Power Dissipation Calculator
Find power dissipated by a resistor.
Current flowing through the resistor.
Results update as you type.
How it works
Power dissipation is the rate at which a resistor turns electrical energy into heat, and this power dissipation calculator finds that figure from the current flowing through the component and its resistance. Every resistor, wire, and load with resistance dissipates power whenever current passes through it, and that energy leaves as heat rather than doing useful work, so knowing the amount is essential for keeping components within their thermal limits. This matters because every resistor carries a power rating, often a quarter or half a watt, and exceeding it makes the part overheat, drift in value, or burn out. Designers use a power dissipation calculator to choose resistors with enough headroom, to size heat sinks, and to estimate how much energy a circuit wastes. It also explains why long cables warm up and why high-current wiring must be thick. The formula works because power equals voltage times current, and Ohm's law lets you replace the voltage across a resistor with current times resistance, giving current squared times resistance. Because current is squared, small increases in current cause large jumps in heat. Electronics engineers, hobbyists, and students rely on a power dissipation calculator to keep parts cool and circuits reliable.
P = I² × R
Power dissipation follows Joule's law: the power converted to heat in a resistor equals the current squared multiplied by the resistance. Current appears squared because power is voltage times current and, by Ohm's law, the voltage across the resistor is itself current times resistance. Doubling the current therefore quadruples the heat, which is why current, not resistance, usually dominates thermal problems in real circuits.
Source:Joule's law (P = I²R)
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Worked example
How to use this calculator
- 1
Enter your values
Enter current and resistance.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Power dissipated updates instantly as you type — no button to press.
Frequently asked questions
How is power dissipation calculated?
Power dissipation in a resistor equals the current through it squared, multiplied by its resistance, written P equals I squared R. This comes from Joule's law and Ohm's law combined. For example, three amps through a four-ohm resistor dissipates nine times four, or thirty-six watts, entirely as heat. Because current is squared, it has a far stronger effect on heating than resistance does.
Why is the current term squared?
Power equals voltage multiplied by current, and by Ohm's law the voltage across a resistor equals current times resistance. Substituting that voltage into the power equation replaces one current with resistance times current, leaving current squared times resistance. This squaring means doubling the current quadruples the heat produced, which is why high-current circuits demand careful attention to conductor size and cooling far more than low-current ones do.
What is a resistor's power rating?
A resistor's power rating is the maximum power it can safely dissipate as heat without damage, commonly an eighth, quarter, half, one, or two watts for through-hole parts. If the calculated dissipation approaches or exceeds this figure, the resistor overheats, drifts in value, or fails. Good practice keeps the actual dissipation below about half the rating to leave a safety margin for reliability and long life.
Does power dissipation cause energy loss?
Yes. The power a resistor dissipates leaves the circuit as heat and cannot be recovered as useful work, so it represents wasted energy. In power electronics and long cable runs, minimising this loss improves efficiency and reduces cooling needs. That is why engineers select low-resistance conductors for high current and why energy-conscious designs try to avoid dropping voltage across resistive elements unnecessarily.
Can I calculate power from voltage instead?
Yes. If you know the voltage across the resistor rather than the current, power equals voltage squared divided by resistance, or simply voltage times current. All three forms give the same result and follow from Ohm's law. This calculator uses the current-and-resistance form because current and resistance are often the known quantities, but you can convert your figures using Ohm's law first if needed.