Resistor Wattage Calculator

You need any two of three values: voltage (V), current (I), or resistance (R). From those, you can calculate power using P = V × I, P = V² / R, or P = I² × R. Once you know the dissipated power, multiply by a safety factor (typically 2×) to select the correct rated resistor.

There are three equivalent formulas: P = V × I (voltage times current), P = V² / R (voltage squared divided by resistance), and P = I² × R (current squared times resistance). All three give the same result — use whichever two quantities you already know.

Measure or calculate the voltage across and current through the resistor, then multiply them: P = V × I. If you only know resistance and one other value, use P = V²/R or P = I²×R. The result is the heat the resistor must safely dissipate.

Resistors running near their maximum rated wattage overheat, degrade, and can fail. Industry best practice is to derate by 50% — meaning you choose a resistor rated at least 2× the calculated power. For high-reliability applications (military, medical), a 3× factor is common.

Common standard ratings are 1/8 W, 1/4 W, 1/2 W, 1 W, 2 W, 3 W, 5 W, 10 W, 20 W, and 50 W. After calculating your required power with the safety factor, choose the next standard rating above that value.

For purely resistive loads in AC circuits, the same formulas apply if you use RMS values for voltage and current. However, if the circuit contains reactive components (capacitors, inductors), the real power dissipated in the resistor depends on the power factor, and you should use P = V_rms × I_rms × cos(φ).

No. Resistors are passive components that only dissipate (consume) power, converting electrical energy into heat. They cannot generate or supply power to a circuit.

At a fixed voltage, higher resistance means lower current and therefore less power dissipated (P = V²/R). At a fixed current, higher resistance means more voltage drop and more power dissipated (P = I²×R). The relationship depends on which quantity is held constant in your circuit.