Should one say 200 kelvins or 200 kelvin?

Please provide a source to the answer, as that would be much appreciated.

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    Related on Physics SE. – user178049 May 11 at 12:00
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    I wonder if there's a BrE/AmE divide here. I don't have a source to contradict the two current answers, but during my Physics degree here in the UK I pretty much only heard kelvin, without an s – Tristan May 12 at 9:11
  • I suspect people who say "two hundred kelvin" without an S are influenced by Celsius and Fahrenheit where you say "two hundred Celsius" etc (Celsius is considered a scale not a unit). The formal rule expressed in the answers here is to say "kelvins" just like you say "two hundred meters" or "two hundred volts", but people seldom follow rules exactly. Common usage appears to be "two hundred kelvin" even if ISO and NIST demand "kelvins". If you look on Merriam-Webster, none of the examples they give match the rules. merriam-webster.com/dictionary/kelvin – Stuart F May 12 at 9:57
  • @StuartF even working in physics, hearing the plural is rare (also we'd be more likely to say "Two hundred kay" than use "kelvin" in full) – Chris H May 12 at 12:13

You should say "two hundred kelvins", and write "200 kelvins" with a small 'k', or "200 K", with a capital 'K'.

The revised SI (November 2018) says

The kelvin, symbol K, is the SI unit of thermodynamic temperature; its magnitude is set by fixing the numerical value of the Boltzmann constant to be equal to exactly 1.380649 × 10-23...J K-1[joules per kelvin]

The US National Institute of Standards and Technology (NIST) page about this says:

The kelvin unit is not expressed in degrees like Celsius or Fahrenheit are. It is used by itself to describe temperature. For example, “mercury loses all electrical resistance at a temperature of 4.2 kelvins.”

Kelvin: Introduction

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    In German, you should be writing "200 K" (with a thin non-breaking space), not "200 K" (with a normal, breaking space). Is that not the case in English as well? (Note that the lexicographic rules of the International System of Units only talk about "space" without further specifying which kind of space.) – Jörg W Mittag May 12 at 4:03
  • I can't talk to the size of the space, but there is a space between numerals and units. There is no space between units (eg J/K) or between prefixes and units (eg mJ). There is also no space between numerals and the symbols for degree, minute, and seconds (of angle) bipm.org/documents/20126/41483022/SI-Brochure-9-EN.pdf – masher May 12 at 4:43
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    @JörgWMittag With my US English keyboard layout, I have no idea how I would choose between two different kinds of space. Perhaps academics and scientists know how to do it, and/or they use some kind of typesetting like LaTeX. IMHO that’s not an English language thing, it’s a science-specific thing. – Todd Wilcox May 12 at 6:02
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    @ToddWilcox ctrl+shift+space gives a non-breaking space in Word etc. – Chris H May 12 at 8:52
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    @StuartF - rules (or, perhaps, conventions) are often ignored, but it is probably sensible for a learner to adhere to them unless there is a very good reason. – Michael Harvey May 12 at 10:55

Technically, the unit is "a kelvin", and we pluralize them like any other unit - for example, the NIST uses kelvins ("mercury loses all electrical resistance at a temperature of 4.2 kelvins") - but it is still often written as "degrees Kelvin".

The nose temperature of both the pearlite and bainite transformations range from 920 to 980 and 570 to 610 degrees Kelvin for low carbon iron...

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    it may be often written as "degrees Kelvin", but as that NIST page clearly says, that is wrong usage: "The kelvin unit is not expressed in degrees like Celsius or Fahrenheit are." – ilkkachu May 11 at 20:17
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    I'm always suspicious of NIST pronouncements on SI - for a country that still uses inches they seem to have a lot to say about the rules of SI. Why not look to BIPM? bipm.org/en/history-si/kelvin – D Duck May 11 at 23:36
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    @DDuck My understanding is that the populace in the USA largely uses Imperial units, but the scientific community uses SI. Every recent scientific document I've read from the USA is 100% metric. NIST is a well-respected scientific organisation, although they would certainly spell it organization, which actually makes more sense. – RockPaperLz- Mask it or Casket May 12 at 0:38
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    Speaking of not trusting: the official NIST pronunciation guide for "giga" uses the soft "g" (or "j") at the start -> "jiga" (nist.gov/pml/weights-and-measures/metric-si-prefixes) – masher May 12 at 4:37
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    @masher So that's where Doc Brown got his pronunciation from! – Barmar May 12 at 15:05

As someone whose training is in condensed matter physics and who has attended a lot of talks, seminars, lectures, etc. in which people talk about temperature dependence, I've only heard "kelvin" without the s. I don't know what the official SI rule is, but "kelvin" is definitely the de facto standard.

Edit: I checked a reference I thought might write this out (An Introduction to Thermal Physics by Schroeder), and in the one place I could find it he uses "kelvins":

For this reason it's usually wise to convert temperatures to kelvins before plugging them into any formula.

I stand by my answer in that if you go around saying "kelvins" in spoken language, you may look a little silly.

  • Yup. I'm nowhere near as much into science as that but "kelvins" feels wrong and I think this question is the first time I've ever seen it. With all temperature scales you plural "degree(s)", not the actual unit. Same as you have 2 liters of water, not 2 liter of waters. – Loren Pechtel May 12 at 19:00
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    @LorenPechtel - as has been already said, a kelvin is a unit, like a metre or a gramme. – Michael Harvey May 12 at 19:19
  • Is it possible that people often say "kelvin", but write "kelvins"? Often people are more accurate and precise in writing. – RockPaperLz- Mask it or Casket May 13 at 14:01
  • @RockPaperLz-MaskitorCasket I'm not sure. I can only recall seeing the unit symbol K in writing, not "kelvin" or "kelvins" written out. – d_b May 13 at 21:57
  • That's a good point. It is usually just written as "K". I bet if it was spelled out (not that I would want that!), people would be more likely to say it "properly" when speaking. – RockPaperLz- Mask it or Casket May 13 at 21:59

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