Component Derating Calculator

Derating is running a part below its rating so that it survives. The factors live in documents this page will not reproduce, so you type in your own — and what you get back is the arithmetic a single-factor calculation misses: the manufacturer’s own temperature curve has already taken part of the rating away before your factor is applied, and the two compound.

Last updated 24 Sep 2026

Applied stress against the derated limit, with the curve applied

Rating + your factor + the curve → margin

Part typeThis selects the advice, not the arithmetic — the arithmetic is the same for every part. It decides which stress usually binds, which document clause to look the factor up in, and which interactions the notes below warn about. No derating factor is supplied for any of them: that comes from your own programme.

Which stress are you derating?Most parts have several and each has its own factor. A MOSFET has a drain-source voltage, a drain current, a dissipation and a junction temperature and all four are derated separately; run the page once per stress and the binding one is whichever comes out worst.

The part’s rating in that quantityVolts, amperes or watts, at the manufacturer’s stated reference conditions — which for a power rating is almost always a stated ambient or case temperature, not room temperature. 0.25 W is a 1206 thick-film chip resistor. Use the figures from your part’s datasheet; typical values vary widely between manufacturers.

Applied stress, in the same unitsThe worst case your circuit actually produces, including tolerance, transients and the end of life. Not the nominal.

Derating factor from YOUR standard (% of rating)This page does not supply it. ECSS-Q-ST-30-11C Rev.2 puts the load ratios in clause 6, one table per component family — 6-1 to 6-9 for capacitors, 6-25 to 6-31 for resistors, 6-13 to 6-15 for diodes, 6-35 to 6-38 for transistors, 6-24 for relays, 6-10 for connectors. NASA EEE-INST-002 and MIL-STD-1547 hold their own. All are copyrighted or controlled and none is reproduced here. The 60% default is the generic figure NASA GSFC’s Preferred Reliability Practice PD-ED-1201 Table 1 gives for resistors — a US Government work that may be reproduced — and it is an EXAMPLE to replace with the figure your programme requires.

Apply the manufacturer’s temperature curve as well?This is the whole point of the page. A rating quoted at 70 °C is not available at 85 °C, and your programme’s factor is applied on top of what the curve leaves, not instead of it.

Knee temperature: the curve is flat below this (°C)The temperature up to which the full rating applies. 70 °C for most film and thick-film resistors, 85 °C for a great many capacitors, 25 °C for some wirewound and power parts. Use the figures from your part’s datasheet; typical values vary widely between manufacturers.

Temperature at the end of the curve (°C)Where the straight line stops. 155 °C is the maximum film temperature for a common thick-film resistor, at which the allowed power is zero; 125 °C is the usual end for a solid tantalum, at which a fraction of the rated voltage remains. Use the figures from your part’s datasheet; typical values vary widely between manufacturers.

What is left at that temperature (% of rating)Zero for a resistor’s power rating — the line runs to the maximum film temperature and stops. Not zero for a voltage rating: Vishay’s technical note on MnO₂ tantalum derating describes the maximum working voltage as equal to the rated voltage from −55 °C to +85 °C and then falling linearly to two thirds of it at 125 °C, so that case is 66.7 here.

Ambient or reference temperature at the part (°C)Not the room. The air, or the mounting surface, next to this part with everything else running, on the hottest case the box will see. In vacuum there is no air at all and the reference is whatever the part conducts to.

Thermal resistance to that reference (K/W), 0 = derive itOnly used when the stress is power. Leave at 0 and the page derives it from the curve itself: at the full rating in the knee-temperature ambient the hot spot must be exactly at the end temperature, so R_th = (T_end − T_knee) ÷ rating. For a 0.25 W part on a 70/155 curve that is 340 K/W. A measured figure, or one from the data sheet, is better.

Hot-spot or junction temperature you allow (°C)Your own limit, not the part’s absolute maximum. PD-ED-1201 Table 1 — the NASA GSFC practice, a US Government work — caps semiconductor junctions at 110 °C and microcircuit junctions at 100 °C and says those must not be exceeded at any time in any ground, test or flight exposure; for a film resistor the physical cap is the end of its own power curve, 155 °C for a common thick-film part, and the 125 °C default here is a margin below that. Your own programme’s figure replaces it.

