Power Dissipation Calculator
Enter a current and a resistance to get the power dissipated by a resistor in watts — plus the voltage drop — and see why the heat climbs with the square of the current.
Power and voltage drop at once
Enter the current and resistance and this power dissipation calculator returns the power (I²R) in watts and the voltage drop (IR) in volts together.
Use SI units
Current in amperes and resistance in ohms give power in watts — make sure you read the steady (DC or RMS) current, not a peak value.
What is power dissipation?
The heat a resistor gives off
Power dissipation is the rate at which a resistor turns electrical energy into heat. Whenever a current flows through a resistance, the resistor warms up, and that wasted heat is the dissipated power, measured in watts. This power dissipation calculator turns two measurements, the current in amperes and the resistance in ohms, into the power in watts, alongside the voltage drop across the resistor (current times resistance). It is the number that decides which power rating a resistor needs, how warm a circuit will run, and how much energy a load wastes as heat instead of useful work.
Enter a current in amperes and a resistance in ohms to get the power dissipated in watts and the voltage drop instantly.
Power dissipation is the current squared multiplied by the resistance, and the voltage drop is simply the current times the resistance.
P = I² × RThe current is squared, so it dominates the result: a small change in current produces a large change in power. The voltage drop (V = I × R) keeps the current to the first power, so it rises more gently. Use amperes and ohms and the power comes back in watts and the voltage drop in volts.
Suppose a current of 2 A flows through a 10 Ω resistor.
Square the current
2² = 4 — the squared current that drives the heat.
Multiply by the resistance
4 × 10 = 40 W — the power dissipated by the resistor.
Find the voltage drop
2 × 10 = 20 V — the potential difference across the resistor.
The two outputs answer two different questions. The power dissipation (40 W for the resistor above) is how much heat the component must shed every second — the figure you compare against a resistor's power rating so it does not overheat or burn out. The voltage drop (20 V) is the potential difference the resistor creates in the circuit and is what shares the supply voltage between components. The crucial insight is that power scales with the square of the current: double the current from 2 to 4 A and the dissipated power jumps fourfold, from 40 to 160 W, while the voltage drop only doubles. That is exactly why an overloaded resistor heats up so dramatically, why fuses and wire gauges are sized around current, and why even a modest current spike can push a part past its rating. Higher resistance raises the heat too, but only in direct proportion — current is the lever that moves the result the most.
The formula is exact, but a couple of practical points are worth keeping in mind.
Ohmic resistors and consistent units
This calculator assumes an ideal ohmic resistor whose resistance does not change with temperature or frequency; it does not model diodes, capacitors, inductors, or components whose resistance drifts as they heat up. Keep your units consistent — amperes for current and ohms for resistance — or the watts will be wrong, and use the steady DC or RMS current rather than a peak value.