How to Calculate Power Dissipation
Whenever electric current flows through a resistive component, some of that electrical energy converts into heat. The rate at which this happens — the power dissipation — is measured in watts and is what determines how hot a resistor, wire, or other component gets, and whether it needs a heat sink or a higher wattage rating. This calculator finds power dissipation from any two of voltage, current, and resistance, and checks the result against a component's power rating.
The power dissipation formulas (Joule's Law)
The base relationship is P = V × I, where P is power in watts, V is voltage in volts, and I is current in amps. Combining this with Ohm's Law (V = I × R) lets you substitute out either variable to get two more equivalent forms: P = I²R (useful when you know current and resistance) and P = V²/R (useful when you know voltage and resistance). All three formulas describe the same physical quantity — pick whichever matches the values you actually measured.
Using the calculator: solving for a missing quantity
Enter any two of Voltage, Current, and Resistance and leave the third field blank — the calculator applies Ohm's Law to solve for the missing value, then computes power from the complete set. If you enter all three, it checks that V = I × R holds (within rounding) and flags a mismatch if your numbers are inconsistent. Optionally enter the component's printed power rating to see what percentage of that rating your dissipation represents — a useful check before running a resistor continuously near its limit.
Frequently Asked Questions
What is the formula for power dissipation?
How do I find power dissipation from current and resistance only?
How do I find power dissipation from voltage and resistance only?
Why does a component's power rating matter?
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Common Measurement Conversions for Everyday Use
| Quantity | US / Imperial | Metric Equivalent |
|---|---|---|
| 1 cup | 8 fl oz | 237 mL |
| 1 pint | 16 fl oz | 473 mL |
| 1 quart | 32 fl oz | 946 mL |
| 1 gallon | 128 fl oz | 3.785 L |
| 1 lb | 16 oz | 453.6 g |
| 1 ft | 12 in | 30.48 cm |