Heatsink Thermal Resistance Calculator

The heatsink a power transistor, regulator or diode needs: the maximum sink-to-ambient thermal resistance (θSA) from the power, temperatures, θJC and the mounting interface — and the junction temperature and margin with the heatsink you pick.

Last updated 22 Sep 2026

Required heatsink θSA

Power + temperatures → °C/W

Power dissipated in the device (W)Conduction plus switching loss for a MOSFET; (Vin − Vout) × I for a linear regulator.

Maximum junction temperature you allow (°C)Datasheet absolute maximum is often 150 or 175 °C; designing to 125 °C or lower leaves margin and extends life.

Ambient air temperature (°C)The air around the heatsink, inside the enclosure, on the hottest day — not room temperature.

θJC, junction to case (°C/W)From the device datasheet (RθJC or RthJC). Use the figures from your part’s datasheet; typical values vary widely between manufacturers.

θCS, case to heatsink: package and interfaceTypical values from onsemi AN1040/D at the recommended screw torque. Mica is the classic insulator; grease halves its resistance.

θCS from your pad’s datasheet (°C/W), overrides the list0 = use the list. Silicone pads and phase-change materials publish their own figure for a given package.

θSA of the heatsink you are considering (°C/W)From the heatsink datasheet, at your airflow. Natural-convection ratings assume vertical fins and free air.

The heat path drawn as a circuit: power (W) flows like current, thermal resistances (°C/W) are resistors and temperatures are voltages, measured up from the ambient air. The heatsink turns amber within 10 % of your junction limit and red at or over it.

5.40°C/WExample

10 W in a TO-220 device, θJC 1.5 °C/W, mica + grease (1.6 °C/W), Tj limit 125 °C, ambient 40 °C, candidate heatsink 4.5 °C/W

The thermal resistance chain

θ_SA ≤ (T_J,max − T_A) ÷ P − θ_JC − θ_CS; T_J = T_A + P × (θ_JC + θ_CS + θ_SA)

θ_JC: junction to case, from the device datasheet (measured per JEDEC JESD51-14 for most power packages)
θ_CS: case to sink: the interface — grease, pad or insulator — and the mounting pressure
θ_SA: sink to ambient: the heatsink itself, at your airflow
P: power dissipated in the device, in watts

Worked example

10 W in a TO-220 device, θJC 1.5 °C/W, mica + grease (1.6 °C/W), Tj limit 125 °C, ambient 40 °C, candidate heatsink 4.5 °C/W
Budget = (125 − 40) ÷ 10 = 8.50 °C/W from junction to air
θSA ≤ 8.50 − 1.5 − 1.6 = 5.40 °C/W
With the 4.5 °C/W heatsink: Tj = 40 + 10 × (1.5 + 1.6 + 4.5) = 116.0 °C, a margin of 9.0 °C
Case 101.0 °C, heatsink 85.0 °C; that heatsink handles up to 11.18 W

Required θSA as the power rises

The worked example’s device: θJC 1.5, mica + grease 1.6, limit 125 °C, ambient 40 °C. Above 27.4 W no heatsink is enough.
Power	Budget (°C/W)	Heatsink θSA needed (°C/W)	Tj with a 4.5 °C/W heatsink
2 W	42.50	39.40	55.2 °C
5 W	17.00	13.90	78.0 °C
10 W	8.50	5.40	116.0 °C
15 W	5.67	2.57	154.0 °C
20 W	4.25	1.15	192.0 °C
25 W	3.40	0.30	230.0 °C
30 W	2.83	none can do it	268.0 °C

Typical case-to-sink interface resistance

onsemi AN1040/D, Table 1: TO-220 at 8 in-lb screw torque, TO-3 (TO-204AA) at 6 in-lb. Values are approximate; a pad or phase-change material’s datasheet gives its own.
Package and interface	θCS (°C/W)
TO-220, metal to metal, dry	1.2
TO-220, metal to metal, thermal grease	1
TO-220, 2-mil mica, dry	3.4
TO-220, 2-mil mica + grease	1.6
TO-3, metal to metal, dry	0.5
TO-3, metal to metal, thermal grease	0.1
TO-3, 3-mil mica, dry	1.3
TO-3, 3-mil mica + grease	0.36

How to size a heatsink

Heat leaves a power semiconductor through a chain of thermal resistances, each in °C per watt: from the silicon junction to the package case (θJC), across the interface to the heatsink (θCS), and from the heatsink into the surrounding air (θSA). They add like resistors in series, and the temperature rise is the power times their sum. So the junction runs at Ta + P × (θJC + θCS + θSA). Rearranged, the largest θSA you can accept is the temperature budget per watt, (Tj max − Ta) ÷ P, minus the two resistances you cannot change with the heatsink. In the example, 10 W with 85 °C to spare leaves 8.50 °C/W; the device and its mica washer take 3.1, so the heatsink must be 5.40 °C/W or better.

