Thermal Derating Calculator
Calculate generic datasheet thermal derating for power, current, load or another rated quantity. Enter the rated value, reference temperature, slope, zero-rating endpoint, or two datasheet points to estimate allowable rating at operating temperature.
THM-004 does not invent a component derating curve. Use manufacturer datasheets or qualified engineering data, and keep ambient, case, board and junction temperature bases separate.
Engineering tool
Thermal Derating Calculator
Calculate generic datasheet linear thermal derating for power, current, load, or another rated quantity.
Label for the generic rated quantity.
Display unit for rating values, such as W, A, %, or load units.
Used as a label only. This calculator does not convert ambient, case, board, and junction temperatures.
Absolute temperature unit for all temperature fields.
Rated power at or below the reference temperature.
Temperature where derating begins.
Temperature at which the derated rating is evaluated.
Absolute slope in W per temperature interval.
Result console
- Allowable Rating
- 4W
- Remaining Rating
- 40%
- Derating Amount
- 6W
- Raw Mathematical Rating
- 4W
- Temperature Above Reference
- 30°C
- Status
- Within modeled derated rating
- Zero-Rating Temperature
- 120°C
The allowable rating follows the entered linear datasheet derating model.
Formula
Formula reference
Linear thermal derating formulas
The calculator assumes full rating at or below the reference temperature and clamps negative user-facing allowable ratings to zero.
Xallow = Xrated for T <= TrefXallow = Xrated - D(T - Tref) for T > TrefPercentage: Xallow = Xrated × [1 - (d/100)(T - Tref)]Two point: D = (X1 - X2) / (T2 - T1)Zero endpoint: D = Xrated / (Tzero - Tref)Tzero = Tref + Xrated / DTmax = Tref + (Xrated - Xrequired) / DMargin = Xallow - XappliedUtilization = Xapplied / Xallow × 100%Variable definitions
- Xrated
- rated value at or below reference temperature
- Xallow
- allowable derated value
- D
- absolute derating slope
- d
- percentage-point derating slope
- Tref
- derating reference temperature
- Tzero
- zero-rating endpoint
Thermal Derating Formula Audit
| Linear Derating Formula | Xallow = Xrated - D(T - Tref) for T > Tref. |
|---|---|
| Percentage Derating Formula | Xallow = Xrated × [1 - (d/100)(T - Tref)]. |
| Percentage Semantics | d is percentage points of original rated value per degree. |
| Two-Point Slope Formula | D = (X1 - X2) / (T2 - T1). |
| Zero-Rating Slope Formula | D = Xrated / (Tzero - Tref). |
| Zero-Rating Temperature Formula | Tzero = Tref + Xrated / D for D > 0. |
| Maximum Operating Temperature Formula | Tmax = Tref + (Xrated - Xrequired) / D. |
| Rating Margin Formula | Margin = Xallow - Xapplied. |
| Utilization Formula | Utilization = Xapplied / Xallow × 100% when Xallow > 0. |
| Below-Reference Policy | Full rating is assumed at or below Tref; rating is not increased. |
| At-Reference Policy | Xallow = Xrated. |
| Zero-Rating Clamp Policy | User-facing allowable rating is clamped to zero. |
| Above-Zero-Rating Policy | Raw negative value is retained for status, not displayed as valid allowance. |
| Zero-Slope Policy | Forward rating remains constant; inverse temperature has no finite derating-derived limit. |
| Negative-Slope Policy | Rejected because V1 models normal thermal derating only. |
| Temperature Basis Policy | Ambient, case, board and junction are labels only; no automatic conversion between them. |
| Absolute Temperature Convention | Absolute temperatures are converted to °C internally. |
| Temperature Interval Convention | Slopes use temperature intervals, not absolute temperature conversion. |
| °C/K Slope Equivalence | unit/°C and unit/K are numerically equal. |
| °F Slope Convention | per-°F slopes are converted using interval scale. |
| Linear-Model Boundary | Nonlinear or piecewise curves must be handled from datasheet segments. |
| Datasheet Authority Boundary | The calculator does not invent derating curves. |
| THM-001 Scope Separation | THM-004 does not calculate ΔT = P × Rθ. |
| THM-002 Scope Separation | THM-004 does not size heatsinks. |
| THM-003 Scope Separation | THM-004 does not combine thermal resistance networks. |
Worked Examples
| Input | Calculation | Result |
|---|---|---|
| 10 W, Tref 70°C, D 0.2 W/°C, T 100°C | ΔT = 30°C, derating = 6 W | 4 W, 40% remaining |
| Same at 70°C | T = Tref | 10 W |
| Same at 50°C | Below Tref | 10 W, not 14 W |
| Same at 120°C | 10 - 0.2(50) | 0 W |
| Same at 130°C | raw = -2 W | displayed 0 W, beyond zero rating |
| 50 W, Tref 50°C, d 1%/°C, T 80°C | 30 percentage points lost | 35 W, 70% remaining |
| Same at 50°C | no excess temperature | 50 W |
| Same at 150°C | 100 percentage points lost | 0 W |
| 70°C → 10 W, 120°C → 0 W | D = 10 / 50 | 0.2 W/°C |
| Same two-point model at 100°C | 10 - 0.2(30) | 4 W |
| 10 W, Tref 70°C, Tzero 120°C | D = 10 / 50 | 0.2 W/°C |
| Same at 95°C | 10 × (120 - 95)/(120 - 70) | 5 W |
| 10 W, Tref 70°C, D 0.2, required 6 W | Tmax = 70 + (10 - 6)/0.2 | 90°C |
| Same, required 10 W | Tmax = Tref | 70°C |
| Required 11 W | required > rated | rejected / invalid requirement |
| Slope = 0, T > Tref | no derating | rated value remains |
| Slope = 0 inverse | division by zero avoided | N/A |
| Allowable 6 W, applied 5 W | margin = 1 W | pass |
| Allowable 6 W, applied 6 W | margin = 0 W | at limit |
| Allowable 6 W, applied 7 W | margin = -1 W | fail |
| Allowable 5 W, applied 4 W | 4 / 5 × 100 | 80% utilization |
| Allowable 0, applied > 0 | division by zero avoided | N/A utilization |
| T2 <= T1 | invalid point order | rejected |
| X2 > X1 | increasing rating | rejected |
| Negative slope | outside derating model | rejected |
| Rated value <= 0 | invalid nameplate rating | rejected |
| Temperature below 0 K | absolute temperature invalid | rejected |
| 1 unit/°C and 1 unit/K | interval scales match | same result |
| 0.18 W/°F | 0.18 × 1.8 | 0.324 W/°C |
| 1%/°C | percentage points, not 0.99^ΔT | linear remaining percent |
Engineering Notes
Thermal Derating
Thermal derating reduces an allowable rating as temperature increases according to a specified model.
