Heat Sink Calculator
Size a heat sink, predict semiconductor junction temperature, or find maximum steady-state power from the thermal resistance path.
Calculator is for informational purposes only. Terms and Conditions
Uses a simplified steady-state series thermal path from junction to case, interface, heat sink, and ambient; verify final designs with component and heat-sink manufacturer data.
Choose the thermal calculation
Select the result you need. Required fields and answer units update automatically.
Enter the thermal path values
Use power actually dissipated as heat and thermal resistances for the intended package, interface, mounting, airflow, and orientation.
Treat the junction temperature entry as your design target. If the datasheet maximum is an absolute limit, enter a lower design target to preserve thermal margin.
Result
The primary thermal design value is followed by the most useful temperature and thermal-budget checks.
Result details
- Check—
Show calculation steps Review conversions, thermal budget, substitutions, assumptions, and reverse checks
- Enter valid values to see the complete calculation.
Thermal Resistance Budget
Compare the junction-to-case, interface, and heat-sink portions of the active steady-state thermal path.
- Enter valid values to populate the chart.
Method, Sources, and Assumptions
Calculation basis, authoritative references, limitations, and final verification requirements.
The calculation treats dissipated power like current through a series thermal-resistance path from junction to case, case to heat sink, and heat sink to ambient. It is a preliminary engineering design check, not a transient thermal model or a substitute for manufacturer qualification data.
- Use component-specific θJC for the intended heat-flow path and heat-sink θSA for the actual airflow, orientation, and installation.
- Do not add datasheet θJA to the θJC + θCS + θSA series path; θJA describes a different overall junction-to-ambient condition.
- The model is steady state. Pulsed loads may require transient thermal impedance Zθ(t), thermal capacitance, duty cycle, and manufacturer transient curves.
- Final thermal design should account for tolerances, interface quality, local ambient, enclosure effects, airflow, neighboring heat sources, contamination, and measured temperatures.
Calculator guide
Understanding Your Heat Sink Calculator Result
The Heat Sink Calculator determines the maximum heat-sink-to-ambient thermal resistance you can allow, the junction temperature produced by a known thermal path, or the maximum steady-state power that path can dissipate. For heat-sink sizing, enter power dissipation, junction-temperature target, worst-case ambient temperature, junction-to-case resistance, and case-to-sink resistance. The key output is usually \(\theta_{SA}\) in °C/W or K/W: a lower value means the heat sink must remove heat more effectively.
The calculation uses a simplified junction → case → interface → heat sink → ambient resistance path. It is most appropriate when the intended case surface and attached heat sink carry most of the device heat. Packages with substantial PCB, exposed-pad, bottom-side, top-side, or parallel heat paths can require a more complete thermal network.
- Best for
- Semiconductor heat-sink sizing and steady-state thermal checks
- Primary sizing output
- Maximum allowable \(\theta_{SA}\) for the heat sink
- Critical interpretation
- Choose a sink rated at the calculated °C/W value or lower under the real cooling conditions
How to Use the Three Calculation Modes
The calculator exposes three solve modes. Choose the one that matches the design question, then use device, interface, and heat-sink values that correspond to the same intended heat-flow path.
-
Size a heat sink with Required heat sink resistance
Use this mode when power dissipation, junction target, ambient temperature, \(\theta_{JC}\), and \(\theta_{CS}\) are known. The result is the maximum allowed \(\theta_{SA}\). A candidate heat sink must have an equal or lower rated thermal resistance under the intended installation conditions.
-
Check a candidate with Junction temperature
Enter power dissipation, ambient temperature, \(\theta_{JC}\), \(\theta_{CS}\), and the candidate heat sink’s \(\theta_{SA}\). The calculator returns the predicted steady-state junction temperature and useful case and sink temperature checks.
-
Find the thermal ceiling with Maximum allowable power
Enter the junction target, ambient temperature, and the complete \(\theta_{JC}+\theta_{CS}+\theta_{SA}\) path. The result is the steady-state power that reaches the entered junction-temperature target. Electrical safe-operating-area, current, voltage, switching, and package limits can require a lower value.
