How to Size a Resistor's Wattage
Every resistor turns some electrical power into heat. Pick one whose power rating is too low and it runs hot, drifts in value, discolours, and eventually fails — sometimes open, sometimes as a short. Sizing the wattage is a two-line calculation plus a margin.
The power a resistor dissipates
Start from P = V × I — power equals the voltage across the resistor times the current through it. Using Ohm's law (V = I × R) to substitute, you get two more forms that are usually more convenient:
P = I² × R (you know the current and the resistance)
P = V² ÷ R (you know the voltage across it and the resistance)
All three give the same answer. Pick whichever matches the two values you already have. The resistor power calculator takes any two of V, I, R, P and returns the dissipation plus a suggested rating.
Why the I² term matters
Power rises with the square of the current. Double the current and the heat quadruples; triple it and the heat goes up nine times. This is why a resistor that's fine at 50 mA can burn out at 150 mA, and why high-current paths need deliberate attention while signal-level resistors almost never do.
Worked examples
LED series resistor. 3.0 V dropped across the resistor at 20 mA: P = 3.0 × 0.02 = 0.06 W. A 1/8 W (0.125 W) part has ample margin. (The LED resistor calculator does this whole job.)
Linear voltage drop. Dropping 9 V at 100 mA: P = 9 × 0.1 = 0.9 W. You need a 2 W resistor, and it will still get warm. This is the moment to ask whether a switching regulator would avoid the heat entirely.
Current-sense resistor. 0.1 Ω carrying 2 A: P = I²R = 4 × 0.1 = 0.4 W. Use a 1 W part.
How much headroom
Running a resistor continuously at its rated wattage shortens its life and pushes its temperature — and therefore its value — around. The standard rule is to choose a rating at least twice the calculated dissipation. Increase that margin when:
- the resistor is in a sealed or unventilated enclosure,
- it sits next to other hot parts,
- the ambient temperature is high — datasheets "derate" the allowed power above about 70 °C, sometimes to zero by 150 °C,
- reliability matters more than cost.
Standard axial ratings step 1/8, 1/4, 1/2, 1, 2, 3, 5 W. Pick the first one above 2× your calculated power.
Physical size follows wattage
A higher-power resistor is physically bigger because it needs more surface area to shed heat. A 1/4 W axial part is about 6 mm long; a 2 W part about 15 mm; beyond that you move to wirewound or aluminium-clad resistors bolted to a heatsink. If a calculation calls for many watts in a small space, that's a signal to change the circuit rather than the resistor.
Pulses are different
The power rating assumes steady-state dissipation. A resistor can absorb far more energy in a short pulse than its continuous rating suggests — but how much depends on the pulse width and duty cycle, and it's specified by a pulse-power curve in the datasheet. Don't use the continuous rating for a resistor that only sees brief surges (an inrush limiter, a snubber, a discharge resistor).
To read the value off a resistor you already have, use the resistor color code; for the underlying V/I/R relationships, the Ohm's law calculator and the Ohm's law guide.