Whichever part type you selected, in the simplest circuit that puts the stress on it. The part turns amber as the applied stress passes four fifths of the derated limit and red past it. Everything is written as a percentage of the nameplate rating, because the stress can be a voltage, a current or a power and the arithmetic is the same for all three — and because the comparison the page exists for is between the third line and the second: a single-factor calculation allows what your factor times the nameplate gives, and the real allowance is smaller because the manufacturer's temperature curve has already taken part of the rating away.

64.8%Example

a 0.25 W chip resistor whose curve is flat to 70 °C and runs to zero at 155 °C, sitting in an 85 °C box, dissipating 80 mW, with a 60% derating factor typed in from the reader’s own document

Two factors, and they multiply

frac(T) = 1 for T ≤ T_knee; 1 − (1 − k_end)·(T − T_knee) ÷ (T_end − T_knee) between; k_end above
allowance(T) = rating × frac(T)
derated(T) = rating × frac(T) × D
and a single-factor calculation returns rating × D, which is larger by 1 ÷ frac(T)

D: your programme’s derating factor, from your own copy of your own standard. This page will not supply it
frac(T): the straight line on the data sheet: full rating to a knee temperature, then falling to an end value. For a resistor’s power that end value is zero; for a tantalum’s voltage it is a fraction of the rating
R_th: (T_end − T_knee) ÷ rating when it is derived from the curve, because at full rating in the knee ambient the hot spot is by definition at the end temperature

Worked example

a 0.25 W chip resistor whose curve is flat to 70 °C and runs to zero at 155 °C, sitting in an 85 °C box, dissipating 80 mW, with a 60% derating factor typed in from the reader's own document
At 85 °C the curve has already fallen to (155 − 85) ÷ (155 − 70) = 82.4% of the rating, so the manufacturer's own allowance there is 205.9 mW rather than 250 mW
The 60% factor applies to that, not to the nameplate: 205.9 mW × 0.60 = 123.5 mW
A single multiplication would have given 250 mW × 0.60 = 150 mW — 1.214× larger than the allowance that actually exists, and the whole of that gap is the temperature the reader is running at
So 80 mW is 64.8% of the real limit, where the single-factor answer would have said 53.3%. Headroom is 1.54×, and the ambient at which this dissipation uses the whole derated allowance is 109.7 °C
Separately, the curve implies a thermal resistance of (155 − 70) ÷ 0.25 = 340 K/W, so 80 mW lifts the film 27.2 °C above its surroundings to 112.2 °C, which is 12.8 °C below the 125 °C limit entered. A part can be inside its derated power and still too hot, and the two are separate findings

Where the factor comes from, and where it does not

The distinction that matters is between a document that may be quoted and one that may only be cited. PD-ED-1201 is a NASA Preferred Reliability Practice and a US Government work, so its Table 1 is reproduced above in full. The derating standards a flight programme actually imposes are not, and a page that invented their numbers would be worse than no page.
Document	Status	What it gives	How this page treats it
ECSS-Q-ST-30-11C Rev.2, 23 June 2021	European Cooperation for Space Standardization; downloadable free, copyrighted	clause 5 sets out the principles and clause 6 holds the load ratios, one table per family — 6-1 to 6-9 capacitors, 6-13 to 6-15 diodes, 6-18 inductors and transformers, 6-24 relays and switches, 6-25 to 6-31 resistors, 6-35 to 6-38 transistors	cited by clause and table number; no value reproduced
NASA EEE-INST-002	NASA Goddard instruction, controlled	part selection, screening, qualification and derating for three quality levels	cited; no value reproduced
MIL-STD-1547	US military standard for space and launch vehicles	parts, materials and processes, with derating requirements	cited; no value reproduced
NASA GSFC PD-ED-1201, Preferred Reliability Practices	US Government work, may be reproduced	a single generic Table 1 — 60% of rated voltage for capacitors, 60% of rated power for resistors, 50% of rated power and 75% of rated voltage for semiconductors with a 110 °C junction cap, 80% of supply voltage and 75% of rated power for microcircuits with a 100 °C junction cap, 50% of rated voltage for inductive devices, 50% of rated current for relays and connectors	quoted, because it may be — and it is the source of this page’s default. It is a generic starting point, not your programme’s requirement
The part’s own data sheet	the manufacturer’s	the temperature curve: the knee, the end temperature and what is left there	taken as three reader inputs, because no two families share a curve

