Which is correct?

a. Images were obtained at a distance of 4, 8, 12, 16 and 20 mm from the bottom of the plate.

b. Images were obtained at a distance of 4 mm, 8 mm, 12 mm, 16 mm and 20 mm from the bottom of the plate.

c. Images were obtained at a distance of 4mm, 8mm, 12mm, 16mm and 20mm from the bottom of the plate.

  • Does the style guide in your field (example: APA) say anything about this? – TaliesinMerlin Feb 1 '19 at 15:38
  • It really doesn’t matter. If this is for publication in a scientific journal the sub-editor will put it into house style. (Although I’m fairly sure that c will be changed to b.) – David Feb 1 '19 at 20:07
  • Also, if you have an Fig of this in your paper, consider writing something like “between 4 and 20 mm”. If all variants to sentence have problems, the answer is often to rewrite the sentence. – David Feb 1 '19 at 20:12
  • You seem to have forgotten some commas. – tchrist Feb 3 '19 at 3:28

The first two examples are acceptable. I prefer the first, but that's only an opinion.

The third is incorrect by most style guides I know. As an "authoritative" reference in case you don't have a specific style guide to follow, I'd cite the NIST Guide for the Use of the International System of Units, p 8:

There is a space between the numerical value and unit symbol, even when the value is used as an adjective, except in the case of superscript units for plane angle. (See Sec. 7.2.)

a 25 kg sphere but not: a 25-kg sphere

an angle of 2º3'4" but not: an angle of 2 º3 '4 "

If the spelled-out name of a unit is used, the normal rules of English are applied: “a roll of 35-millimeter film.” (See Sec. 7.6, note 3.)

(Which is also interesting because the style guide for my college's theses called for a dash between number and unit when used as an adjective)

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  • Dear 'The Photon' - this is so useful - thank you very much indeed. And thank you as well for alerting me to the NIST Guide. Really helpful. Best regards, M-L Archer – M-L Archer Feb 1 '19 at 19:21
  • There is a dash if the the numbers are spelled out: A twenty-five-kilogram sphere. – Jason Bassford Feb 1 '19 at 21:38

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