Heatsink Thermal Resistance Calculator

Solve for:

Choose the thermal question that matches the datasheet or prototype check.

Package preset:

Use a preset as a starting point, then keep the numeric fields tied to the actual part.

{{ powerFieldLabel }}:

Enter the worst-case device heat load in watts.

Ambient temperature:

Worst-case local air temperature around the heat sink.

deg C

Maximum junction temperature:

Datasheet junction limit for the selected device.

deg C

Design margin below Tj max:

The calculator sizes to Tj max minus this margin.

deg C

Junction-to-case thetaJC:

Package thermal resistance controlled by the part selection.

deg C/W

Interface path:

Pick a typical interface, then adjust thetaCS when the material data sheet gives a value.

Case-to-sink thetaCS:

Thermal resistance of the mounting interface.

deg C/W

Heat-sink candidate:

Use an installed or candidate thetaSA for verification, charting, and maximum-power estimates.

Installed or candidate thetaSA:

Enter the heat-sink-to-air rating to check an actual design.

deg C/W

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Heat-sink rating context:

Use neutral free-air unless the datasheet rating and the product airflow are known to differ.

Preferred sink base ceiling:

Guidance threshold for the estimated heat sink base temperature.

deg C

Display precision:

Adjust output precision without changing the calculation.

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Heat-sink selection starts with a temperature budget. A power device can look electrically safe while the silicon inside it is running too hot, because the metal tab, package surface, and surrounding air are all separated from the junction by thermal resistance. That resistance is measured in C/W, meaning degrees Celsius of temperature rise per watt of heat.

The useful question is not whether a heat sink is large, but whether the whole path can carry the expected heat while staying below the junction-temperature target. Heat leaves the junction, moves through the package, crosses the mounting interface, spreads into the heat sink, and finally leaves through air. A small mistake early in that path can consume the same thermal budget as a visibly undersized heat sink.

Power dissipation is the heat created inside the device. For a linear regulator, that may be voltage drop times current. For an LED, transistor, motor driver, processor, or power module, it is the loss power after electrical efficiency has already been considered. Ambient temperature is also specific: it is the air near the heat sink inside the finished product, not the room temperature several feet away.

A junction-to-air heat path showing package, interface, heat sink, and ambient resistance segments.

Three terms appear repeatedly in heat-sink work. ThetaJC is the junction-to-case resistance from the device data sheet. ThetaCS is the case-to-sink resistance created by paste, pads, insulation, surface flatness, and pressure. ThetaSA is the heat-sink-to-ambient resistance, usually supplied by the heat-sink manufacturer under stated airflow and mounting conditions.

A heat-sink rating is not a universal promise. Fins turned sideways, blocked inlet air, a sealed enclosure, warm nearby components, or a fan failure can turn a good paper value into a tight assembly. A thermal-resistance calculation is therefore a planning and screening method. Final confidence still comes from the part data sheet, the heat-sink data sheet, and temperature measurement on the real board in the real enclosure.

How to Use This Tool:

Use the fields in working order, starting with the thermal question and then replacing presets with real data-sheet numbers wherever possible.

Choose Solve for. Required heat-sink thetaSA sizes the maximum C/W rating you can accept, Check installed heat sink tests a candidate, and Maximum power on installed sink estimates the safe heat load for a fixed sink.
Pick a Package preset only as a starting point. Confirm Maximum junction temperature and Junction-to-case thetaJC against the device data sheet before making a hardware choice.
Enter Power dissipated, Ambient temperature, and Design margin below Tj max. Use worst-case device heat and the hottest air around the sink.
Choose an Interface path or type the actual Case-to-sink thetaCS. A pad or insulator stack can change the answer more than a small heat-sink upgrade.
For installed-sink checks, choose a Heat-sink candidate or enter Installed or candidate thetaSA. Use the manufacturer rating that matches airflow, fin direction, and mounting as closely as possible.
Open Advanced when the entered sink rating needs context. Heat-sink rating context adjusts the effective thetaSA for open air, horizontal fins, enclosure restriction, or forced airflow.
Set Preferred sink base ceiling when touch temperature, plastics, wiring, or nearby parts matter. The ceiling affects guidance, not the junction-temperature formula.
Read Thermal Stack Ledger first, then use Fit Guidance, Junction Margin Curve, Sink Class Margin, and JSON for deeper review.

