Heatsink Thermal Resistance Calculator
Highest heatsink thermal resistance that keeps a device junction within its limit. With the formula, a worked example and typical values.
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Estimate only. Not engineering or safety advice. Check the result against the applicable standard and the equipment data, and have a competent engineer review it before it is relied on. Terms of use.
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About this calculator
This page calculates total allowed θja and heatsink to ambient θsa, at most from power dissipated, ambient temperature, maximum junction temperature, junction to case θjc and 1 more input. Change any value or its unit and the result updates. The steps below show the working for the default values, so you can check each line against your own method.
Limits of this calculator
Leave margin below the junction limit. Heatsink data sheets give θsa for natural convection or a stated airflow, so match that condition.
Cross-check: 6 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review.
Verification status: Indicative
Indicative. Estimation only; further engineering checks are required. Basis: Steady-state thermal estimate.
What this calculator checks
- The arithmetic of the simplified model for the values you entered.
What remains unverified
- Whether the simplifying assumptions listed on this page hold for your case.
- Any standard, manufacturer or site data. Treat the result as an estimate and confirm it by a further engineering check.
Verification register entry
| Calculation author | Rahul Sharma (author) |
|---|---|
| Technical review | No independent technical review has been carried out |
| Individual professional approval | Not recorded |
| Formula reference | not recorded |
| Standard and edition | Not applicable: The thermal-resistance network is a general heat-transfer calculation. Heatsink and semiconductor limits are manufacturer-specific; no single standard governs this simplified equation. |
| Standard review state | Not applicable |
| Scope-screening note | not recorded |
| Cross-check test cases | 6 reference vectors (tests/reference_vectors.json) |
| Review date | not recorded |
| Next review date | not recorded |
| Status | Indicative |
Needed before this calculator can be marked Verified: reviewer name, qualification and review date; exact formula reference; formula source (textbook or standard); next review date.
Formula
Worked example
| Input | Value |
|---|---|
| Power dissipated | 15 W |
| Ambient temperature (°C) | 40 |
| Maximum junction temperature (°C) | 125 |
| Junction to case θjc (°C/W) | 1.5 |
| Case to sink θcs (°C/W) | 0.5 |
- θja(max) = (125 − 40) / 15 = 5.667 °C/W
- θsa(max) = 5.667 − 1.5 − 0.5 = 3.667 °C/W
Assumptions
- Steady-state dissipation
- Single heat path from junction through case and sink to ambient
Leave margin below the junction limit. Heatsink data sheets give θsa for natural convection or a stated airflow, so match that condition.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| Power dissipated | W | greater than 0 W | |
| Ambient temperature (°C) | (number) | -50 or more, up to 150 | |
| Maximum junction temperature (°C) | (number) | 0 or more, up to 300 | |
| Junction to case θjc (°C/W) | (number) | 0 or more | From the device data sheet |
| Case to sink θcs (°C/W) | (number) | 0 or more | Depends on the pad or paste; 0.1 to 1 is common |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| Total allowed θja | (number) | Shown to 2 decimal places | |
| Heatsink to ambient θsa, at most (°C/W) | (number) | Shown to 2 decimal places |
How it is calculated
Each input is converted to the base unit of its quantity, the formula above is applied in the order shown in the worked example, and each result is shown in the unit you choose. Results are held at full precision and rounded only for display, except where a step selects a standard size, as the worked example shows. A value outside its allowed range is refused with a message instead of giving a result.
How the result changes with power dissipated
| Power dissipated (W) | Total allowed θja | Heatsink to ambient θsa, at most |
|---|---|---|
| 1.5 | 56.67 | 54.67 |
| 3.75 | 22.67 | 20.67 |
| 7.5 | 11.33 | 9.33 |
| 15 | 5.67 | 3.67 |
| 30 | 2.83 | 0.83 |
| 60 | 1.42 | -0.58 |
| 150 | 0.57 | -1.43 |
Other inputs are the same as in the worked example above.
Standards and further reading
These are textbook circuit relationships. The component data sheet sets the real limits, tolerances and ratings, and takes priority over the calculated value.
Frequently asked questions
What do I need to enter?
Enter power dissipated, ambient temperature (°C), maximum junction temperature (°C), junction to case θjc (°C/W) and case to sink θcs (°C/W). Change the unit of a value with its selector and the result stays the same.
What does this calculator assume?
Steady-state dissipation. Single heat path from junction through case and sink to ambient.
Which calculators should I use next?
Common next steps are Power from Voltage and Current. The related links at the bottom of this page open them.
How accurate are the results?
Every figure comes from the same tested engine as the Elecmetric calculators and is meant for estimating. 6 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review. Check design values against the standard and the equipment data you work to.
Formula guides and reference
Longer explanations with worked examples and tables, written for the same topic.
- Heat Sink Formulas: Junction Temperature: Heat sink formulas: junction temperature from thermal resistances
- Thermal Time Constant Formulas: Heating and cooling curve formulas for a cable, motor or transformer: the temperature rise after a time
- MOSFET Loss Formulas: Conduction and Switching: MOSFET power loss formulas for a switching converter: conduction loss from Rds(on)
More Electronics and Instrumentation calculatorsAll engineering calculatorsHeatsink Thermal Resistance in Elecmetric