What formula does this calculator use?

Rth(sink-to-ambient) = (Tj max - Tambient) / Pdissipation - Rth(junction-to-case) - Rth(case-to-sink). The result is the maximum thermal resistance a heatsink can have; any heatsink rated at or below that number keeps the junction under its temperature limit.

What junction-to-case value should I use for TO-220 or TO-3?

Datasheet values vary by device, but typical figures are around 1.5 degrees C per watt for TO-220, 1.4 for TO-3, and 0.6 to 0.8 for the larger TO-247 and TO-3P packages. Always use the actual datasheet value when available; the presets here are representative starting points, not a substitute.

What case-to-sink resistance should I assume?

A thin layer of thermal grease on bare metal-to-metal contact is typically 0.2 to 0.5 degrees C per watt. A silicone thermal pad or a mica insulator with grease (needed when the case is electrically live) typically runs 0.5 to 1.5. Dry contact with no interface material can exceed 1.5 and is not recommended.

The calculator says the required resistance is zero or negative. What does that mean?

It means the junction-to-case and case-to-sink resistances alone already consume the entire thermal budget, so no heatsink, however large, keeps the junction at or below the target temperature. Reduce the power dissipation, lower the ambient temperature, choose a lower-resistance package or interface, or accept a higher junction temperature if the device allows it.

Should I design to the maximum rated junction temperature?

Running at the absolute maximum leaves no margin for tolerance stack-up, aging, or a hotter-than-expected day. A common practice is to design to 25 to 40 degrees C below the datasheet maximum, which this calculator supports by simply entering a lower target in the junction temperature field.

Does this account for airflow or heatsink fin geometry?

No. The output is a required thermal resistance rating in degrees C per watt. Heatsink datasheets publish that rating for stated conditions, usually natural convection or a specific airflow speed, so match the heatsink you buy to the airflow you actually have; forced air can substantially lower the same heatsink's effective resistance.

Heatsink Size Calculator

Enter power dissipation, max junction temperature, ambient temperature, and the package thermal resistances to get the maximum sink-to-ambient thermal resistance your heatsink is allowed to have.

Device and Thermal Parameters

Power Dissipation (W)

Max Junction Temp (°C)

Ambient Temp (°C)

Package Preset

Rth Junction-to-Case (°C/W)

Rth Case-to-Sink (°C/W)

Grease on bare contact is roughly 0.2 to 0.5; a pad or mica insulator is roughly 0.5 to 1.5.

Required Heatsink Rating

Maximum Rth Sink-to-Ambient

—

Thermal Budget

0.00 °C/W

(Tj max − Tambient) ÷ P

Junction-to-Case

0.00 °C/W

subtracted

Case-to-Sink

0.00 °C/W

subtracted

Shopping Guidance

Enter values on the left to see a recommendation.

Summary

A heatsink is only correctly sized when its rated sink-to-ambient thermal resistance is at or below the value the circuit actually needs. This calculator works backward from the maximum allowed junction temperature, the ambient air temperature, and the power the device dissipates, subtracting the package junction-to-case resistance and the case-to-sink interface resistance, to give the maximum Rth(sink-to-ambient) a heatsink can have before the junction overheats.

How it works

Enter the power the device dissipates in watts, from a datasheet loss calculation or measurement.
Enter the maximum junction temperature the device is rated for, or a lower design target for margin.
Enter the worst-case ambient air temperature the heatsink will see, not room temperature.
Pick a package preset or enter the junction-to-case thermal resistance from the datasheet.
Enter the case-to-sink interface resistance for your mounting method (grease, pad, or dry contact).
Read the required heatsink rating and shop for a heatsink rated at or below that value.

Use cases

Sizing a heatsink for a TO-220 voltage regulator or MOSFET in a hobby project.
Checking whether a stock heatsink is adequate before finalizing an enclosure design.
Comparing how much a thermal pad versus grease changes the required heatsink rating.
Deciding between natural convection and a fan-cooled heatsink for a power supply build.
Verifying a linear regulator will not thermally shut down at full load and high ambient.
Teaching the junction-to-ambient thermal resistance chain in an electronics course.

Frequently Asked Questions

Last updated: 2026-09-09 · Reviewed by Nham Vu

Heatsink Size Calculator

Use the Heatsink Size Calculator

Device and Thermal Parameters

Required Heatsink Rating

Summary

How it works

Use cases

Frequently Asked Questions