Pipe OD, ID and wall thickness: what to check
Distinguish nominal pipe sizes from actual dimensions before calculating weight or velocity.
Nominal sizes are designations
DN and NPS identify nominal pipe sizes. Their numbers cannot simply be entered as millimeter outside diameters. Check the actual outside diameter and thickness in the applicable dimensional table. Different standards and wall schedules may give different internal bores for the same nominal designation.
Subtract the wall on both sides
A circular pipe has wall thickness on both sides, so ID = OD − 2t. For a hypothetical pipe with OD 100 mm and wall thickness 10 mm, ID is 80 mm. This example is not a claim about a standard product size. Confirm whether a dimension on a drawing is an actual diameter or a nominal label.
Choose the correct area
Average fluid velocity uses the internal area πID²/4. Theoretical pipe mass uses the metal area π(OD²−ID²)/4. In the example, these are approximately 5,026.548 mm² and 2,827.433 mm² respectively. The calculation purpose determines which area is appropriate.
Theoretical and supplied mass
Theoretical mass assumes uniform wall thickness and density. Manufacturing tolerances, welds, coatings, linings and corrosion can affect actual mass. For purchasing, lifting and transport, compare the result with manufacturer data and project criteria. Account for valves, fittings and insulation separately.