writing - Is it correct to write "a 5-mm-thick layer"?

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

Answer

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.