Enter values and press Calculate.
Heat flows from the silicon junction → device case → heat sink → ambient air. Each step has a thermal resistance in °C/W, and they add up in series like electrical resistors. The temperature rise above ambient equals the power dissipated times the total thermal resistance.
| Symbol | Meaning |
|---|---|
| θJC | Junction-to-case (from the device datasheet) |
| θCS | Case-to-sink (thermal pad / paste, ~0.2–1 °C/W) |
| θSA | Sink-to-ambient (the heat sink's rating) |
| TJ | TA + P × (θJC+θCS+θSA) |
A lower θSA means a bigger/better heat sink. If the required θSA comes out negative, no heat sink can keep the part cool — you must reduce power, lower ambient, or pick a device with lower θJC.
Yes for anything above a watt or two. Paste (or a pad) fills microscopic air gaps between the case and sink, dropping θCS from ~1 °C/W dry to ~0.2–0.5 °C/W.
Still-air heat sinks are quiet and reliable. Adding a fan (forced convection) can cut the effective θSA by 2–4×, letting a smaller sink handle the same power — useful when space is tight.
More surface area and fins mean a lower θSA (better cooling). Datasheets rate sinks in °C/W — pick one at or below the value this calculator gives.
Yes, but their heat adds up. Sum the power dissipations for the θSA calculation, and remember each device still has its own θJC and pad in series with the shared sink.
Compute the junction temp with no sink (θSA very large). If TJ stays below the maximum with margin, you don't. Otherwise size one with the first tab.
Dry metal-to-metal ≈ 0.5 °C/W; with thermal paste ≈ 0.2–0.4 °C/W; with an insulating mica/silicone pad ≈ 0.5–1.2 °C/W.
Keep TJ at least 20–30 °C below the datasheet maximum for reliability and to allow for hot days and aging.