innerdiameter

Inner diameter, abbreviated ID, is the internal distance across a hollow object, defined as the diameter of the bore measured through the centerline. It is the counterpart to outer diameter (OD) and is a fundamental dimension for tubes, pipes, bushings, and cylinders used in fluid conveyance and mechanical assemblies. ID determines capacity, flow, and clearance with mating parts.

For a cylindrical part with uniform wall thickness t, ID = OD − 2t. If the wall is not uniform, ID must be specified at a reference location and may vary along the length. In piping, ID is influenced by the chosen wall thickness, manufacturing process, and standard schedules or tube specifications.

Measurement of the inner diameter requires gauges that reach the bore interior. Methods include bore gauges, internal micrometers, calipers, or coordinate measuring machines. Measurements are taken at multiple positions to assess straightness and roundness and to ensure conformance with tolerances. Wear, corrosion, or deposits can reduce effective ID and alter performance.

ID is critical for calculating flow capacity, internal volume, and clearance fits. In engineering drawings, it is commonly labeled as ID or id and expressed in millimeters or inches. In pipes, the relationship between ID, OD, and wall thickness determines selection of pipe schedules and tube grades.

Example: for a tube with ID 30 mm and length L, cross-sectional area is A = π*(ID/2)^2 = π*(15 mm)^2, and volume is A×L. The hydraulic diameter equals the ID for circular sections, making ID a key parameter in hydraulic and thermal analyses.