Calculates the power a resistor dissipates as heat for a given current — useful for sizing resistors correctly in a circuit.
How it works
The current squared is multiplied by the resistance to give power dissipated.
What this does not include
This does not include the general power-over-time relationship — for that broader calculation, use this site’s power calculator instead, which handles energy divided by time for any situation.
How to use this calculator
- Enter the current flowing through the resistor and its resistance.
A worked example
2 amps through a 10Ω resistor: power = I²R = 2² × 10 = 40 W.
0.5 amps through a 100Ω resistor: power = 25 W.
What the variables mean
| Variable | Meaning |
|---|---|
| Current | Current flowing through the resistor, in amps |
| Resistance | Resistor value, in ohms |
Edge cases worth knowing
Power dissipation scales with the square of current, not linearly. Doubling the current quadruples the heat generated — a key reason why even small overcurrents can cause disproportionate heating in a circuit.
A negative resistance has no physical meaning, so the calculator declines to show a result for one.
Frequently asked questions
Why is current squared in this formula?
Power dissipation combines Ohm’s law (V=IR) with the basic power equation (P=VI), and substituting one into the other naturally squares the current term.
Why does this matter for choosing a resistor?
Every resistor has a maximum power rating — exceeding it causes overheating and potential failure, so calculating expected dissipation helps select an appropriately rated component.
Does a higher resistance always mean more heat?
Only for the same current — for a fixed voltage instead, a higher resistance actually reduces both current and power dissipated, since current falls faster than resistance rises.