When no heatsink can help. If θJC + θCS alone exceed the budget, the junction overheats even on an infinite heatsink. The example device reaches that point at 27.4 W. Then the only fixes are less power, a cooler ambient, a better interface (bare metal with grease instead of mica raises the example’s allowance to 6.00 °C/W) or a part with a lower θJC — often a larger package or two devices sharing the load.

Getting the inputs right. θJC comes from the device datasheet; JEDEC’s JESD51 series defines how it is measured, and JESD51-12 warns that θJA, the junction-to-ambient figure on the front page of many datasheets, belongs to a standard test board and is no guide to your heatsink. Texas Instruments’ SPRA953 explains the difference between these metrics. θCS depends on the interface: onsemi’s AN1040 gives about 1.0 °C/W for a greased TO-220 on bare metal and 1.6 °C/W with a mica insulator and grease, rising to 3.4 dry. For ambient, use the air around the heatsink inside the enclosure on the hottest day. For the power, a switching MOSFET’s conduction plus switching loss — the chart above shows how the junction temperature climbs if the power turns out higher than you thought.

Reading heatsink datasheets. A heatsink’s θSA is quoted at a stated airflow and orientation; natural-convection figures assume vertical fins in free air and get worse in a cramped box. Many heatsinks are rated at a particular temperature rise, and θSA improves slightly as they get hotter. Choose one comfortably below the requirement. The limit you enter is a design choice: the absolute maximum junction temperature (often 150 or 175 °C) is not a target, and every 10 °C cooler roughly improves long-term reliability. The model is steady state: short pulses are handled with the datasheet’s transient thermal impedance curve, not θJC. To check the resistor in a linear circuit, the Ohm’s law calculator gives its dissipation.

Frequently asked questions

How do I calculate the heatsink I need?

θSA ≤ (Tj max − Ta) ÷ P − θJC − θCS. For 10 W, a 125 °C limit, 40 °C ambient, θJC 1.5 °C/W and a mica washer with grease (1.6 °C/W): 8.5 − 1.5 − 1.6 = 5.40 °C/W or lower.

What is thermal resistance in °C/W?

How many degrees the temperature rises for each watt flowing through that part of the path. A 4.5 °C/W heatsink carrying 10 W sits 45 °C above the air around it.

What does it mean if the required θSA is negative?

The junction-to-case and case-to-sink resistances already use up the whole temperature budget, so even a perfect heatsink cannot keep the junction below your limit. Reduce the power, the ambient temperature, or the θJC and θCS.

Does thermal paste make a difference?

Yes. onsemi AN1040 gives a TO-220 at 1.2 °C/W dry on bare metal and 1.0 °C/W greased; with a mica insulator, 3.4 °C/W dry against 1.6 °C/W greased. On a tight budget that difference decides the heatsink.

Can I use θJA from the datasheet instead?

Only for a device without a heatsink, and only as a rough guide: θJA is measured on a standard JEDEC test board and changes with your board and airflow. With a heatsink, use θJC + θCS + θSA.

Related calculators

References

JEDEC. JESD51-12: Guidelines for Reporting and Using Electronic Package Thermal Information; JESD51-14: Transient Dual Interface Test Method for the Measurement of the Thermal Resistance Junction to Case of Semiconductor Devices with Heat Flow Through a Single Path. JEDEC Solid State Technology Association.
onsemi. AN1040/D: Mounting Considerations for Power Semiconductors, Rev. 5, August 2021. Table 1: approximate values for interface thermal resistance.
Texas Instruments. SPRA953: Semiconductor and IC Package Thermal Metrics. Application report.
Horowitz P, Hill W. The Art of Electronics, 3rd ed. Cambridge University Press, 2015. Chapter 1: voltage, current and resistance; Ohm’s law; power in resistors; voltage dividers and Thévenin equivalents.