Heatsink Calculator
Required heatsink thermal resistance (Rθsa) for a given die.
How to Use
- Enter Pd, Tj,max, Ta, Rθjc and Rθcs.
- Needed Rθsa = (Tj − Ta)/P − Rθjc − Rθcs.
Show Work
Formulas
History of Heat Sinking
Thermal management of semiconductor devices became a formal engineering discipline in the 1950s as the first germanium and silicon power transistors shipped. Early power devices in TO-3 metal cans were screw-mounted directly to chassis; the chassis itself was the heatsink. Extruded aluminum fin-stock heatsinks appeared in the 1960s, and TIM (thermal interface materials) — silicone grease, later graphite pads, and then phase-change materials — evolved alongside to fill microscopic air gaps between mating surfaces.
The thermal-resistance model (Rθ in °C/W, added in series like electrical resistances) was formalized by the Electronic Industries Association (EIA) in the 1970s and remains the primary analytical tool used in datasheets and design guides today. Its elegance: each stage of the path from junction to ambient can be characterized independently (junction-to-case, case-to-sink, sink-to-ambient) and summed, giving a simple additive calculation for the full thermal chain.
Modern high-power electronics — GPUs, EV motor drives, photovoltaic inverters — push heatsinks into active cooling territory. Direct liquid cooling (cold plates bolted to the package lid), vapor chambers (heat pipes with wick+fluid+fin), and even phase-change immersion cooling are now standard for 300+ W applications. The humble fin-stock extrusion persists for everything up to ~50 W continuous dissipation.
About This Calculator
Enter the power dissipated, target maximum junction temperature (usually 85°C below Tj,max for reliability margin), ambient air temperature, junction-to-case thermal resistance RθJC (from datasheet), and case-to-sink resistance RθCS (TIM — typically 0.1–1 °C/W). The tool returns the required heatsink thermal resistance RθSA — pick a heatsink with equal or lower rating.
For natural convection in still air, typical finned heatsinks deliver 5–15 °C/W. Add a fan to drop into the 1–5 °C/W range. If RθSA needed is below 1 °C/W, consider liquid cooling or direct die-cooling techniques. Everything runs client-side; no values leave your browser.
About the Heatsink Calculator
Working on electronics and circuit design? The Heatsink Calculator is a free browser tool that gives you the answer in seconds. Required heatsink thermal resistance (Rθsa) for a given die.
How it works
Put each value in its box and read the answer as you go. Because it recalculates live, you can play with the inputs to see how each one moves the result — handy for checking your own working or planning ahead. Everything happens on your device, so it is fast and private.
Want the deeper story? The Knowledge Base explains the ideas behind the tools in more detail.
Frequently Asked Questions
Units?
Thermal resistance in °C/W.
Derate?
Keep Tj ≥20 °C below rated max for reliability.
How do I use the Heatsink Calculator?
Simply type your numbers and read the result, which refreshes the instant you change something. There is nothing to submit and nothing to wait for.
Do I need to install or sign up for anything?
Not at all — it runs in the browser with nothing to install and no account. After it loads once, it even works without an internet connection.
Is my information private?
Yes. Everything happens in your browser. Nothing you type is sent to a server or saved anywhere.
Common Use Cases
Linear Reg
LM78xx dissipating 10 W.
MOSFET
Pick sink for switching loss.
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