Thermal resistance stack calculator

Enter junction-to-case, case-to-sink, and sink-to-ambient thermal resistance in °C/W together with steady-state power in watts. The calculator returns the total path resistance and the temperature rise across every segment. Add an ambient temperature when you also want estimated heat-sink, case, and junction temperatures for a quick thermal design check.

Build the complete junction-to-ambient path

A data sheet often gives junction-to-case resistance, while a thermal interface material or mounting test supplies case-to-sink resistance and a heat-sink specification supplies sink-to-ambient resistance. These three values describe consecutive sections of one heat-flow path, so they add directly when the same steady heat rate passes through each section. Enter every resistance in degrees Celsius per watt and power in watts. The calculator reports their sum as total thermal resistance in degrees Celsius per watt. It also multiplies each section by power, making the contribution of the package, interface, and heat sink visible instead of hiding everything inside one total. This separation is valuable when deciding where an improvement will matter. Reducing an already tiny interface resistance may barely change junction temperature if sink-to-ambient resistance dominates. Conversely, an excellent heat sink cannot compensate for an unexpectedly poor package or mounting interface. Use values that describe the same physical assembly and operating conditions; mixing a natural-convection heat-sink rating with forced-air assumptions elsewhere does not represent a coherent stack.

Interpret temperature rise and node temperatures

For steady one-dimensional heat flow, temperature rise equals dissipated power multiplied by thermal resistance: ΔT = P × Rθ. The total rise is therefore power times the sum of junction-to-case, case-to-sink, and sink-to-ambient resistance. The individual rises show how much temperature is lost across each part of the path. If ambient temperature is supplied, the calculator works back through the stack: sink temperature equals ambient plus the sink-to-ambient rise, case temperature adds the case-to-sink rise, and junction temperature adds the junction-to-case rise. This is an equilibrium estimate, not a warm-up curve. It assumes constant power, stable ambient conditions, and resistance values appropriate to the installation. It does not include thermal capacitance, transient impedance curves, parallel heat paths through leads or a circuit board, temperature-dependent power, radiation changes, or airflow variation. Treat the computed junction temperature as a design estimate and preserve margin below the manufacturer’s maximum rating, especially when resistance tolerances and worst-case ambient conditions are uncertain.

Choose inputs and check design margin

Use dissipated heat rather than electrical input power unless nearly all input power becomes heat in the modeled device. For a regulator, for example, estimate dissipation from its voltage drop and load current plus relevant quiescent losses. Obtain junction-to-case resistance for the correct package and heat-flow direction. Case-to-sink resistance must reflect the actual pad, grease, insulator, mounting pressure, contact area, and fastener arrangement. Sink-to-ambient resistance depends strongly on orientation, enclosure, altitude, airflow, nearby surfaces, and whether the published figure assumes natural or forced convection. Run a nominal case first, then repeat with maximum power, maximum ambient temperature, and conservative resistance values. Compare the resulting junction temperature with the data-sheet operating limit and any reliability target your project imposes. The API price is $0.002 per request, while the browser calculation can support quick iteration during component selection. Zero values are accepted because they can represent an idealized segment, but negative resistance or power is rejected as invalid input rather than producing a physically misleading result.