Two factors that multiply, and the one that gets forgotten

Derating is the discipline of running a part below its rating so that it survives — longer, and through excursions the nominal design never sees. It is the single cheapest reliability measure there is, and on a spacecraft or a high-reliability programme it is not optional: the derating requirement is a contractual one, verified part by part in a review.

Why this page has no table in it. The factors live in ECSS-Q-ST-30-11C, in NASA EEE-INST-002 and in MIL-STD-1547, and those documents are copyrighted or controlled. They are also not interchangeable: they differ by part family, by quality level, by whether the application is benign or severe, and by programme. Reproducing a plausible-looking table here would give a reader a number that is wrong for their project and looks authoritative, which is worse than giving them nothing. So the page cites the clauses — ECSS-Q-ST-30-11C Rev.2 puts its principles in clause 5 and its load ratios in clause 6, one table per family — and asks you to type in the figure from your own copy. The one number it offers as a default comes from NASA Goddard’s Preferred Reliability Practice PD-ED-1201, which is a US Government work and may be quoted; its Table 1 is reproduced in full above, and it is a generic starting point rather than anybody’s requirement.

The arithmetic those documents leave to you. A derating requirement is a fraction of a rating, and a rating is a number stated at a condition. Almost every power, current and voltage rating is stated at a reference temperature, and the data sheet then draws a straight line: full rating up to a knee, falling to an end value at the maximum temperature. For a thick-film resistor that line runs from 70 °C to zero power at 155 °C, the maximum film temperature. For a solid tantalum’s voltage it runs from 85 °C to a fraction of the rating at 125 °C. Your factor applies to what the curve leaves, not to the nameplate, and the two compound. A 0.25 W resistor derated to 60% is not allowed 0.15 W in an 85 °C box; the curve has already taken it to 0.206 W, and 60% of that is 0.124 W. The single multiplication overstates the allowance by 1 ÷ frac(T), which at 85 °C on a 70/155 curve is 1.214 times. Run hotter and the gap widens fast.

The curve also tells you the thermal resistance, for free. At the full rating in the knee-temperature ambient the hot spot must by definition be at the end temperature, so R_th = (T_end − T_knee) ÷ rating. That is 340 K/W for a 0.25 W part and 42.5 K/W for a 2 W one — small parts are not merely weaker, they are thermally much worse. It is the number to reach for when a data sheet gives no thermal figure, and the page uses it unless you type a better one in. For a resistor specifically, the resistor power rating calculator does the same curve with the electrical side worked out for you; this page is the general case and the one to use when the stress is a voltage or a current rather than a power, or when the part is not a resistor.

What derating does not do. It does not turn a wear-out mechanism into a stress ratio. An aluminium electrolytic dies because its electrolyte dries out at a rate set by temperature, and no voltage derating changes that — see the ripple current and ESR calculator. A Class II ceramic loses most of its capacitance under DC bias, and derating the voltage changes where on that curve you sit rather than removing it — see the MLCC DC bias calculator. A solid tantalum’s voltage derating is about surge and ignition rather than about lifetime, and the circuit impedance decides how much is enough — see the tantalum derating calculator. Derating is necessary and it is not sufficient, and the failure modes it does not touch are the ones that catch people.

Frequently asked questions

Why does this calculator not just tell me the derating factor?

Because the factors are in ECSS-Q-ST-30-11C, NASA EEE-INST-002 and MIL-STD-1547, which are copyrighted or controlled and are not interchangeable. They differ by part family, by quality level and by programme, and a plausible-looking invented table would be worse than none. The page cites the clauses and does the arithmetic those documents leave to you. The one default it offers comes from NASA GSFC’s PD-ED-1201, which is a US Government work, and it is a generic figure to replace rather than to rely on.

Do I apply my derating factor before or after the temperature curve?

