Power Dissipation in Resistor Calculator
Calculate resistor power dissipation using voltage, current, or resistance values.
What the power dissipation in resistor calculator does
The power dissipation calculator tells you how many watts a resistor turns into heat. Enter any two of voltage across it, current through it and its resistance, leave the third field empty, and press Calculate. The tool picks the right form of the power law and reports the dissipation in watts, which you then compare with the resistor’s power rating.
The three formulas
| Known | Formula | Example |
|---|---|---|
| Voltage and resistance | P = V² / R | 12 V across 100 Ω → 1.44 W |
| Current and resistance | P = I² × R | 50 mA through 220 Ω → 0.55 W |
| Voltage and current | P = V × I | 5 V and 20 mA → 0.1 W |
If all three fields are filled, the calculator uses voltage and resistance. All three formulas follow from Ohm’s law and give the same answer for consistent inputs.
Choosing a resistor rating
A resistor’s rating is the power it can dissipate continuously at 70 °C ambient (25 °C for some series) without exceeding its maximum body temperature. Good practice is to run at no more than half the rating, and a third or less if the resistor is in an enclosure, near other hot parts, or expected to last for years. The rating must also be derated above the reference ambient, typically linearly to zero at 155 °C for film types.
| Calculated dissipation | Minimum rating (50% rule) | Common package |
|---|---|---|
| Up to 0.03 W | 1/16 W | 0402 SMD |
| Up to 0.05 W | 1/10 W | 0603 SMD |
| Up to 0.06 W | 1/8 W | 0805 SMD, small through-hole |
| Up to 0.125 W | 1/4 W | 1206 SMD, standard through-hole |
| Up to 0.25 W | 1/2 W | Larger through-hole |
| Up to 0.5 W | 1 W | 2512 SMD, metal oxide |
| Up to 2.5 W | 5 W | Cement or wirewound |
Worked examples
LED series resistor. 12 V supply, LED drop 2 V, 20 mA. The resistor drops 10 V at 20 mA: P = 10 × 0.02 = 0.2 W. A 1/4 W part is marginal; use 1/2 W.
Bleeder across a capacitor bank. 400 V DC across 100 kΩ: P = 400² / 100,000 = 1.6 W. Use a 3 W or 5 W part, or two 200 kΩ 2 W resistors in parallel to share the heat and the voltage stress.
Current-sense shunt. 10 A through 0.01 Ω: P = 10² × 0.01 = 1 W. A 2 W or 3 W shunt keeps its temperature, and therefore its resistance, stable.
Pull-up resistor. 3.3 V across 10 kΩ: P = 3.3² / 10,000 = 1.1 mW. Any package will do.
Pulse and surge conditions
Short pulses can exceed the continuous rating because the resistor’s mass absorbs the energy before it heats up. Energy per pulse is P × t; a 100 W pulse for 1 ms is 0.1 J. Manufacturers publish pulse-load curves, and wirewound and thick-film pulse-rated types handle far more than thin-film parts of the same size. Conversely, resistors that carry high voltage need a voltage rating check as well: a 1206 chip is typically limited to 200 V regardless of its power.
Common mistakes
- Using the supply voltage instead of the voltage across the resistor. In the LED example only 10 of the 12 V appear across the resistor.
- Entering milliamps as amps. 20 mA is 0.02 A. The error multiplies power by a million because current is squared.
- Running at 100% of rating. Resistors last far longer at half rating, and their value drifts less.
- Forgetting that power depends on the value chosen. Halving R at a fixed voltage doubles the dissipation.
Related tools
The Ohm’s law calculator solves for any of V, I, R and P. The watts to ohms and volts to watts calculators cover single conversions, the voltage divider calculator gives the voltage across each resistor in a divider, and the resistivity converter helps with wire and heating elements.
External references
Frequently asked questions
Can I use two resistors to share the power?
Yes. Two equal resistors in series or in parallel each dissipate half the total. Choose the combination that gives the resistance you need.
Does the resistor’s value change when hot?
Slightly. Metal film parts drift by 50 to 100 ppm per °C; carbon composition by much more. Running cool keeps precision circuits accurate.
What temperature does a resistor reach at full rating?
Often 100 to 150 °C on the body surface, hot enough to discolour a PCB. This is the main reason for the 50% rule.
Is the rating different for AC?
Use RMS voltage or current and the formulas apply unchanged. For peak values divide by √2 first.
Why is power proportional to the square of current?
Doubling the current doubles the voltage drop across the same resistance, so the product V × I quadruples.