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Do I need hyphens? Should I use the indefinite article or zero article?

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It is incorrect to write "a 5-mm-thick layer". Rather, it should be written:

a 5 mm-thick layer

or, better,

a five millimeter-thick layer

5 mm-thick is a compound adjective formed from the adjectives 5 mm and thick, and it modifies layer. Consider:

  • a 5-mm layer/a 5 mm layer
    (as a matter of style, some writers don't hyphenate numerals with letters)
  • a five-millimeter layer
  • a thick layer
  • a 5 mm-thick layer

Note that as a compound adjective in its own right, 5-mm/five-millimeter is hyphenated. When compounded again, though, the first hyphen is dropped: 5 mm-long/five millimeter-wide, etc.

Other similar compound adjectives can be found in contexts similar to the following:

  • a five year-long period
  • a two month-old movie
  • a ten year-old boy

Review this article for more on compound adjectives.

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  • 14
    I'm surprised that a prescriptive rule (I note that no authority is mentioned) requires a ten year-old boy rather than a ten-year-old boy. Certainly Ten year-old boys were monitored for signs of the condition is different from Ten-year-old boys were monitored ... . The best criterion to apply is surely the avoidance of ambiguity. Wikipedia, in a fine article at en.wikipedia.org/wiki/Hyphen , addresses this consideration, and the relaxing of overly precious demands for pet styles of hyphenating.
    – Edwin Ashworth
    Commented Nov 11, 2012 at 16:18
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    @EdwinAshworth You’re right, and I am pretty sure this answer is wrong. See my own.
    – tchrist ♦
    Commented Nov 11, 2012 at 16:36
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    Structurally OP's example is no different to "five year old boy", where skimming through some of the hundreds of thousands of examples in that Google Books link, it's quite clear the vast majority use two hyphens. Even if this answer is "correct" according to some particular style guide (I see none cited) it's obviously out of step with majority usage. In short, it's wrong.
    – FumbleFingers
    Commented Nov 11, 2012 at 16:49
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To answer the question directly: yes, it is perfectly correct to write “a 5-mm-thick layer”. However, when not in prefix position, it would be “a layer that’s 5 mm thick”, this time without any hyphens.

The accepted answer appears to be wrong in its assertion that there is something wrong with writing “a five-millimeter-thick layer”. There isn’t.

Here are actual examples:

  • This will be squeezed into a one-foot-thick ice layer in a century or two. . . . [citation]
  • That layer, in turn, is underlain by a 6- to 9-mile-thick layer of extremely dense rock that is even more iron-rich than the basalt on the surface. [citation]
  • Here the phosphatic rocks are 14.9 feet thick, and a 5.8-foot-thick zone has an average of 22.1 percent P2O5. [citation]
  • The first rapids, just above the Rice River confluence, rates class I. Just downstream from the confluence, the Little Fork flows through a 50-yard-long rapids, flanked by outcrops, which bends sharply left and throws 2-foot waves in high water. [citation]
  • The Dakota Sandstone consists of three distinct stratigraphic units: (1) a 4- to 134-foot-thick, lower conglomeratic. . . . [citation]
  • Starting at the bend of the hook, tie in the white thread and a sparse bunch of 3-inch-long white bucktail. [citation]
  • Tensile strength of lumber laminated from 1/8-inch-thick veneers (title) [citation]
  • Finally, the cylindrical shaft is comprised of structural walls with a 15.5-foot interior core diameter and that increase in thickness from 27 inches to 33 inches halfway along its 74 foot vertical length, coming to rest on a 20-foot-thick rectangular. . . . [citation]
  • I plod step by ponderous step along the bottom of the 37-foot-deep test tank at NASA's National Space Technology. . . . [citation]
  • A twenty-thousand-foot-thick layer of stratified sediments was deposited in a shallow sea over millions of years, creating the Long Island Platform that underlies Long Island. [citation]
  • We were sitting under a six-yard-thick concrete ceiling in a nuclear-bomb-proof bunker, and only the vibrations enabled us to guess whether the missiles had landed nearby or farther away. [citation]
  • In the eastern Thousand Lakes Wilderness, about a dozen cinder cones erupted along a 7-mile-long line (perhaps a fault). [. . .] Quarter-mile-thick glaciers flowed through, exerting great pressure on the valley floor. [citation]

  • A 4- to 6-inch-tall species with 5- to 6-inch-long leaves that are about 1 inch wide. [citation]

    NOTE: There are many, many examples at that citation, several dozen on the referenced page alone: more examples

  • While the main gun rounds of the M60A1 remain unprotected in this picture, the main gun rounds in the M1 are protected by one-inch-thick blast doors, which are closed in this image. [citation] more examples

I believe that should establish the “correctness” of the fully hyphenated form. Certainly it occurs with plenty of frequency in written English, although I cannot speak to non-English usage.

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    Per my comment to the (incorrect) accepted answer, I think this is one of those cases where citing authorities/style guides isn't really necessary. Actual usage so overwhelmingly favours using both hyphens it would be meaningless to claim that's somehow "wrong".
    – FumbleFingers
    Commented Nov 11, 2012 at 16:54
  • Really? A quick (but adjusted for repeats) Google search for "mm thick layer" shows a preponderence of non-hyphenated forms, followed some way behind by the doubly-hyphenated. Interestingly, one source ( www.ncbi.nlm.nih.gov/pubmed/ ...) uses both alternatives.
    – Edwin Ashworth
    Commented Nov 11, 2012 at 19:42
  • @EdwinAshworth Yes, really. Now try with “six-mile-long”, “three-foot-wide”, etc. The hyphenated forms clearly dominate, and often eclipse.
    – tchrist ♦
    Commented Nov 11, 2012 at 19:54
  • Two-inch-thick layer: 18 × "two-inch-thick layer" + 27 × "two-inch thick layer" + 7 × "two inch thick layer". Run the same on "2-inch-thick layer" and you get 27 × "2-inch-thick layer" + 6 × "two-inch thick layer" + 10 × "two inch thick layer". See what I mean now?
    – tchrist ♦
    Commented Nov 11, 2012 at 20:18
  • tchrist, Sometimes there is safety in numbers. But i wouldn't place too much confidence in your list of citations demonstrating first-hyphen preservation in multi compound-adjective terms. You cite McClelland & Steward (Concrete Toronto), who, in the same passage, use "comprised of". This doesn't prove anything either way about hyphenating of course. I am, however, under the impression that the first hyphen gets dropped when a compound adjective is joined by another adjective.
    – user53094
    Commented Sep 30, 2013 at 2:46
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According to excerpts from the Chicago Manual of Style, 5-mm-thick layer is correct: http://www.apsstylemanual.org/oldmanual/mechanics/hyphens.htm http://www.chicagomanualofstyle.org/16/images/ch07_tab01.pdf

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  • We like references, but you should quote the relevant parts in your answer.
    – Laurel ♦
    Commented May 14, 2017 at 17:00
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I would have said that there is no need for a hyphen between 5 and mm; 5mm is normally written thus.

So it would be:

A 5mm-thick layer.

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  • 5
    The SI brochure mandates a space between the number and the unit, so it should be 5 mm rather than 5mm.
    – Brian Nixon
    Commented Jan 13, 2011 at 14:15

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