-
Keep temperature and resistance units distinct
The calculator supports °C, °F, and K for absolute temperatures; °C/W, K/W, and °F/W for thermal resistance; and W, mW, and kW for power. A thermal resistance of 1 °C/W equals 1 K/W but equals 1.8 °F/W, so °F/W cannot be relabeled without conversion.
How to determine power dissipation
Use the heat actually generated in the device, not its headline electrical rating. For a simple linear regulator, a useful first estimate is \(P_D\approx(V_{IN}-V_{OUT})I_{OUT}\), plus any additional losses that materially affect the device. For MOSFET conduction loss, \(P_{cond}=I_{RMS}^2R_{DS(on)}\), but total switching-device loss can also include switching, diode, reverse-recovery, gate-drive, and output-capacitance losses. If output power and efficiency are known, \(P_{loss}=P_{out}\left(\frac{1}{\eta}-1\right)\) is another useful relationship.
Heat Sink Thermal Resistance Equations
The calculator uses a closed-form, steady-state thermal-resistance model analogous to an electrical series-resistance network. Texas Instruments’ Semiconductor and IC Package Thermal Metrics explains the limits of standardized thermal metrics and the use of junction-to-case, case-to-sink, and sink-to-ambient resistances for heat-sink analysis. TI’s Linear Regulator Design Guide for LDOs gives the direct heat-sink sizing relationship used here.
Required heat-sink thermal resistance
Plain language: divide the allowable junction-to-ambient temperature rise by device power, then subtract the package and interface resistances. The remainder is the maximum resistance available for the heat sink.
This form assumes the modeled junction-to-case-to-sink path is an appropriate dominant path for the heat being analyzed and that conditions are at steady state.
Check junction temperature
Plain language: add the temperature rise through all three series resistances to the local ambient temperature.
Maximum steady-state power
Plain language: the available temperature rise divided by the complete series thermal resistance gives the thermal power ceiling for this simplified model.
- \(\theta_{SA}\)
- Heat-sink-to-ambient thermal resistance Resistance from the heat sink to the surrounding air under the specified cooling conditions.
- \(\theta_{JC}\)
- Junction-to-case thermal resistance Package resistance from the semiconductor junction to the specified case surface used for the heat-sink path.
- \(\theta_{CS}\)
- Case-to-sink thermal resistance Resistance across the thermal interface and mechanical contact between the package and heat sink.
- \(T_J\)
- Junction temperature Temperature of the semiconductor junction where device losses become heat.
- \(T_A\)
- Ambient temperature Air temperature surrounding the heat sink at the location relevant to its thermal rating.
- \(P_D\)
- Power dissipation Electrical loss converted to heat in the device, not the device’s headline power rating or load output.
Worked Heat Sink Sizing Example
Consider a semiconductor dissipating 10 W in a 40°C local ambient. The design target for junction temperature is 125°C, the datasheet junction-to-case resistance is 1.5°C/W, and the case-to-sink interface is estimated at 0.5°C/W.
Find the total thermal budget
The junction can rise 85°C above ambient. At 10 W, that gives a total junction-to-ambient budget of 8.5°C/W for the modeled series path.
Subtract package and interface resistance
Result
Maximum heat-sink thermal resistance = 6.5°C/W
A candidate heat sink should be rated at 6.5°C/W or lower under the actual airflow, orientation, mounting, and ambient conditions. A lower \(\theta_{SA}\) provides more thermal headroom than the exact calculated limit.
How to Interpret the Result
For heat-sink sizing, the result is an upper limit on thermal resistance. If the calculator returns 6.5°C/W, a 5°C/W heat sink is thermally better than a 7°C/W heat sink when the ratings are measured under comparable conditions.
What does °C/W mean on a heat sink?
