Learning Hub
Glossary
Outside Diameter (OD)
Outside diameter (OD) is the maximum external width of a cylindrical or round part, measured as the distance across the outside surface through the centerline. It is the standard way to specify the “outer size” of components such as rod, wire, tube/pipe, sleeves, bushings, and many fastener features and it’s commonly specified along with inside diameter (ID) and length/thickness to fully define a part.
For fasteners, OD commonly refers to the shank diameter of a bolt or screw, the outer diameter of a nut or washer, the outside diameter of a spacer, or the major diameter of an externally threaded part (the largest diameter measured crest-to-crest). OD is critical for ensuring proper fit in holes and counterbores, maintaining required bearing surface area, controlling clearances, and meeting dimensional standards and tolerance requirements.