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Which one is the correct one?

  1. 100l There is no space between number and unit. Unit is written in small letter.
  2. 100L There is no space between number and unit. Unit is written in big letter.
  3. 100 l There is a space and unit in small letter.
  4. 100 L There is a space and unit in big letter.
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Then there is the question of how much space if there is space? For example, would a \thinspace make sense instead of a "full" space? – morbusg Jan 6 '11 at 9:46
@morbusg. Indeed there is. A thin space emphasises the 'single entity' nature of the number-unit combination, and is the siunitx default. – Joseph Wright Jan 6 '11 at 13:38
up vote 9 down vote accepted

The capitalization depends in which part of the world you live!

The pump in the US is pumping \SI{100}{\liter\per\second}, 
whereas in the rest of the world is pumping \SI{100}{\litre\per\second}.

As usual Joseph's siunitx package will typeset it correctly, except in South Africa, where the convention was and is to a large extend to use a cursive (l). Personally I never capitalize the l, after all you wouldn't capitalize in, so why would one capitalize litres and run the risk of the lb's complaining? In addition SI units only capitalize units that are derived from a person's name. As for the space you unquestionably need it!

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I'd agree entirely with Yiannis here: you definitely need a space, whereas the symbol is variable. (I always stick to cubic metres or derivatives thereof, which nicely sidesteps the issue.) @Yiannis. I've not come across the South African convention: can you point to a ref. for this? – Joseph Wright Jan 6 '11 at 7:03
@Joseph This was in the standard when I was living there! The standard is SABS M33a:1992 (I am not sure if there is an update), but here is a link mecheng.sun.ac.za/Forstudents/docs/General/SIguide.pdf I also avoid using litres as much as I can! – Yiannis Lazarides Jan 6 '11 at 8:24

It's not clear this is really a TeX question. But you can get an answer here http://www.bipm.org/utils/common/pdf/si_summary_en.pdf. Either l or L is permitted. SI seems to like a space; others don't.

The siunitx package gives you various options, and is a must if you are concerned about consistency in your documents.

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