The cultivation was performed in a 1000-L bioreactor.
The cultivation was performed in a 1000 L bioreactor.

Which is better? I find conflicting examples online. On Grammar Book I found this:

According to The Chicago Manual of Style, hyphens are never used between the numeral and the abbreviation or symbol, even when they are in adjectival form. Therefore write 12 mm focal length.

But then on the APS House Style page I found this:

Hyphenate when a number and a single unit of measure is used as a modifier or when a number and unit of measure is part of a compound adjective, e.g., "a 12-kDa fragment," "3 × 4-cm strip," or "2-μm-diameter tip."

So there's no definitive authority on this, and it's a matter of personal taste and corporate rules?

  • 1
    My personal taste would be to write the word out in full (a 1000-litre bioreactor, with or without hyphen). Sure, you could refer to your car's 2L or 2-L engine, but it's only a convention to use upper case for the abbreviation anyway, and you'd just create confusion if you used the written form a 2l engine. – FumbleFingers Apr 7 '17 at 18:24
  • You will find "1000L", "1000l", "1000 L", "1000 l", "1kL", "1kl", "1KL", "1 kl", and "1 KL", and probably some other variants. I use the form in Lawrence's answer. – Tᴚoɯɐuo Apr 7 '17 at 19:56

One of the keepers of the International System of Units is the International Bureau of Weights and Measures (Bureau international des poids et mesures). Its website says, among many other things:

... a space is always used to separate the unit from the number ... Even when the value of a quantity is used as an adjective, a space is left between the numerical value and the unit symbol. Only when the name of the unit is spelled out would the ordinary rules of grammar apply, so that in English a hyphen would be used to separate the number from the unit.

It gives the example of 'a 35-millimetre film'. I assume that 'a 35 mm film' would be interchangeable.

'Litre' is a 'Non-SI unit accepted for use with the International System of Units' and has the alternative abbreviations L and l (link), with a footnote:

(f) The litre, and the symbol lower-case l, were adopted ... in 1879 ... The alternative symbol, capital L, was adopted [in] 1979 ..., in order to avoid the risk of confusion between the letter l (el) and the numeral 1 (one).

(Generally speaking, only units based on people's names are in upper case.)

So, your original example would be either:

a 1000-litre reactor


a 1000 L reactor

(With the the possibility of kilolitre or kL.)

That said, if you are formally or informally using a published or house style guide, then you would go with that. And it might make a difference whether you are writing for a specialist or general reader.


Numeric metric expressions with the abbreviated metric measurement name should be written after the number with no space.

The cultivation was performed in a 1000L bioreactor.

I bought 250kg of X, 250ml of Y, 250cm of Z, etc.

3cm x 4cm strip.

  • I wonder if there's some authority I could cite on this in future. Some book or other document. – CowperKettle Apr 8 '17 at 0:34
  • Major journals (Science, Nature) do use a space (e.g., 473 nm). – Ulrich Stern Apr 25 '19 at 5:05
  • this answer is absolutely contrary to all recommendations that I've seen -- never glue units and number together! – Felipe G. Nievinski Jun 1 '20 at 20:41

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