Work out V squared over R, then pick a part rated for at least twice it. 5 V across 220 ohms is 0.114 W — arithmetic says an eighth-watt part, practice says a quarter.
Manufacturer ratings are measured in still air at 25 degrees C with the part on its own. Inside an enclosure, beside other components, on a board that is already warm, the real limit is lower and the part drifts before it fails. Derating is not caution, it is correcting for the fact that the quoted figure was measured somewhere your circuit is not.
It is a rule for continuous dissipation. A resistor absorbing a brief surge — an inrush limiter, a snubber, a discharge path — is governed by pulse energy and thermal mass instead, and a part will survive many times its rated power for a few milliseconds. Conversely, precision parts drift with temperature long before they are anywhere near their rating, so anything setting a reference wants far more headroom than a factor of two.
Estimate with the rule, then check it against the calculator that models it properly.
Open Resistor Colour Code Calculator →Work out V squared over R, then pick a part rated for at least twice it. 5 V across 220 ohms is 0.114 W — arithmetic says an eighth-watt part, practice says a quarter. Manufacturer ratings are measured in still air at 25 degrees C with the part on its own.