DIA means diameter — the distance across a circle through its centre, twice the radius. Written Ø on mechanical sets, DIA on architectural and civil sheets.
DIA is short for diameter: the distance right across a circular feature, measured through its centre. It is twice the radius. On a drawing it sizes holes, round bar, dowels, bolts, pipes, cores, bored piers and round columns.
The one mistake that costs money is reading it as a radius. A note saying 100 DIA and a note saying R100 describe circles that differ by a factor of two.
How DIA appears on a drawing
Three forms, all meaning the same thing:
- The symbol Ø in front of the value — the ISO-based form, standard on mechanical sets:
Ø20 - DIA after the value, still very common on architectural and civil sheets:
20 DIA - DIA in front, mostly on older or hand-lettered drawings:
DIA 20
A worked example — holes in a steel base plate:
4 - Ø26 HOLES @ 150 CRS EW FOR M20 H.D. BOLTS, 75x75x10 PLATE WASHERS
Four holes, each 26 mm in diameter, at 150 mm centres each way, for M20 holding-down bolts. The leading 4 - is a quantity, not part of the size.
Note the hole size. Base plates are the exception to the usual bolt-plus-2 mm clearance. In Australian practice the holes are made up to 6 mm oversize to absorb holding-down bolt setting tolerance, with plate washers under the nut. A Ø22 hole on an M20 base plate is a site problem waiting to happen. How much oversize is allowed, and how it is detailed, varies between codes and offices — take the hole size off the connection detail and the steel general notes rather than from habit.
And a civil one:
P1: Ø600 BORED PIER, 900 MIN SOCKET INTO ROCK
On a metric set the millimetres are assumed and not written. If you see units on a diameter, you are probably on an imperial or mixed-unit drawing, so slow down.
Terms it gets confused with
- R / RAD — radius. Half of it.
- ID and OD — inside and outside diameter. Matters wherever wall thickness does:
Ø114.3 OD x 4.5 WALL CHS - DN / NB — diameter nominal and nominal bore, used for pipe.
DN150is a size label, not a measurement; the real bore and outside diameter depend on the material and pressure class. - NOM DIA — nominal diameter. Same warning.
- PCD — pitch circle diameter: the circle the bolts sit on, not the size of the bolts.
- Reinforcement callouts — in Australia a bar is named by grade and diameter together, so
N16is a 16 mm deformed bar. You rarely see “16 DIA” for rebar; if you do, read it the same way. - Ø rendered badly — a missing font turns Ø into a slashed zero, a degree symbol or an empty box. If a hole size reads
020, suspect the font before you suspect the designer.
AU/UK versus US
The meaning does not change. The habits do.
- AU, NZ, UK — Ø on mechanical and ISO-based sets, DIA still widely used on architectural and civil sheets, millimetres implied and units not shown.
- US — Ø is standard on mechanical drawings; architectural and structural sheets often spell it out, in inches and fractions:
3/4" DIA THRU. ASME Y14.38 is the abbreviations standard those sets point to. - Pipe — DN in AU and the UK, NPS in the US. DN150 corresponds to NPS 6.
- Rebar — US bar numbers run in eighths of an inch, so a #4 bar is nominally 1/2 in diameter. Never read a US bar number as a millimetre diameter.
Where to check on your own set
- The abbreviation legend, usually on the first architectural or structural sheet, or on a drawing conventions sheet.
- Schedules — hole, pile, bar, penetration — which override plan callouts in most offices.
- The specification, for the pipe sizing convention in use on that job.
- The dimension itself, if the symbol is missing. A diameter dimension runs across the full circle; a radius dimension has one arrowhead and runs from the arc to the centre.
Still ambiguous? Raise an RFI. Nobody has ever been criticised for confirming a hole size. A fuller list of this shorthand sits in the A-Z of engineering drawing abbreviations.
Other meanings of DIA
Outside drafting, DIA is the Defense Intelligence Agency (US), a common local name for Denver International Airport, and the Detroit Institute of Arts. On older drawings, DIAG (not DIA) is the abbreviation for diagram or diagonal.