Heat Sink and Thermal Resistance Calculator

The heat sink thermal resistance a power device needs, or the junction temperature it reaches with a given heat sink, from the power, the ambient temperature and the thermal resistances.

How it works

Heat flows from the junction of a device through the case, an interface (paste or pad), the heat sink and into the air. Each step has a thermal resistance in kelvin per watt, and, like resistors in series, they add. The temperatures then follow from Ohm’s law for heat: ΔT = P × R.

Tj = Ta + P × (Rjc + Rcs + Rsa)

Rearranged, the heat sink needed is Rsa = (Tj − Ta) ÷ P − Rjc − Rcs. A heat sink’s Rsa depends on its size, its fins and the airflow, and is given for natural convection or at a specified air speed. How the board itself conducts heat is in Thermal Design on the Board, and regulators and power stages in Electronics Fundamentals.

Worked example

A power transistor in a TO-220 package dissipates 8 W. The ambient temperature inside the case reaches 50 °C, the junction must stay below 110 °C, Rjc = 1.5 K/W and a silicone pad gives Rcs = 0.5 K/W.

  • Allowed total resistance = (110 − 50) ÷ 8 = 7.5 K/W.
  • Heat sink Rsa = 7.5 − 1.5 − 0.5 = 5.5 K/W or less: choose a sink rated at 5 K/W or better.
  • With a 4 K/W heat sink instead: Tj = 50 + 8 × (1.5 + 0.5 + 4) = 98 °C; the case is at 86 °C and the sink at 82 °C (hot to the touch). That arrangement can dissipate up to 10.0 W for a 110 °C junction.

Things to know

Use the worst case: highest ambient temperature, the highest power the device will dissipate (average power for slow thermal time constants, peak for short pulses) and the maximum Rjc from the data sheet.

Heat sink ratings depend strongly on airflow and orientation. A rating “in natural convection” assumes vertical fins in free air: fins blocked by a case or mounted horizontally perform worse by 20 to 40 percent.

Pulsed loads can use the transient thermal impedance Zth of the device, which is lower than the steady-state resistance for short pulses.

Junction temperature governs life: every 10 °C of extra temperature roughly halves the life of many semiconductors. Keep a margin below the absolute maximum.

Thermal paste or a pad is essential: an unfilled air gap between a package and a sink can have 10 times the resistance of paste.

Common questions

What does K/W mean?

Kelvin of temperature rise per watt of heat flow. A heat sink of 2 K/W rises 2 °C for every watt it dissipates (above the ambient).

Where do I find Rjc?

In the data sheet of the device, as θJC or RθJC. It is small for packages with a metal tab (TO-220, TO-247, D2PAK) and large for plastic-only packages.

Do I need a heat sink?

If the junction temperature from Tj = Ta + P × Rja (Rja from the data sheet without a sink) is above the allowed value, yes.

Sources

  • JEDEC JESD51 series (thermal characterization of packages); manufacturers’ heat sink data (Aavid, Wakefield-Vette, Fischer Elektronik).
  • Lienhard, J. H. IV and V, A Heat Transfer Textbook, Phlogiston Press; Texas Instruments, Semiconductor and IC Package Thermal Metrics (SPRA953).

Last reviewed: November 2, 2026