After — the factor applies to what the manufacturer’s curve leaves at your temperature. The two compound, and the page shows what the single multiplication overstates the allowance by. Some programmes instead take the smaller of the two limits rather than the product; the page reports that reading as well, so you can see which your document means.

My rating is quoted at 25 °C and my part runs at 85 °C. What do I use?

Set the knee to the temperature the rating is quoted at, the end temperature and end value to whatever the data sheet’s curve shows, and the reference temperature to the temperature at the part. The page then gives the allowance that actually exists there. A rating taken off the front page of a data sheet and used at 85 °C without the curve is the commonest single mistake in this whole subject.

What temperature should I type in — the ambient or the case?

Whatever the curve is drawn against, which the data sheet states, and measured where the curve says. For most small parts it is the ambient immediately around the part with everything else in the box running, not the room. In vacuum there is no ambient air at all: the reference is whatever the part conducts into, which is what the conduction-cooled electronics page is for.

Does derating a part make it last longer, or just make it safer?

Both, and they are different arguments. The safety argument is margin against an excursion the design did not anticipate. The lifetime argument is that most failure rates fall steeply with stress ratio — which is what PD-ED-1201’s Figure 1 shows. But derating does not touch a wear-out mechanism that is driven by something else: an electrolytic’s drying, a ceramic’s ageing, a solder joint’s thermal cycling.

Can I derate one stress and ignore the others?

No, and this is where parts get through review and fail in the field. A MOSFET has a drain-source voltage, a drain current, a dissipation and a junction temperature, and each is derated separately; the binding one is whichever comes out worst and it is often not the one the schematic draws attention to. Run the page once per stress.

Related calculators

References

NASA Goddard Space Flight Center. Preferred Reliability Practices: EEE Parts Derating, Practice No. PD-ED-1201. A US Government work, freely distributed and reproducible. Table 1 gives generic derating guidelines — 60% of rated voltage for capacitors, 60% of rated power for resistors, 50% of rated power and 75% of rated voltage for semiconductor devices with a 110 °C junction cap, 80% of rated supply voltage and 75% of rated power for microcircuits with a 100 °C junction cap, 50% of rated voltage for inductive devices, 50% of rated current for relays and connectors — and Figure 1 shows piece-part failure rate against temperature for stress ratios from 1.0 down to 0.6. Verified against two independently hosted copies of the practice, which agree line for line. This is the source of this page’s default factor and junction-temperature limit.
ECSS-Q-ST-30-11C Rev.2, 23 June 2021, Space product assurance — Derating — EEE components. Clause 5 sets out the principles of derating (5.2), applicability and component selection (5.3), the derating parameters and the requirements for transient and surge conditions (5.4). Clause 6 holds the tables of load ratios, one per family: 6-1 to 6-9 capacitors, 6-10 to 6-11 connectors, 6-13 to 6-15 diodes, 6-18 inductors and transformers, 6-24 relays and switches, 6-25 to 6-31 resistors, 6-35 to 6-38 transistors, 6-41 and 6-42 harness bundle derating. Clause structure verified against the published document; the values are copyrighted and are not reproduced here.
NASA. EEE-INST-002: Instructions for EEE Parts Selection, Screening, Qualification, and Derating. The NASA derating instruction referenced by many US civil space programmes, with derating requirements by part family and quality level. Cited; not reproduced.
MIL-STD-1547, Electronic Parts, Materials, and Processes for Space and Launch Vehicles. The US military standard whose derating requirements sit alongside the above on defence space programmes. Cited; not reproduced.
Vishay. Solid Tantalum Capacitors (With MnO₂ Electrolyte) Voltage Derating, document 40246, revision 07 September 2022. The source of the shape of a voltage derating curve used as this page’s non-zero end-value example: the maximum working voltage equals the rated voltage from −55 °C to +85 °C and then falls linearly to two thirds of it at the 125 °C maximum working temperature.
JEDEC. JESD51-12: Guidelines for Reporting and Using Electronic Package Thermal Information. The reason a thermal resistance has to be qualified by what it was measured against; the junction-to-ambient figure on a data sheet’s front page belongs to a standard test board. Cited by number; copyrighted and not reproduced.