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Junction Temperature Calculator

Work out junction temperature instantly with clear inputs, formula shown and shareable results.

About Junction Temperature Calculator

Junction temperature is ambient plus power times total thermal resistance. Reliability degrades exponentially with junction temperature — the common rule is that each 10 degree rise halves expected life — so designing to the absolute maximum is never appropriate.

Formula

Junction temperature

Tj = Tambient + P x Rja; P_max = (Tj_max - Tambient) / Rja

Worked example

With the default values below, Junction Temperature Calculator returns Junction temperature of 75.75. Change any input above and every figure here is recalculated in your browser.

Inputs used

Power dissipated
65 W
Junction-to-ambient resistance
0.55 °C/W
Ambient temperature
40 °C
Maximum junction temperature
105 °C

Results

Junction temperature
75.75 °C
Margin to the limit
29.25 °C
Maximum power at this ambient
118.18 W
Assessment
Comfortable thermal margin

Step by step

  1. 1Values used
  2. 2Junction temperature
  3. 3Junction temperature= 75.75 °C
  4. 4Margin to the limit= 29.25 °C
  5. 5Maximum power at this ambient= 118.18 W
  6. 6Assessment= Comfortable thermal margin

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.

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

Frequently Asked Questions

How much margin should I leave?

At least 10-15 degrees below the rated maximum at worst-case ambient, and more if the part is safety-related or expected to last many years.

Why does life halve every 10 degrees?

Most wear-out mechanisms — electromigration, dielectric breakdown, solder fatigue — follow Arrhenius kinetics with activation energies that give roughly that doubling rate.

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