Resistor Power Rating Calculator
Resistor Power Rating Calculator
The power a resistor really dissipates from its resistance and either the voltage across it or the current through it, the next standard wattage to buy after derating, what the manufacturer’s derating curve allows at your ambient temperature, and how hot the part itself gets.
Resistor power and rating
100 Ω with 12 V across it, a 50% derating, a 2 W part at 25 °C
Power, derating and temperature
rating to buy: P ÷ (derating ÷ 100), rounded up to a standard wattage
the manufacturer’s curve: Pallowed = Prated × (Tmax − Tamb) ÷ (Tmax − Trated), capped at Prated, with Trated = 70 °C and Tmax = 155 °C
hot spot: T = Tamb + P × Rth, and the curve itself implies Rth = (Tmax − Trated) ÷ Prated
- P rated
- the nameplate wattage, which is only valid at the stated ambient — 70 °C for most film parts
- R th
- thermal resistance from the resistor’s hot spot to the air around it, in kelvin per watt
- derating
- the fraction of the rating you allow yourself. 50% is the common rule; it is a margin, not a limit
Worked example
100 Ω with 12 V across it, a 50% derating, a 2 W part at 25 °C
Current = 12 ÷ 100 = 120 mA
P = 12² ÷ 100 = 1.44 W — the same as I²R and as V × I
At 50% derating the rating needed is 1.44 ÷ 0.5 = 2.88 W, so buy the next standard size: 3 W
The 2 W part you have is running at 72.0% of its nameplate — past the 50% you asked for
The derating curve at 25 °C still allows the full 2 W, because the curve is flat below 70 °C
That rating implies Rth = (155 − 70) ÷ 2 = 42.5 K/W, so the part sits about 61.2 K above ambient — roughly 86 °C. Hot enough to burn a finger, and hot enough to matter to whatever is next to it
Standard wattages, and what each one is really worth
| Rating | Implied thermal resistance | Allowed at 25 °C | Allowed at 85 °C | Allowed at 125 °C | At the 50% rule |
|---|---|---|---|---|---|
| 0.125 W | 680.0 K/W | 0.125 W | 0.103 W | 0.044 W | 0.0625 W |
| 0.25 W | 340.0 K/W | 0.250 W | 0.206 W | 0.088 W | 0.1250 W |
| 0.5 W | 170.0 K/W | 0.500 W | 0.412 W | 0.176 W | 0.2500 W |
| 1 W | 85.0 K/W | 1.000 W | 0.824 W | 0.353 W | 0.5000 W |
| 2 W | 42.5 K/W | 2.000 W | 1.647 W | 0.706 W | 1.0000 W |
| 3 W | 28.3 K/W | 3.000 W | 2.471 W | 1.059 W | 1.5000 W |
| 5 W | 17.0 K/W | 5.000 W | 4.118 W | 1.765 W | 2.5000 W |
| 10 W | 8.5 K/W | 10.000 W | 8.235 W | 3.529 W | 5.0000 W |
| 20 W | 4.3 K/W | 20.000 W | 16.471 W | 7.059 W | 10.0000 W |
| 25 W | 3.4 K/W | 25.000 W | 20.588 W | 8.824 W | 12.5000 W |
| 50 W | 1.7 K/W | 50.000 W | 41.176 W | 17.647 W | 25.0000 W |
What a wattage rating actually promises
The electrical part is trivial and exact: a resistor with V across it and I through it turns V × I watts into heat, which is also V² ÷ R and I² R. Every hard question on this page is thermal. A resistor’s wattage rating is not a property of the resistor alone — it is the power it may dissipate while its own hot spot stays below a stated maximum, in a stated ambient, mounted in a stated way. Change any of those and the number changes.
The derating curve is the real limit. Most film resistors are rated at a 70 °C ambient and derate linearly to zero power at 155 °C, the maximum film temperature. That single line does a lot of work. It says a 1/4 W part in a 70 °C box may have 0.25 W; in a 125 °C one, 0.088 W; at 155 °C, nothing at all. It also implies the part’s thermal resistance, because at rated power in a 70 °C ambient the hot spot must be exactly at 155 °C: Rth = (155 − 70) ÷ Prated, which is 340 K/W for a 1/4 W part and 42.5 K/W for a 2 W one. Small parts are not just weaker, they are thermally much worse, and that is the number to reach for when a datasheet gives no thermal figure. Use the real one when you have it: the mounting, the copper it is soldered to and the air moving past all change it, and a chip resistor on a generous copper pour can beat its datasheet comfortably.
