Junction Thermal Resistance Calculator
Heat from a semiconductor flows from its junction, through the package and any heatsink, and finally into the surrounding air. Each step in that path resists heat flow, measured in degrees Celsius per watt, and they add in series like electrical resistors. This calculator sums the junction-to-case, case-to-sink, and sink-to-ambient thermal resistances, then finds the junction temperature for a given power and ambient, the total junction-to-ambient resistance, and the maximum power the part can dissipate before reaching its rated junction temperature.
Thermal resistance formula
theta JA = theta JC + theta CS + theta SA
Tj = Tambient + Pd * theta JA
Pd_max = (Tj_max - Tambient) / theta JA
All theta values in degrees Celsius per watt
The three thermal resistances add in series because heat passes through each path in turn. Lower total resistance keeps the junction cooler for the same power.
Thermal design context
- Junction-to-case comes from the datasheet; case-to-sink from the interface; sink-to-ambient from the heatsink rating.
- Resistances in series add, so a better heatsink (lower theta SA) lowers the whole chain.
- Design with margin below the rated maximum junction temperature for long life.
- Higher ambient temperature directly raises junction temperature for the same power.
- Maximum safe power falls as ambient temperature rises, a process called derating.
Thermal resistance: frequently asked questions
How is junction temperature calculated?
Junction temperature equals ambient temperature plus power times the total junction-to-ambient thermal resistance: Tj = Tambient + Pd times theta_JA. The total theta_JA is the sum of the junction-to-case, case-to-sink, and sink-to-ambient resistances, all in degrees Celsius per watt.
What is thermal resistance in C/W?
Thermal resistance is how many degrees Celsius the temperature rises for each watt of heat flowing through a path. A value of 5 C/W means every watt raises the temperature 5 degrees across that path. Lower values move heat more easily, like a thicker copper bar.
How do I find the maximum power a part can dissipate?
Rearrange the equation: Pd_max = (Tj_max minus Tambient) divided by theta_JA. This is the most power the device can handle before its junction reaches the rated maximum at the given ambient temperature and thermal path.
Why add several thermal resistances in series?
Heat flows from the silicon junction through the package to the case, through the thermal interface to the heatsink, then from the heatsink into the air. Each step has its own resistance, and they add in series just like electrical resistors, giving the total junction-to-ambient resistance.
Where do the thermal resistance values come from?
Junction-to-case is on the device datasheet. Case-to-sink depends on the interface material and mounting pressure. Sink-to-ambient is the heatsink rating at a given airflow. Because they are device- and design-specific, all three are user-editable inputs here.
Official sources
- Texas Instruments: semiconductor thermal characteristics documentation.
- NIST: thermal metrology and SI units.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.