ASME/ANSI B36.10/19
As the schedule number increases, the wall thickness increases, and the actual bore is reduced. For example: A 4 inches (100 mm) Schedule 40 pipe has an outside diameter of 4.500 inches (114.30 mm), a wall thickness of 0.237 inches (6.02 mm), giving a bore of 4.026 inches (102.26 mm) A 4 inches (100 mm) Schedule 80 pipe has an outside …