power increases with voltage squared. Doubling voltage = 4× power.
P = I²×R
Use when you know current and resistance. Power increases with current squared. Doubling current = 4× power.
P = V×I
Direct formula. Voltage drop × current flow. Useful when both quantities are known or measured.
Power Ratings
Common SMD: 1/16W, 1/10W, 1/8W. Through-hole: 1/4W, 1/2W, 1W. Power: 2W, 5W, 10W+. Wirewound for high power.
Teaching Example: 12V across a 100Ω resistor.
P = V²/R = 12²/100 = 144/100 = 1.44W.
Recommended rating: 1.44W × 2 = 2.88W → Use 3W or 5W resistor. A 1/4W resistor would overheat and fail!
Applications
Power SuppliesLED DriversAudio AmplifiersMotor ControlHeating Elements
Frequently Asked Questions
How to calculate resistor wattage?▼
P = V²/R, P = I²×R, or P = V×I. Always derate by using at least 2× the calculated value for the resistor rating.
What power rating resistor do I need?▼
Calculate power dissipation, then double it. Choose the next standard rating: 1/8W, 1/4W, 1/2W, 1W, 2W, 5W, 10W etc.
Can I use a higher wattage resistor?▼
Yes, always safe. A higher wattage resistor runs cooler and lasts longer. The only downside is larger physical size and slightly higher cost.
What happens if wattage is too low?▼
The resistor overheats. It may smoke, catch fire, change value, or fail open-circuit. Always design with adequate margin (2× derating recommended).
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