Heat Sink Calculator
All components generate heat that must be dissipated to prevent failure. Enter the power, temperatures, and internal package resistances below. The calculator will output the maximum allowable heat sink thermal resistance (Rth in °C/W) to keep your part operating safely.
What Is Thermal Resistance?
Thermal resistance (Rth) describes how hard it is for heat to move through a material or across a junction between two materials. It's measured in °C/W — the temperature rise you get for every watt of heat flowing through. A low Rth means heat passes through easily; a high Rth means it backs up, and temperature rises fast.
The comparison to electrical resistance isn't just a teaching device — the math is identical. Heat is current, temperature difference is voltage, and thermal resistance behaves exactly like electrical resistance, including how it adds up when resistances are stacked in series.
In a typical component, heat has to cross three separate boundaries before it reaches the air:
| Stage | What's happening | Who controls it |
|---|---|---|
| Junction → Case | Heat moves from the silicon die to the outer package | Fixed by the manufacturer (Rth-JC on the datasheet) |
| Case → Sink | Heat crosses the interface between the package and the heat sink | You, via mounting pressure and thermal interface material |
| Sink → Ambient | Heat leaves the fins and enters the surrounding air | You, via heat sink size, material, and airflow |
A heat sink's job is to make that last stage as easy as possible, so the total resistance from junction to air stays low enough to keep the silicon under its rated maximum.
The Thermal Formula
The governing equation for steady-state heat transfer in electronics is:
TJ = TA + (Pdiss × Rth_total)
Where:
| Symbol | Meaning | Typical value |
|---|---|---|
| TJ | Junction temperature — the silicon die itself | Max 125°C or 150°C, per datasheet |
| TA | Ambient temperature — the surrounding air | 25°C in open air, up to 50–60°C inside an enclosure |
| Pdiss | Power dissipated as heat | Watts (W), calculated from the circuit |
| Rth_total | Sum of every thermal resistance in the path | °C/W — Rth-JC + Rth-CS + Rth-SA |
Rearranged to solve for the maximum allowable heat sink resistance, this becomes:
Rth_sink(max) = [(TJ − TA) / Pdiss] − Rth-JC − Rth-CS
That's the calculation running behind the fields above. Rth-JC comes straight from the datasheet and can't be changed. Rth-CS depends on your mounting hardware and thermal paste or pad. Whatever margin is left over is the maximum resistance your heat sink is allowed to have.
Practical Applications
| Scenario | Power | Target | Typical Rth needed | Notes |
|---|---|---|---|---|
| CPU cooling | ~65–100 W | Keep TJ under 85–90°C | ~0.2–0.4 °C/W | Needs heat pipes, a large fin stack, and active airflow |
| Linear regulator (7805, TO-220) | ~7 W (12 V → 5 V at 1 A) | Keep TJ under 125°C | Bare TO-220 alone is ~65 °C/W — nowhere close | Without a heat sink, junction temperature would calculate out to roughly 480°C; the part fails long before reaching that number |
| High-power LED | 1–10 W per die | Keep junction under ~85°C for rated lifespan | Depends on package, often under 5 °C/W | Usually handled with an aluminum MCPCB conducting heat into a metal chassis |
| Power MOSFET (switching) | 5–20 W | Keep TJ under 150°C with margin | 1–3 °C/W, sometimes lower with forced air | Rth-JC is small; most of the resistance is in the case-to-sink interface and the sink itself |
The regulator example is worth sitting with for a second: 7 W doesn't sound like much, but a TO-220 package on its own has a junction-to-case-to-air resistance around 65°C/W. Run the numbers and the projected junction temperature is nowhere near survivable — which is exactly why every 7805 circuit you've ever seen has either a heat sink bolted to it or a large copper pour doing the same job.
Video Reference
For a walkthrough of heat sink sizing that goes beyond the formula and into how engineers actually pick a part off the shelf, EEVblog's thermal design tutorial is a solid watch:
EEVblog #105 – Electronics Thermal Heatsink Design Tutorial https://www.youtube.com/watch?v=8ruFVmxf0zs
It covers the same junction-case-sink-ambient chain used in the calculator above, with real component examples and a look at how datasheet numbers translate into a physical heat sink choice.
FAQ
Why do I need thermal paste?
Even a heat sink base and a component case that both look flat are rough at a microscopic level. Clamp them together and you still get tiny air gaps at the contact surface. Air conducts heat poorly, so those gaps act like an extra layer of insulation sitting right where you need heat to flow freely. Thermal paste (or a thermal pad) fills the gaps with a material that conducts far better than air, which is what actually lowers Rth-CS in practice, not just on paper.
Active vs. passive cooling — which one do I need?
Passive cooling relies on natural convection: air near the fins heats up, becomes less dense, and rises on its own, pulling cooler air in behind it. It's silent and has no moving parts to fail, but it needs a physically larger heat sink to move the same amount of heat. Active cooling adds a fan to force air across the fins, which strips heat away much faster and allows a smaller heat sink for the same power — at the cost of noise, extra current draw, and a fan that can eventually fail. The choice usually comes down to available space versus tolerance for noise and moving parts.
What is Rth-JC, and can I improve it?
Rth-JC is the junction-to-case thermal resistance — how well heat moves from the silicon die to the outside of the package. It's baked into the datasheet by the manufacturer and depends on the package's internal construction: die size, lead frame material, how much silicon is exposed. You can't change it after the part is made. As a rule of thumb, bigger packages have lower Rth-JC because there's more material and surface area for heat to spread through — a TO-247 might sit around 0.5–1°C/W, while a tiny SOT-23 can be well over 100°C/W. If a design needs a very low junction-to-case resistance, that constraint has to be factored in at the component-selection stage, not fixed later with a bigger heat sink.
What happens if my heat sink's Rth is higher than the calculated maximum?
Junction temperature rises above the safe target. In the short term that usually just means the part runs hotter than it should; over time, elevated junction temperature accelerates wear-out mechanisms (electromigration, thermal cycling fatigue, dielectric breakdown) and shortens the component's usable life. In the worst case — especially with power semiconductors — thermal runaway can set in, where rising temperature increases leakage current, which generates more heat, which raises temperature further, until the part fails outright.
Does airflow direction matter for a heat sink?
Yes. Fins are usually oriented to align with the direction of airflow, whether that's from a fan or natural convection. Air flowing across the fins (parallel to them) moves heat away efficiently; air blocked by fins running the wrong way barely improves on a flat plate, regardless of how large the heat sink looks.