If an input issue appears, fix it before using the result. Common causes are zero or negative power, a working junction target at or below ambient, negative resistance values, a missing candidate thetaSA in installed-sink modes, or a sink base ceiling that is not above ambient.

Interpreting Results:

Required thetaSA is the highest heat-sink-to-air resistance that still meets the working junction target after package and interface losses are removed. In this context, a smaller C/W number means a stronger heat sink. If the required thetaSA is negative, the package and interface already consume more thermal budget than the design allows.

Installed margin compares the estimated junction temperature with the working target, which is Tj max after the selected design margin has been subtracted. A margin of at least 10 C is labeled Installed sink has margin. A margin from 0 C up to but not including 10 C is Installed sink is tight. A negative margin is Installed sink runs hot.

How to interpret heat-sink thermal resistance outputs
Output	Best use	Do not overread
Working junction target	Shows the actual temperature target after design margin.	It is not the same as the absolute data-sheet maximum.
Estimated junction	Checks the steady-state silicon temperature for the entered stack.	It does not cover startup pulses, thermal transients, or uneven die heating.
Estimated sink base	Flags handling and enclosure-temperature concerns.	A cool base does not prove the junction is cool when thetaJC or thetaCS is high.
Maximum power on candidate	Estimates heat load before the working target is reached.	It assumes the same ambient, interface, and effective thetaSA stay valid.
Sink Class Margin	Compares rough heat-sink classes under the entered assumptions.	It is a screening comparison, not a substitute for a selected part's data sheet.

Use the charts to test sensitivity. A design that only works at one power point or with one optimistic airflow assumption deserves a prototype temperature check before it becomes a board or enclosure decision.

Technical Details:

Steady-state thermal resistance treats heat flow like a series resistance path. Power dissipation plays the role of current, temperature rise plays the role of voltage difference, and each thermal-resistance segment adds to the total. The method is useful when the device has reached a stable operating temperature and the main heat path is from junction to case, through the interface, into the heat sink, and out to air.

The calculation separates the allowable total junction-to-air resistance from the part of the path that the heat sink cannot fix. ThetaJC belongs to the device package. ThetaCS belongs to the interface and mounting. ThetaSA is the heat sink's remaining allowance.

Formula Core:

The heat-sink target is found by subtracting package and interface resistance from the allowed total junction-to-air resistance.

\begin{array}{rcl} T_{target} & = & T_{j max} - M \\ θ_{JA allowed} & = & \frac{T_{target} - T_{A}}{P} \\ θ_{SA required} & = & θ_{JA allowed} - θ_{JC} - θ_{CS} \\ T_{j installed} & = & T_{A} + P (θ_{JC} + θ_{CS} + θ_{SA effective}) \\ P_{max} & = & \frac{T_{target} - T_{A}}{θ_{JC} + θ_{CS} + θ_{SA effective}} \end{array}

Here P is device heat in watts, TA is local ambient temperature, M is the design margin below Tj max, thetaJC is junction-to-case resistance, thetaCS is case-to-sink resistance, and effective thetaSA is the entered heat-sink rating after any airflow or enclosure context multiplier. Display precision changes rounded output only.

With the default planning values of 7 W, 50 C ambient, 125 C Tj max, 20 C margin, 4 C/W thetaJC, and 0.2 C/W thetaCS, the working target is 105 C. The allowed rise is 55 C, so thetaJA allowed is 55 / 7 = 7.857 C/W. Subtracting 4.2 C/W leaves a required thetaSA of 3.657 C/W.

Heat-sink calculation boundary rules
Rule	Boundary	Meaning
Power dissipated	Must be > 0 W	Thermal resistance needs positive heat flow.
Ambient range	-80 C to 175 C accepted by validation	Values outside that range are treated as unrealistic for this estimate.
Working target	Tj max - margin must be > ambient	No steady-state heat-rise budget remains when the target is at or below local air temperature.
Thermal resistance terms	thetaJC, thetaCS, and thetaSA must be >= 0 C/W	Negative resistance is not physically meaningful in this series model.
Installed margin band	>= 10 C, 0 to < 10 C, or < 0 C	Those bands drive the installed-sink status labels.

