Which one is the correct way of writing:

  1. The sugar concentration was 20 g/mL or
  2. The sugar concentration was 20 g/ mL

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  • 1
    There's no space. The slash (oblique symbol) represents "per" as also division. The question of discretion, if any would only be about space between 20 and g/ml as some style recommendations say a space may be acceptable to avoid ambiguity in some instances (between 1 --one, and l --ell, for instance. ) – Kris Jan 3 '15 at 14:59

g/mL or g/ml

The International Standard allows L as an alternative to the canonical l and different national bodies and style guides differ as to whether they recommend, allow, prohibit or require the capital L, so if you have a style-guide you are writing to then use it, otherwise use L with American English (to match NIST guidelines) and l otherwise.

Space within a unit like that means multiplication (as does the clearer mid-dot ·). 20 g/ml being the same as 20 kg/l is hence the same as 20 kg l-1 or 20 kg·l-1 but a space after a solidus makes no sense.

(You might for typographical reasons use a very thin space like a hair-space in the final lay-out if it aids appearance, but such spaces are so thin that they don't look like spaces so much as like slightly different kerning).

  • 1
    "a very thin space" may be used on both sides, not just one, of the slash. – Kris Jan 3 '15 at 14:59
  • 1
    The SI style guide (official pdf can be found under that name) also makes the point that the space between the numerals and the units must be (a) present and (b) non-breaking. The style guide, while dry, is well worth reading if you make a habit writing with quantities and units in any technical sense - the reasoning behind many points is well set out and quite instructive. – Chris H Jan 3 '15 at 15:36
  • True, @ChrisH but the national standards for one's own country will have further rules which may restrict within choices the SI leave open (alternative spellings, whether to use L or l for litre, and so on). – Jon Hanna Jan 4 '15 at 0:35
  • In the context of a paper surely it's the journal's house style that applies rather than any national rules. Most international journals including those based in the US, at least in physical sciences and the bits of engineering I've read, follow the SI guidance either explicitly or implicitly. I can't however speak for US journals with little international reach or other fields - I would expect that in e.g. agriculture the customary bizarre units are still prevalent. – Chris H Jan 4 '15 at 17:30

"Sugar concentration was 20g/mL." is what we usually see, but I don't think there's a rule for that.

Edit: After reading Jon Hanna's answer, I've searched the web and found that most scientists follow an International Standard in their scientific papers.

  • 2
    I prefer 20g/ml – Martin Jan 3 '15 at 14:27
  • @Martin, that is preferred by the SI standard but L may be used for clarity and to disambiguate (against 1). The space between numbers and units isn't optional though (see my other comment). – Chris H Jan 3 '15 at 15:38
  • The one that gets me is using an upper case "k" in kg or kW – Martin Jan 3 '15 at 16:58

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