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Heat Sink Size Calculator

Work out heat sink size instantly with clear inputs, formula shown and shareable results.

About Heat Sink Size Calculator

The thermal chain is junction to case, case to sink through the interface, then sink to ambient. Total allowable thermal resistance is the permitted temperature rise divided by the power, so the heat sink must supply whatever is left after subtracting the junction-to-case and interface resistances.

Formula

Thermal sizing

Rth(total) = (Tj(max) - Tambient) / P; Rth(sink) = Rth(total) - Rth(jc) - Rth(interface)

Worked example

With the default values below, Heat Sink Size Calculator returns Heat sink thermal resistance required of 4.97. Change any input above and every figure here is recalculated in your browser.

Inputs used

Power to dissipate
12 W
Maximum junction temperature
125 °C
Ambient temperature
45 °C
Junction-to-case thermal resistance
1.2 °C/W

Results

Heat sink thermal resistance required
4.97 °C/W
Total allowable thermal resistance
6.67 °C/W
Case temperature
110.60 °C
Temperature rise available
80.00 °C

Step by step

  1. 1Values used
  2. 2Thermal sizing
  3. 3Heat sink thermal resistance required= 4.97 °C/W
  4. 4Total allowable thermal resistance= 6.67 °C/W
  5. 5Case temperature= 110.60 °C
  6. 6Temperature rise available= 80.00 °C

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

Frequently Asked Questions

What if the required sink resistance is negative?

The device cannot dissipate that power at that ambient, no matter what sink is fitted. Reduce the power, use a better package, or add forced air or liquid cooling.

Should I design to the maximum junction temperature?

No. Derate to 80% or less of Tj(max) — reliability and lifetime fall roughly exponentially with junction temperature.

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