Heatsink Thermal Resistance Calculator
Heat leaves a power semiconductor through a chain of thermal resistances: from the silicon junction to the package case, through the mounting interface to the heatsink, and from the heatsink into the surrounding air. These resistances add like electrical resistors. To keep the junction below its rated maximum, the heatsink must be good enough that the whole chain stays inside the temperature budget. This calculator finds the maximum allowed heatsink-to-ambient thermal resistance, and the resulting junction temperature for a heatsink you specify.
Heatsink thermal formula
Max Rsa = (Tj_max - Ta) / P - Rjc - Rcs
Total R = Rjc + Rcs + Rsa (chosen)
Junction temp Tj = Ta + P * Total R
Temperature rise = P * Total R
The three series resistances sum to the total junction-to-ambient resistance. Multiplying by the dissipated power gives the temperature rise above ambient. The maximum heatsink resistance is whatever is left in the budget after the fixed junction and interface resistances.
Thermal design facts
- Thermal resistances in series add, exactly like electrical resistors.
- Thermal resistance is measured in degrees Celsius per watt.
- Forced airflow can cut heatsink-to-ambient resistance several-fold.
- Thermal interface material lowers case-to-sink resistance.
- A negative maximum heatsink resistance means passive cooling is impossible at that power.
Heatsink thermal resistance: frequently asked questions
How is heatsink thermal resistance calculated?
The junction temperature equals the ambient temperature plus the dissipated power times the total thermal resistance from junction to ambient. Rearranging for the heatsink, the maximum allowed heatsink-to-ambient resistance is (Tj_max minus Ta) divided by power, minus the junction-to-case and case-to-sink resistances.
What is the thermal resistance chain?
Heat flows from the silicon junction to the case (Rjc), through the mounting interface to the heatsink (Rcs), and from the heatsink to ambient air (Rsa). These three resistances add in series, just like electrical resistors, and are measured in degrees Celsius per watt.
What does a lower thermal resistance mean?
A lower thermal resistance in degrees Celsius per watt means the path carries heat more easily, so a given dissipation produces a smaller temperature rise. A larger heatsink, forced airflow, or better thermal interface material all lower the resistance and keep the junction cooler.
What if the required heatsink resistance is negative?
A negative result means no passive heatsink can keep the junction below its maximum at this power and ambient. The junction-to-case and case-to-sink resistances alone already exceed the budget. You must reduce power, lower ambient, improve the interface, or add forced cooling.
What junction-to-case resistance should I use?
Use the value from the device datasheet for your specific package; it is a measured property of the part. This calculator exposes it as an editable input, along with case-to-sink resistance, which depends on the thermal pad or grease and mounting pressure you use.
Official sources
- NIST: Physical Measurement Laboratory: thermal measurement.
- JEDEC: JEDEC standards for semiconductor thermal characterisation.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.