Heatsink Thermal Resistance Calculator
Heat flows from a semiconductor junction to the surrounding air through a chain of thermal resistances: junction to case, case to heatsink, and heatsink to ambient. To keep the junction below its rated temperature you must pick a heatsink with a low enough sink-to-ambient resistance. Enter the dissipated power, the maximum junction temperature, the ambient temperature, and the junction-to-case and case-to-sink resistances to find the maximum allowable heatsink resistance.
Thermal resistance formula
Max total Rja = (Tj_max - Ta) / P
Max heatsink Rsa = Rja - Rjc - Rcs
where P = power (W), temperatures in C, resistances in C/W
The junction, case, sink, and ambient form a series thermal path. The heatsink must supply whatever resistance budget remains after the fixed junction-to-case and case-to-sink terms.
Worked example
A device dissipates 15 W, with a 125 C junction limit, 40 C ambient, Rjc = 1.5 C/W, and Rcs = 0.5 C/W. Max total Rja = (125 - 40) / 15 = 85 / 15 = 5.67 C/W. Max heatsink Rsa = 5.67 - 1.5 - 0.5 = 3.67 C/W. Any heatsink rated at or below 3.67 C/W keeps the junction within limits.
Heatsink thermal resistance: frequently asked questions
How is the required heatsink thermal resistance found?
The required sink-to-ambient resistance is Rsa = (Tj_max - Ta) / P - Rjc - Rcs. It is the total allowable thermal resistance from junction to ambient minus the fixed junction-to-case and case-to-sink resistances.
What do the thermal resistances mean?
Rjc is the junction-to-case resistance from the device datasheet, Rcs is the case-to-heatsink resistance set by the thermal interface (grease or pad), and Rsa is the heatsink's resistance to ambient. They add in series: each degree Celsius per watt restricts how much heat can flow.
What units are used?
Power in watts, temperatures in degrees Celsius, and thermal resistances in degrees Celsius per watt (C/W). The junction temperature is the sum of ambient plus power times the total thermal resistance.
What does a negative required resistance mean?
A negative result means even a perfect zero-resistance heatsink cannot hold the junction below the limit at that power. You must reduce the power, lower the ambient temperature, or improve the junction-to-case path.
Sources
- NIST: SI units (watt, degree Celsius).
- The series thermal resistance model is a standard steady-state heat-transfer result. Device resistances come from your datasheet.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.