Heat Sink Size Calculator

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 extruded aluminum, that increases the surface area available for shedding that heat into the air.

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 or paste, typically 0.5 to 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 = 0.7 °C/W (thermal pad)
• Ambient temperature: 40°C

Required R_sa = (150 − 40) / 25 − 1.5 − 0.7 = 4.4 − 2.2 = 2.2 °C/W

You would need a heat sink rated at 2.2 °C/W or better (lower number = better cooling).

Now do it properly. Running silicon at its absolute maximum junction temperature is a specification limit, not a design target, and lifetime drops sharply as you approach it. Design to 80% instead, which is 120°C here:

Required R_sa = (120 − 40) / 25 − 1.5 − 0.7 = 3.2 − 2.2 = 1.0 °C/W

That is more than twice as demanding, and it is the figure worth buying against. The gap between those two numbers is the difference between a part that works and a part that works for ten years.

Typical Heat Sink Thermal Resistance Values

• Small clip-on finned heat sinks: 10 to 50 °C/W
• Medium extruded aluminum heat sinks: 2 to 10 °C/W
• Large finned heat sinks with a fan: 0.5 to 3 °C/W
• Large passive heat sinks: 0.5 to 2 °C/W
• Liquid cooling blocks: 0.05 to 0.5 °C/W

Always Apply Thermal Interface Material

Between the component case and the heat sink, always use thermal paste, a pad, or grease to fill the microscopic air gaps. Dry metal-to-metal contact runs 2 to 5 °C/W; with a decent interface material that drops to 0.3 to 1.0 °C/W.

Work out what that is worth. Going from a dry joint at 3 °C/W to paste at 0.5 °C/W saves 2.5 °C/W, and on a 25 W part that is 2.5 × 25 = 62°C off the junction temperature. It is the cheapest thermal improvement available and the one most often skipped.