Why 6 inch pipe is not 6 inches
Nominal Pipe Size is a label, not a measurement. NPS 6 pipe has an outside diameter of 6.625 inches, and it keeps that outside diameter in every schedule made. What the schedule number changes is the wall: at extra strong (xs) the wall is 0.432 inches, so the bore comes out at 5.761 inches.
That is the practical consequence. Fittings, flanges, supports and threading dies are sized to the outside diameter, so they fit any schedule of the same trade size. Flow, pressure drop and anything that has to pass through the pipe depend on the inside diameter, which changes with every schedule. Order by trade size and schedule; work your hydraulics from the bore.
Frequently asked questions
What is the wall thickness of 6 inch extra strong (xs) pipe?
0.432 inches (10.97 mm). The outside diameter is 6.625 inches (168.27 mm) whatever the schedule, so this wall leaves an inside diameter of 5.761 inches.
What is the inside diameter of 6 inch extra strong (xs) pipe?
5.761 inches (146.33 mm). Inside diameter is outside diameter minus twice the wall: 6.625 - 2 x 0.432 = 5.761 inches.
Is 6 inch pipe actually 6 inches?
No. NPS 6 is a trade size, not a measurement. The pipe measures 6.625 inches across the outside and 5.761 inches across the bore in this schedule. Only the outside diameter is fixed across schedules.
How much does 6 inch extra strong (xs) pipe weigh?
28.6 pounds per foot plain end (42.56 kg per metre), for carbon steel. Stainless of the same dimensions runs a little lighter, and the figure excludes coatings, linings and fittings.