Heatsink Thermal Resistance Calculator
Find the heatsink thermal resistance you need from the power dissipated and temperature limits, and check whether you need one at all.
Example: a 7805 regulator (TO-220) dropping 12 V to 5 V at 0.3 A dissipates about 2 W.
Formula and cautions
θsa = (Tj − Ta) / P − θjc − θcs. Heat flows from the die to the case, through the heatsink and into the air, and the sum of those resistances must stay within budget. Pick a heatsink with a lower (better) thermal resistance than the result, and keep your junction temperature target below the datasheet maximum.
Frequently asked questions
What does °C/W mean for a heatsink?
It is how many degrees the heatsink warms above the air for each watt it sheds. A 10 °C/W heatsink carrying 2 W runs 20 °C above ambient. Lower numbers mean better cooling.
Does thermal paste matter?
Yes. The interface between the part and the heatsink (θcs) can add around 0.5 to 1 °C/W with paste and much more with a dry joint or a thick pad. On high-power parts it can decide whether a design works.