The 50% rule of thumb. Never running a resistor past half its rating is a margin engineers impose on top of the curve, and it is the same thing as buying twice the wattage — which is exactly the convention this site’s LED series resistor calculator uses when it tells you what to order. Why bother, when the curve already has margin in it? Because the ambient next to the part is usually hotter than you think, the resistance drifts with temperature and with time under load, the noise and the voltage coefficient both get worse hot, and because a part running at 40% of rating lasts very much longer than one at 90%. Enter whatever margin you actually want above — this page applies your figure and the manufacturer’s curve separately and tells you which of the two is biting.
Pulses are a different question, and the datasheet answers it. A resistor survives brief overloads far beyond its continuous rating, because for short enough pulses the heat goes into the film’s own thermal mass rather than flowing out to the air. The generic specification, IEC 60115-1, has a short-time overload test that every part is qualified against: 2.5 times rated power for five seconds, with the resistance allowed to shift by only a couple of tenths of a per cent. Beyond that there is no general rule at all. The real pulse capability is a curve in the datasheet, plotted as permissible pulse power against pulse duration, and it varies enormously between constructions — a thin-film chip is poor, a thick-film pulse-proof part is far better, a bulk metal or wirewound part better still, and a laser-trimmed spiral in a high-value thick film can fail on a single spike that leaves the average power a tiny fraction of the rating. If you have an inrush, a discharge or a surge, do not compute an average and call it safe; find the pulse curve for the exact part. Vishay’s Pulse Load Capabilities of Film Resistors note is the usual starting point.
Voltage, not just power. Every part also has a maximum working voltage, and on high-value small parts it bites long before the wattage does — 75 V on an 0603, 200 V on a 1206. A 10 MΩ 0603 at 75 V is dissipating 0.6 mW against a 0.125 W rating and is nevertheless at its limit. For the power in a network rather than one part, the series and parallel calculator flags the hottest resistor, and the current divider gives the share each branch of a parallel group carries. Above a few watts the problem stops being the resistor and starts being where the heat goes: the heatsink calculator is the next step.
Frequently asked questions
How do I calculate the power a resistor dissipates?
P = V² ÷ R if you know the voltage across it, P = I² R if you know the current through it, or simply P = V × I. All three are the same number. 12 V across 100 Ω is 1.44 W.
What wattage resistor should I use?
Take the dissipated power, divide by the fraction of the rating you are willing to use — 50% is the usual rule, which means doubling the power — and round up to a standard size: 1/8, 1/4, 1/2, 1, 2, 3, 5 W and upwards. 1.44 W at 50% needs 2.88 W, so fit a 3 W part.
What does derating a resistor mean?
Two different things, and it is worth keeping them apart. The manufacturer’s derating curve is a limit: the rating holds to about 70 °C ambient and falls linearly to zero at the maximum film temperature, around 155 °C. The 50% rule is a margin you choose on top of that, for drift, ageing and the ambient being hotter than you assumed.
How hot does a resistor get?
Ambient plus P × R_th. If the datasheet gives no thermal resistance, the derating curve implies one: (155 − 70) ÷ rating, which is 340 K/W for a 1/4 W part. At its full rating any resistor sits at its maximum film temperature, which is why parts at their rating are always too hot to touch.
Can a resistor take more than its rated power for a short time?
Yes, by a lot — but how much is a property of the specific part, not a rule. IEC 60115-1 qualifies parts with a short-time overload of 2.5 times rated power for five seconds. Anything beyond that needs the pulse-power curve from the datasheet, which differs by more than an order of magnitude between thin-film, thick-film, pulse-proof and wirewound constructions.
Why did my resistor change value instead of burning out?
That is the normal failure mode for a film resistor that has been run too hot: the film oxidises and the value climbs, often permanently, long before anything goes open circuit. A resistor that measures 10% high and looks discoloured has been telling you about a thermal problem for a while.
Related calculators
References
- Vishay. D/CRCW e3 Standard Thick Film Chip Resistors, datasheet document 20035. Power ratings per case size at 70 °C (0402 0.10 W, 0603 0.125 W, 0805 and 1206 0.25 W, 2512 1 W), a 155 °C maximum film temperature with the rating derated linearly to zero there, maximum working voltages of 75 V (0603) to 500 V (2512), and the short-time overload condition U = √(2.5 × P70 × R) for 5 s — which is 2.5 times rated power. Qualified to EN 60115-1 and EN 60115-8.
- IEC 60115-1. Fixed resistors for use in electronic equipment — Part 1: Generic specification. Defines rated dissipation at a stated ambient, the derating curve, and the short-time overload test. Vishay’s General Information for Fixed Film Resistors (document 20103) states rated dissipation as the “maximum load at a well-defined ambient temperature, e.g. 70 °C” and names IEC 60115-1 for the voltage and insulation tests; the standard itself is paywalled and was not read directly.
- Vishay. Pulse Load Capabilities of Film Resistors, application note document 50060. The pulse-versus-continuous distinction, and why permissible pulse power has to be read from a curve for the specific construction rather than scaled from the continuous rating.