The rating-context multiplier changes the entered thetaSA before installed temperature and maximum-power estimates are calculated. Neutral open-air mounting uses 1.00x, horizontal fins or a weak chimney path use 1.20x, enclosed or obstructed airflow uses 1.45x, and reliable forced airflow uses 0.65x. These are planning factors, not replacements for manufacturer airflow curves.

Accuracy and Privacy Notes:

The result is a steady-state estimate for design screening. It does not model thermal capacitance, pulsed loads, spreading resistance inside a complex module, heat through the PCB, radiation, altitude, dust, fan aging, or nearby hot components unless those effects are folded into the entered numbers.

Verify thetaJC, Tj max, and power dissipation from the device data sheet and operating conditions.
Verify thetaCS from the thermal interface material, isolation requirement, and mounting pressure.
Verify thetaSA from the heat-sink data sheet under matching airflow and orientation.
Measure the prototype in the final enclosure before treating a tight margin as acceptable.
No part number or project name is required, but copied tables and exported files can still reveal design assumptions.

Advanced Tips:

Use Required heat-sink thetaSA before shopping for parts, then switch to Check installed heat sink with the selected part's real thetaSA.
Raise Design margin below Tj max for sealed, hot, long-life, or safety-critical designs; lowering it can make a marginal sink look acceptable.
Use the enclosed-airflow context when a heat-sink data sheet was measured in open air but the product sits in a case.
Compare Junction Margin Curve against expected power spread, not only the nominal load point.
Watch Estimated sink base when users can touch the enclosure or when nearby plastics, adhesives, and wiring have temperature limits.

Worked Examples:

Sizing a natural-convection sink

A 7 W device in 50 C local air with a 125 C junction limit and 20 C margin has a 105 C working target. With 4 C/W thetaJC and 0.2 C/W thetaCS, the required heat-sink allowance is 3.657 C/W, so a 2.5 C/W large extrusion is thermally stronger than the target before enclosure effects are considered.

Checking the same sink in restricted air

The same 2.5 C/W sink becomes 3.625 C/W effective thetaSA under the enclosed or obstructed airflow context. The estimated junction is about 104.8 C against the 105 C working target, leaving almost no margin. That result should push the design toward better airflow, lower power, or a lower-C/W sink.

Using forced airflow for a higher-power package

A 10 W device in 45 C air with a 150 C junction limit, 25 C margin, 1.2 C/W thetaJC, 0.7 C/W thetaCS, and a 1 C/W fan-cooled sink has 0.65 C/W effective thetaSA. The total stack is 2.55 C/W, the estimated junction is about 70.5 C, and the maximum-power estimate is about 31.4 W under the same assumptions.

FAQ:

Is a lower thetaSA always better?

For the same airflow and mounting conditions, yes. Lower thetaSA means less temperature rise from heat sink to ambient for each watt. Cost, size, electrical isolation, mounting force, and airflow noise still matter.

Why does the candidate pass in open air but fail in an enclosure?

The rating context changes the effective thetaSA. Restricted air raises the C/W value, so the same physical heat sink can produce a hotter junction estimate when the enclosure blocks convection.

What does a negative required thetaSA mean?

It means the package and interface resistance are already too high for the selected power, ambient, and junction target. A heat sink alone cannot fix that stack; reduce power, ambient, margin demand, thetaJC, or thetaCS.

Why am I seeing an input issue?

Check for zero power, a working junction target at or below ambient, negative theta values, a missing installed thetaSA in candidate modes, or a sink base ceiling that is not above ambient.

Glossary:

Junction temperature: The temperature of the active semiconductor region inside the device.
ThetaJC: Junction-to-case thermal resistance from the silicon to a specified package surface.
ThetaCS: Case-to-sink thermal resistance across paste, pads, insulation, and mounting contact.
ThetaSA: Heat-sink-to-ambient thermal resistance from the heat sink into surrounding air.
Design margin: Temperature subtracted from Tj max to create a lower working junction target.
Effective thetaSA: The entered heat-sink rating after the selected airflow or enclosure multiplier is applied.

References:

Thermal Characterization of IC Packages, Analog Devices.
Thermal Design Guide for DSP and ARM Application Processors, Texas Instruments, revised August 2017.
Practical Design Techniques for Power and Thermal Management, Analog Devices.
Thermal Resistance, Analog Devices.