Resistor Power Rating
Known values
Enter any two — the third and the power are computed automatically.
Power & recommendation
How to use this calculator
Fill in any two of voltage, current and resistance — the calculator solves for the missing one using Ohm's law and then computes the dissipated power with P = V·I (equivalently V²/R or I²·R). Results update as you type. It then recommends a standard power rating (⅛ W up to 100 W) using a 2× safety margin, so the part runs cool rather than at its limit.
What the value means in a circuit
Power is the rate a resistor converts electrical energy into heat (P = V·I = I²R = V²/R). A resistor rated for less power than it actually dissipates will overheat, drift in value, and can fail or catch fire — which is exactly why the rating must exceed the calculated dissipation, not merely match it. The 2× derating rule of thumb keeps the part well within its thermal budget across tolerance, voltage spikes and ambient temperature, and is the same margin used in professional design reviews. Use the recommended rating to pick a physically appropriate resistor (a 5 W part is far larger than a ¼ W film resistor).