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Heat Sink Calculator

Find the heat sink thermal resistance you need

Required Heat Sink

8.00 °C/W

Tj

125.0°C

Margin

0.0°C

From device datasheet (typically 0.5–5 °C/W)

Thermal pad: 0.2–1 °C/W

Enter to compute actual Tj with your chosen heat sink

Required θsa (Sink to Ambient)

8.00 °C/W

Junction Temp

125.0°C

Case Temp

110.0°C

Margin

0.0°C

Total θ

10.00

Example Calculations

110W device, 25°C ambient, 125°C max

Inputs

Power10 W
Ambient25 °C
Max Tj125 °C
θjc1.5 °C/W
θcs0.5 °C/W

Result

Required θsa8.00 °C/W
Tj125.0°C
Margin0.0°C

θsa = (125-25)/10 - 1.5 - 0.5 = 10 - 2 = 8.0°C/W.

225W MOSFET, 40°C ambient

Inputs

Power25 W
Ambient40 °C
Max Tj150 °C
θjc0.5 °C/W
θcs0.3 °C/W

Result

Required θsa3.60 °C/W

θsa = (150-40)/25 - 0.5 - 0.3 = 4.4 - 0.8 = 3.6°C/W.

Frequently Asked Questions

Q

How do I calculate heat sink thermal resistance?

θsa = (Tj_max - Ta)/P - θjc - θcs. For 10W, 25°C ambient, 125°C max junction, θjc=1.5, θcs=0.5: θsa = (125-25)/10 - 1.5 - 0.5 = 8.0°C/W.

  • Thermal chain: Tj = Ta + P×(θjc + θcs + θsa)
  • Solve for θsa (sink-to-ambient)
  • θjc from device datasheet
  • θcs depends on thermal pad/grease
Power (W)AmbientMax TjRequired θsa
5W25°C125°C18.0 °C/W
10W25°C125°C8.0 °C/W
20W25°C125°C3.0 °C/W
50W40°C150°C0.2 °C/W
Q

What is θjc (junction-to-case thermal resistance)?

θjc is the thermal resistance from the semiconductor junction to the outside of the package. It's fixed by the IC design and listed in the datasheet. Typical: TO-220 = 1-3°C/W, TO-92 = 80-150°C/W.

  • TO-220: 1–3 °C/W
  • TO-247: 0.4–1 °C/W
  • TO-92: 80–150 °C/W (no heat sink practical)
  • QFN packages: 2–10 °C/W
Q

What is θcs (case-to-sink thermal resistance)?

θcs depends on the thermal interface between IC and heat sink. Thermal grease: 0.2–0.5°C/W. Thermal pad: 0.5–1.0°C/W. Dry contact: 1–2°C/W.

  • Thermal grease: 0.2–0.5 °C/W
  • Silicone thermal pad: 0.5–1.0 °C/W
  • Mica insulator + grease: 0.4–0.9 °C/W
  • Dry contact (no TIM): 1–2 °C/W
Q

What if my required θsa is negative?

A negative θsa means no heat sink can keep the junction below Tj_max. Solutions: reduce power, improve airflow, use a lower θjc package, or increase acceptable Tj.

  • Negative θsa = impossible cooling
  • Reduce power dissipation
  • Use forced-air cooling
  • Switch to better thermal package
  • Consider liquid cooling for high power

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Last Updated: Mar 21, 2026

This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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