A heat-sink thermal resistance of 4°C/W means the sink rises approximately 4°C above its reference ambient for every watt flowing through that sink-to-ambient path under the stated rating conditions. For example, at 10 W, a 4°C/W sink has an approximate 40°C sink-to-ambient rise. If the surrounding air is 30°C, the sink would be about 70°C in this simplified steady-state interpretation.
Heat sink temperature vs junction temperature
The heat sink surface is not the semiconductor junction. In the simple series model, \(T_S=T_A+P_D\theta_{SA}\), \(T_C=T_S+P_D\theta_{CS}\), and \(T_J=T_C+P_D\theta_{JC}\). With positive heat flow and positive resistances, the junction is hotter than the case, the case is hotter than the sink, and the sink is hotter than ambient.
Lower °C/W is better
Thermal resistance is temperature rise per unit heat flow. A lower value means less temperature rise for the same dissipated power.
Ambient temperature consumes the budget
Holding the default 10 W power, 125°C target, 1.5°C/W \(\theta_{JC}\), and 0.5°C/W \(\theta_{CS}\) constant, raising ambient from 40°C to 50°C reduces allowable \(\theta_{SA}\) from 6.5 to 5.5°C/W.
Negative or zero sizing results are a stop signal
If the package and interface consume the entire thermal budget, no positive heat-sink resistance can satisfy the inputs. Reduce heat generation, improve the upstream thermal path, lower ambient, or revisit the design target if the device limits permit.
What Changes Heat Sink Performance in Practice
A heat sink’s catalog thermal resistance is not a geometry-only constant. The physical assembly and test conditions determine how closely the real system follows the simplified series model.
Airflow and fin orientation
Natural convection and forced air can produce very different sink-to-ambient performance. Check whether the manufacturer rating is for still air or a stated air velocity, and orient the fins so the intended flow path is not blocked. A rating such as 2.0°C/W at 400 LFM is not equivalent to 2.0°C/W in natural convection.
LFM, m/s, and CFM are not the same quantity
LFM and m/s describe air velocity; CFM describes volumetric flow. Approximately 1 m/s = 196.85 LFM. Converting CFM to velocity requires the flow area, so a fan’s CFM rating alone does not directly provide the air velocity through a heat sink.
Local ambient, not room temperature
The relevant \(T_A\) is the air around the heat sink. Enclosure recirculation, nearby components, restricted vents, or preheated fan inlet air can make local ambient materially higher than room temperature.
Thermal interface quality
Case-to-sink resistance changes with interface material, thickness, pressure, flatness, contact area, insulation, and assembly. TI’s thermal-metrics guidance notes that \(R_{\theta CS}\) calculated from material thickness and conductivity omits interfacial contact resistance, so measurement or supplier data is preferable when the interface matters.
PCB and parallel heat paths
Many modern packages dissipate significant heat through the PCB rather than through one idealized case surface. Standardized junction-to-ambient behavior can depend strongly on the package, board, airflow, and system construction, so a simple series heat-sink path should not be forced onto a package whose dominant heat flow is elsewhere.
Pulsed loads and transient heating
For short MOSFET or IGBT power pulses, steady-state resistance alone can misrepresent peak junction temperature. Transient thermal impedance \(Z_{th}(t)\) includes thermal capacitance and varies with pulse width and duty cycle. Infineon’s dynamic thermal behavior note explains how transient \(Z_{th}\) is applied to pulse calculations.
Heat spreading and source footprint
A small, concentrated heat source on a large heat-sink base can create extra spreading resistance that the lumped \(\theta_{SA}\) value may not isolate. Check the heat-sink manufacturer’s mounting-area guidance and validate concentrated high-heat-flux designs with detailed thermal modeling or test data.
How to Select a Real Heat Sink
Translate the calculator output into a performance specification first, then choose a physical part whose published data satisfies that specification under conditions that resemble the application.
Calculated requirement
If the result is \(\theta_{SA,\max}=3.0\ ^\circ\mathrm{C/W}\), the thermal model requires a sink-to-ambient resistance no greater than 3.0°C/W. That number is the boundary produced by the entered thermal budget.
