What does o.c. stand for and what does this mean?

These are some examples of its use:

Space end bells approximately 250 mm o.c. for 125-mm ducts.

Space fasteners at 10" (250 mm) o.c. on intermediate supports for single layer floors and 12" (300 mm) o.c. for subfloors.

Cold joints in flat insulating concrete form foundation walls shall be reinforced with no less than one 15M bar spaced at not more than 600 mm o.c. and embedded 300 mm on both sides of the joint.

It seems to be impossible to find a definition for this term, but it's widely used in construction-related papers.

  • It probably means 'outside circumference' or 'outside corners'. Where have you looked? / Yes, AcronymFinder lists the former (frequency level 4) and the latter (level 3). Sep 19, 2016 at 10:39
  • Given that it's talking about spacing of components, centre or centre-line would seem a reasonable guess for c.
    – Andrew Leach
    Sep 19, 2016 at 11:10
  • @AndrewLeach The tag descriptions say "Abbr is shortened word" and "Acronym is made up of initial letters". I had the impression o.c. was made up of initial letters, can you clarify your edit? Sep 19, 2016 at 11:51
  • Yes, an acronym is something like SCUBA or UNESCO, pronounced as a word. TUC (Trades Union Congress) and UN are initialisms as they're pronounced as individual letters. Your o.c., while comprising two initial letters, doesn't make a pronounceable word, and isn't an initialism as those are generally capitalised and refer to organisations. Each of the two words is abbreviated; it's an abbreviation. We might have a question on the difference, but if not, why not ask one?
    – Andrew Leach
    Sep 19, 2016 at 12:00

1 Answer 1


O.C. stands for on center.

From About's woodworking glossary:

Definition: abbreviation for "on-center", which means that an object is placed at the center of the mark. For instance, in building a wall, a building plan may call for the wall studs to be places every 16-inches OC. This means that the center of each stud is placed every sixteen inches.

On center measurement results in different spacing than using the same space between the edges of the pieces:

Space end bells approximately 250 mm o.c. for 125-mm ducts.

If you left 250 mm and weren't spacing on center, then four ducts would require 1.25m (250mm for each of 3 spaces and 125mm for each of 4 ducts.)

Spacing on center, 4 ducts would fit into 875mm: 250mm for the 3 spaces (which the ducts fit into) and 125mm for the edges of the ducts on the ends.

  • Your example calculation is really helpful to understand what's meant here, thanks. Sep 19, 2016 at 11:56
  • Yes, that's frequency level 1 in Acronym Finder. This is rather technical for ELU, though. Sep 19, 2016 at 12:41

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