Heat Sink Size Calculator
Calculate the required thermal resistance of a heat sink for any electronic component.
Prevent overheating in your circuits and designs.
When electronic components like transistors, voltage regulators, and power amplifiers operate, they dissipate power as heat. If this heat is not removed efficiently, junction temperatures rise until the component fails. A heat sink is a thermally conductive device (usually aluminium) that increases the surface area for heat dissipation.
The Thermal Resistance Model
Heat flow in electronics is analogous to Ohm’s law: heat flows from hot to cold like current flows from high to low voltage. Thermal resistance (measured in °C/W) describes how much temperature rises per watt of heat dissipated.
The thermal circuit for a device with a heat sink is:
T_junction = T_ambient + P × (R_jc + R_cs + R_sa)
Where:
- T_junction = component junction temperature (°C) — must stay below maximum rating
- T_ambient = ambient air temperature (°C)
- P = power dissipated (Watts)
- R_jc = junction-to-case thermal resistance (from component datasheet, °C/W)
- R_cs = case-to-heat sink resistance (from thermal pad/paste, typically 0.5–1.5 °C/W)
- R_sa = heat sink-to-ambient resistance (what we calculate, °C/W)
Calculating Required Heat Sink Resistance
Rearranging the equation:
R_sa = (T_junction_max − T_ambient) / P − R_jc − R_cs
Example Calculation
A power MOSFET with:
- Maximum junction temperature: 150°C
- Power dissipation: 25W
- R_jc = 1.5 °C/W (from datasheet)
- R_cs = 0.5 °C/W (with thermal paste)
- Ambient temperature: 40°C
Required R_sa = (150 − 40) / 25 − 1.5 − 0.5 = 4.4 − 2.0 = 2.4 °C/W
You would need a heat sink rated at 2.4 °C/W or better (lower number = better cooling).
Typical Heat Sink Thermal Resistance Values
- Small clip-on finned heat sinks: 10–50 °C/W
- Medium extruded aluminium heat sinks: 2–10 °C/W
- Large finned heat sinks with fan: 0.5–3 °C/W
- Large passive heat sinks: 0.5–2 °C/W
- Liquid cooling blocks: 0.05–0.5 °C/W
Always Apply Thermal Interface Material
Between the component case and heat sink, always use thermal paste, pad, or grease to fill microscopic air gaps. This reduces R_cs from 2–5 °C/W (dry contact) to 0.3–1.0 °C/W.