Selected heat sink
A catalog part rated 2.4°C/W can satisfy the calculated resistance criterion if that 2.4°C/W rating applies to the intended airflow, orientation, mounting, and environment. A part rated 3.6°C/W does not satisfy a 3.0°C/W requirement under comparable conditions.
| Check | Why it matters |
|---|---|
| Rated \(\theta_{SA}\) | Must be at or below the calculator’s maximum allowed resistance for the relevant conditions. |
| Airflow condition | A forced-air rating cannot be assumed in still air; use the manufacturer’s natural-convection or stated-velocity data. |
| Orientation and clearance | Blocked fins and poor natural-convection flow paths can increase actual thermal resistance. |
| Package contact area | The heat source must mount to the sink in a way compatible with the manufacturer’s test or mounting guidance. |
| TIM and isolation | Thermal pads, grease, insulators, and mounting pressure contribute to \(\theta_{CS}\) and can consume thermal margin. |
| Temperature validation | After selecting a candidate, calculate junction temperature and verify the assembled hardware under credible worst-case conditions. |
Common Heat Sink Calculation Mistakes
Most bad heat-sink results come from using the wrong thermal path or entering values that describe a different operating condition than the hardware being designed.
Using \(\theta_{JA}\) as though it were \(\theta_{JC}\)
Datasheet junction-to-ambient resistance describes an overall standardized test condition influenced by the package, PCB, and environment. Junction-to-case resistance refers to a specified package conduction path. Do not add a datasheet \(\theta_{JA}\) to \(\theta_{JC}+\theta_{CS}+\theta_{SA}\).
Entering component rating instead of actual loss
Power dissipation means heat generated in the device. A transistor’s voltage, current, or advertised power capability is not automatically the heat load. Use circuit loss calculations or measured dissipation for the operating condition.
Using room temperature for \(T_A\)
If the heat sink lives inside a warm enclosure, using 25°C because the room is 25°C overstates the available thermal budget. Use the credible worst-case local air temperature around the heat sink.
Comparing °F/W and °C/W as if they were numerically equal
Temperature differences in °F scale by 1.8 relative to °C or K. Therefore, 1°C/W = 1 K/W = 1.8°F/W.
Assuming heat-sink temperature equals junction temperature
The junction is hotter than the sink by the temperature rise through \(\theta_{JC}\) and \(\theta_{CS}\). A comfortable-to-touch sink surface does not prove the silicon junction is cool.
Using steady-state resistance for short pulses
Short-duration losses can require transient thermal impedance and duty-cycle analysis. A steady-state calculator is appropriate only when the thermal system has effectively reached or is being conservatively treated as steady state.
Assumptions and Limits of the Calculator
This is a preliminary engineering thermal model. Its equations are exact for the stated lumped series network, but the real hardware may not behave like a single one-dimensional resistance chain.
Steady-state operation
The model assumes the temperatures can be represented by steady resistances. It does not solve time-dependent heat storage or pulse-to-pulse thermal cycling.
Dominant modeled heat path
The equations assume the junction → case → interface → sink → ambient path is an appropriate dominant path. Packages with substantial PCB, exposed-pad, bottom-side, top-side, or parallel heat flow can require a fuller thermal network.
Input values belong to the actual assembly
\(\theta_{JC}\), \(\theta_{CS}\), and \(\theta_{SA}\) are not interchangeable generic constants. Use the device datasheet, interface data, and heat-sink performance information for the applicable package, mounting, airflow, and orientation.
Thermal limit is not the complete device limit
Maximum allowable power mode checks only the entered thermal constraint. It does not verify safe-operating-area, current density, voltage stress, switching loss, electrical insulation, mechanical attachment, or product qualification requirements.
Heat Sink Calculator FAQs
These questions address common follow-up decisions after calculating a heat-sink resistance or junction temperature.
What does °C/W mean on a heat sink?
It is thermal resistance: the approximate temperature rise per watt through the stated thermal path under the rating conditions. A 4°C/W heat sink carrying 10 W has an approximate 40°C sink-to-ambient temperature rise.
