Heat Sink Calculator (Thermal Resistance)
By Santaji GadepatilinUpdated October 27, 2026
Power transistors, MOSFETs and regulators turn some energy into heat, and too much heat shortens their life or destroys them. A heat sink with low enough thermal resistance keeps the junction cool — see this heat sink calculation guide.
How to use the heat sink calculator
Enter the power the device dissipates, the design junction temperature, the ambient temperature inside the enclosure, and the device’s Rθjc and interface Rθcs. The result is the highest heat sink thermal resistance you can use.
For more background, read What Is Thermal Conductivity?.
The thermal circuit
Tj = Ta + P × (Rθjc + Rθcs + Rθsa)
Rθsa ≤ (Tj − Ta) ÷ P − Rθjc − Rθcs
Example
10 W in a TO-220 MOSFET (Rθjc 1.5 °C/W), thermal pad 0.5 °C/W, 40 °C ambient, 125 °C target:
- Total allowed = 85 ÷ 10 = 8.5 °C/W
- Rθsa ≤ 8.5 − 1.5 − 0.5 = 6.5 °C/W
Typical package resistances
| Package | Rθjc | Rθja (no heat sink) |
|---|---|---|
| TO-92 | — | 150–200 °C/W |
| SOT-23 | — | 200–350 °C/W |
| D-PAK (on PCB copper) | 1–3 °C/W | 40–80 °C/W |
| TO-220 | 0.5–3 °C/W | 50–65 °C/W |
| TO-247 | 0.3–1 °C/W | 35–45 °C/W |
Typical heat sinks
| Heat sink | Rθsa (natural convection) |
|---|---|
| Small clip-on TO-220 | 15–25 °C/W |
| Extruded fin, 50 mm | 4–8 °C/W |
| Extruded fin, 100 mm | 1.5–3 °C/W |
| Large finned profile | 0.5–1 °C/W |
| With fan airflow | 2–4 times lower |
Mount fins vertically for natural convection, and remember the enclosure air temperature — not room temperature — is the ambient.
Frequently asked questions
How do I choose a heat sink?
Calculate the total allowed thermal resistance, (Tj − Ta) ÷ P, subtract the device's Rθjc and the interface Rθcs, and choose a heat sink with a lower Rθsa than the result.
What if the result is negative?
Even an infinite heat sink cannot keep the junction cool enough. Reduce power, use a device with lower Rθjc, or add forced air cooling.
What junction temperature should I design for?
Stay 20–30 °C below the datasheet maximum (often 150 or 175 °C) for reliability — 110–125 °C is a common design target.
What is a typical case-to-sink resistance?
0.1–0.5 °C/W for a TO-220 with thermal paste; 0.5–1.5 °C/W with a silicone or mica insulating pad.