O/A means overall — the dimension spans the full extent of the item, outside face to outside face, including frames, end plates, linings and projections.
O/A is a hand-lettering habit that survived the move to CAD, and it is still all over Australian and UK drawings. It looks trivial until you realise it is one of three numbers describing the same object.
What O/A means
O/A means overall. The dimension runs the full extent of the item or assembly, outside face to outside face. It includes the frame, the end plates, the trims, the linings and any projections that form part of the thing being dimensioned.
It is a fabrication and setting-out number, not a functional one. It tells you how much space the item occupies, or how long to cut it. It does not tell you how much usable opening you get – that is the job of a clear dimension.
You will meet it most on window and door schedules, joinery, stairs, steel fabrication, precast panels and ductwork – anywhere something is made to a size and then installed into a hole.
How it appears on a drawing
As a suffix on a dimension, typically the long outer dimension string:
2400 O/A
On a fabrication drawing, where the distinction from member length matters:
O/A LENGTH 6500 (MEMBER 6476 + 2 No. 12 END PLATES)
In a joinery run, where the O/A is the sum of the modules plus the end panels:
3 No. @ 600 + 2 No. END PANELS @ 18 = 1836 O/A
And in a schedule column, alongside the opening size so the reader can see they differ:
W12 - O/A FRAME 1810 x 2100 - GLASS 1700 x 1990
That last one shows the layout, not a real product size – how far the glass sits inside the frame depends on the profile, so take those offsets from the manufacturer’s details.
Variants for the same thing: O/ALL, O.A., OA, and OAL for overall length on fabrication drawings. The slash is a lettering convention that made the abbreviation unmistakable in ink.
Terms it gets confused with
- CLR – clear. The usable gap between faces, always smaller than the O/A. A door is the illustration: the leaf, the O/A frame size and the clear opening are three different numbers, and an 850 clear opening commonly needs a 920 leaf and a frame O/A larger again.
- NOM – nominal. A naming size that may not be a real measured dimension.
- RO (rough opening, US) or structural opening – the hole the item goes into, larger than the O/A to allow packing and tolerance.
- OD – outside diameter. Related idea for round sections, not interchangeable.
- O/O – out to out. Same intent as O/A, used more in steelwork and setting out.
- CRS or C/C – centres. Measured centre to centre, which is neither overall nor clear.
AU and UK versus US practice
O/A and O/ALL are common on Australian, New Zealand and UK drawings, especially in joinery, window schedules, steel detailing and stair details.
US drawings are more likely to spell OVERALL out, or to use OAL on fabrication and machining drawings. And there is a real trap: on US building sets, OA without the slash is very often outside air on the mechanical sheets, appearing alongside RA (return air) and SA (supply air), on OA louvres and OA ducts. Both meanings can appear in the same project set on different disciplines, so let the discipline and context decide.
Where to check on your own drawing set
- The abbreviations legend on the cover or general sheet, and whether the office uses O/A, O/ALL or OA.
- The schedule that owns the item, and its column headings. “O/A size” and “opening size” are not the same number, and mixing them up is how windows arrive that do not fit.
- The dimension’s own extension lines. An O/A dimension’s witness lines land on the outermost faces of the assembly. If they land on an internal face, it is not an overall dimension no matter what the suffix says.
- Whether the O/A includes site-applied trims, architraves, flashings or cover strips. This is frequently unstated and worth confirming before ordering.
If you are decoding a schedule with several of these tags at once, our A-Z list of engineering drawing abbreviations covers the neighbouring ones.
Other meanings of O/A
The important collision is within construction itself: OA on mechanical and HVAC drawings means outside air (OA louvre, OA duct, OA damper), not overall. Outside construction, OA is open access in publishing, osteoarthritis in medicine, office automation in IT, and open account in trade finance.