UseCalcPro
Home
MathFinanceHealthConstructionAutoPetsGardenCraftsFood & BrewingToolsSportsMarineEducationTravel
Blog
  1. Home
  2. Tools

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

Related calculators

Power Supply CalculatorPSU wattage from component drawPCB Trace Width CalculatorIPC-2221 trace sizingOhm's Law CalculatorV, I, R, P solverHeat Index CalculatorCalculate the heat index (feels-like temperature) using the NWS Rothfusz regression. Enter air temperature and humidity to assess heat danger and safety.PA System CalculatorCalculate PA system wattage for your audience size and venue. Find required SPL, amplifier power per speaker, and distance loss using the inverse square law.Event Power Calculator — Generator Sizing by Equipment LoadCalculate event power requirements and generator size. Add your equipment loads to get total wattage, amperage, circuit count, and generator recommendation.

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

Related Resources

Power Supply Calculator

PSU wattage sizing

PCB Trace Width Calculator

Trace width for current capacity

Ohm's Law Calculator

V, I, R, P solver

More Utility Calculators

Electronics, outdoor, and everyday tools

View All Tools

Last Updated: Jul 13, 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.

UseCalcPro
FinanceHealthMathPrivacyTerms

© 2026 UseCalcPro