OD
(of a pipe, tube, or shaft) Initialism of outer diameter or outside diameter.
OD: the distance across the outside of a round part
Outer diameter (OD) is the maximum distance measured straight across the outside surface of a cylindrical part, from edge to edge through the centre. For pipes, tubes, shafts, and similar round stock, OD is the first specification you check because it determines whether a part will fit into or around another component. Unlike wall thickness or bore, OD is a single, unambiguous measurement that applies to any cross-section of a uniform cylinder.
In practice, OD is measured with calipers, micrometers, or go-no-go gauges depending on the tolerance required. A shaft nominally 25 mm OD might have a tolerance of plus 0.05 mm, meaning the actual diameter must fall between 25.00 and 25.05 mm. For thin-walled tubes, OD is critical because it controls fit in a bore or bearing race. For thick-walled pipes carrying pressure, OD affects the external envelope and how the pipe connects to flanges or fittings.
OD versus ID and wall thickness
OD is often paired with inner diameter (ID) and wall thickness (WT). A steel tube might be specified as 50 mm OD with a 3 mm wall, meaning the ID is 44 mm. The three measurements relate mathematically: ID equals OD minus twice the wall thickness. Confusion arises because some standards (such as nominal pipe size in plumbing) use an old convention where the nominal size does not match the actual OD; a half-inch NPT pipe has an OD close to 21 mm, not 12.7 mm. This historical quirk means always verifying OD with a ruler or calliper rather than assuming it from the label.
OD appears in every engineering drawing, material specification, and procurement document for round stock. When ordering a replacement bearing, pulley, or coupling, the OD of the shaft it will fit is the starting constraint. Manufacturing tolerances on OD are often tighter than on ID because the outside surface is simpler to machine and control. Common causes of OD failures include wear on journals in bearings, corrosion that reduces or builds up the surface, and incorrect sizing during design that leaves no interference or clearance margin.