Rated Value
The rated value is the datasheet rating at or below the derating start temperature.
Reference Temperature
The reference or derating-start temperature is where the modeled reduction begins.
Derating Slope
The absolute slope is rating units lost per temperature interval.
Percentage Derating
Percentage slope is interpreted as percentage points of original rating per degree.
Zero-Rating Temperature
Tzero is the datasheet endpoint where the modeled rating reaches zero, not automatically Tjmax.
Two-Point Derating
Two datasheet points can define a slope when the curve is linear between them.
Temperature Basis
Ambient, case, board and junction temperature are different quantities.
Linear Derating Model
Use this only where source data specifies or supports a linear relationship.
Datasheet Derating Curve
The calculator does not infer derating data; users must enter values from qualified sources.
Absolute Maximum Rating
Absolute maximum limits and recommended derated operating limits are not the same.
Operating Margin
Applied-load margin keeps its sign so failures remain visible.
Common Mistakes
- Implementing 1%/°C as exponential decay.
- Increasing the rating below the reference temperature.
- Treating Tzero as maximum junction temperature.
- Mixing ambient, case, board and junction temperature bases.
- Using the wrong datasheet temperature basis.
- Displaying negative allowable power as a valid rating.
- Taking the absolute value of rating margin.
- Displaying Infinity utilization when allowable rating is zero.
- Returning Infinity for inverse maximum temperature when slope is zero.
- Using absolute Fahrenheit conversion for slope.
- Treating °C and K absolute temperatures as numerically identical.
- Forcing one straight line onto a nonlinear datasheet curve.
- Treating derated rating as guaranteed lifetime.
- Treating absolute maximum rating as recommended operating rating.
Support reference
FAQ
What is thermal derating?
Thermal derating reduces an allowable rating as temperature increases according to a datasheet or engineering specification.
How do I calculate power derating with temperature?
Use Xallow = Xrated - D(T - Tref) above the reference temperature, then clamp negative allowable values to zero.
What is a derating slope?
A derating slope is the amount of rating lost per degree above a reference temperature, such as watts per °C.
How do I calculate allowable power at a higher temperature?
Subtract slope times temperature rise above the derating reference temperature from the rated value.
What does %/°C derating mean?
%/°C is interpreted as percentage points of the original rated value lost per degree.
Is 1%/°C an exponential reduction?
No. THM-004 treats 1%/°C as one percentage point of original rating per °C, not Xrated × 0.99^ΔT.
How do I calculate a derating slope from two datasheet points?
Use D = (X1 - X2) / (T2 - T1), where T2 must be greater than T1 and X2 must not exceed X1.
How do I calculate zero-rating temperature?
For a positive absolute slope, Tzero = Tref + Xrated / D.
How do I calculate maximum operating temperature for a required load?
Use Tmax = Tref + (Xrated - Xrequired) / D. If required value exceeds rated value, the requirement is invalid under this model.
What happens below the derating reference temperature?
This calculator assumes full rated value at or below the reference temperature and does not increase rating below Tref.
What happens above the zero-rating temperature?
The raw linear result may be negative, but the user-facing allowable rating is clamped to zero.
Can allowable rating become negative?
The raw mathematical value can become negative, but the valid engineering allowable rating is shown as zero.
What is the difference between ambient, case and junction temperature derating?
They are different temperature bases. Use the same basis specified by the datasheet curve.
Can I use a junction-temperature curve with ambient temperature?
Not directly. THM-004 does not convert Ta, Tc, board temperature and Tj. Use the datasheet-defined basis.
Is °C-based derating the same as K-based derating?
For temperature intervals, °C and K slopes are numerically equivalent.
How does °F affect derating slope?
A per-°F slope must be converted by interval scale. 0.18 W/°F equals 0.324 W/°C.
Is thermal derating the same as thermal resistance?
No. Thermal resistance predicts temperature rise from heat flow. Derating interprets an allowable rating versus temperature curve.
Does derating guarantee component lifetime?
No. Derating improves operating margin but does not guarantee lifetime, MTBF, or reliability.
What is the difference between derating and absolute maximum rating?
Absolute maximum ratings are stress limits. Recommended derated operating limits are normally more conservative application limits.
Where should I get the derating parameters?
Use the manufacturer datasheet, application note, qualification data, or an approved engineering specification.
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