Is a lower °C/W heat sink better?
Yes. For the same dissipated power and comparable test conditions, a lower sink-to-ambient thermal resistance produces less heat-sink temperature rise above ambient. If your maximum allowed \(\theta_{SA}\) is 4°C/W, a 3°C/W candidate provides more thermal capability than a 5°C/W candidate.
Does the calculator tell me the physical size of the heat sink?
No. It calculates the thermal performance the cooling solution must achieve. Physical dimensions depend on material, fin geometry, base design, orientation, airflow, heat-source footprint, and installation. Select a real part from verified manufacturer thermal-performance data.
Can I use datasheet \(\theta_{JA}\) instead of \(\theta_{JC}\)?
Not in the same series heat-sink equation. \(\theta_{JA}\) describes junction-to-ambient behavior for a defined package, board, and environment, while \(\theta_{JC}\) characterizes a specified conduction path from junction to case. Use the parameter appropriate to the actual thermal model.
How do I calculate heat sink temperature?
In the simplified series model, \(T_S=T_A+P_D\theta_{SA}\). This gives the heat-sink temperature from ambient, dissipated power, and sink-to-ambient resistance. Junction temperature is higher because the case-to-sink and junction-to-case temperature rises must also be added.
How do I know the power dissipation to enter?
Enter the device loss converted to heat at the operating point. For a simple linear regulator this can be approximated from its voltage drop and load current, while switching devices can have conduction, switching, diode, gate-drive, and other losses. Use the loss model appropriate to the circuit rather than the device’s maximum power rating.
Does a fan lower heat sink thermal resistance?
Forced airflow can reduce sink-to-ambient thermal resistance, but the amount depends on the heat sink, fin geometry, airflow velocity, fan operating point, enclosure resistance, and bypass flow. Use the heat-sink manufacturer’s thermal-resistance-versus-airflow data rather than a universal fan correction factor.
Can two transistors share one heat sink?
They can, but the sink temperature responds to the combined heat load while each transistor has its own package and interface temperature rise. In a simplified shared-sink model, \(P_{sink}=P_1+P_2+\cdots\), then each device’s junction temperature is checked from the common sink temperature plus its own \(P_i(\theta_{CS,i}+\theta_{JC,i})\). Spreading and device placement can require a more detailed model.
Can I use this calculator for a pulsed MOSFET load?
Use caution. The calculator is a steady-state model. Short pulses can require the MOSFET’s transient thermal impedance curve \(Z_{th}(t)\), pulse width, duty cycle, starting temperature, and temperature-dependent electrical losses. The steady-state result can still be a useful long-duration limit, but it is not a complete transient junction-temperature analysis.
Why can my calculated required heat-sink resistance be negative?
A negative sizing result means the specified junction-to-case and case-to-sink resistances already exceed the total thermal-resistance budget available between the junction target and ambient at the entered power. No ordinary positive \(\theta_{SA}\) can fix that combination; reduce the heat load or upstream resistance, lower ambient, or revisit the design target within valid device limits.
Sources and Technical Basis
The guide uses manufacturer technical documentation for the thermal-resistance model, interpretation of package thermal metrics, and transient thermal limitations. The worked example was independently reverse-checked against the junction-temperature equation.
- Texas Instruments — Semiconductor and IC Package Thermal Metrics — Supports interpretation and limitations of \(R_{\theta JA}\), \(R_{\theta JC}\), interface resistance, and heat-sink thermal modeling.
- Texas Instruments — Linear Regulator Design Guide for LDOs — Provides the direct junction-to-case, case-to-sink, and sink-to-ambient heat-sink sizing relationship.
- Infineon — Dynamic Thermal Behavior of MOSFETs — Supports the distinction between steady-state thermal resistance and transient thermal impedance for pulsed operation.
For final hardware selection, use current component and heat-sink manufacturer data for the exact package, mounting method, interface, airflow, orientation, and operating environment.