Why 12 inch pipe is not 12 inches
Nominal Pipe Size is a label, not a measurement. NPS 12 pipe has an outside diameter of 12.750 inches, and it keeps that outside diameter in every schedule made. What the schedule number changes is the wall: at schedule 120 the wall is 1.000 inches, so the bore comes out at 10.750 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 12 inch schedule 120 pipe?
1.000 inches (25.40 mm). The outside diameter is 12.750 inches (323.85 mm) whatever the schedule, so this wall leaves an inside diameter of 10.750 inches.
What is the inside diameter of 12 inch schedule 120 pipe?
10.750 inches (273.05 mm). Inside diameter is outside diameter minus twice the wall: 12.750 - 2 x 1.000 = 10.750 inches.
Is 12 inch pipe actually 12 inches?
No. NPS 12 is a trade size, not a measurement. The pipe measures 12.750 inches across the outside and 10.750 inches across the bore in this schedule. Only the outside diameter is fixed across schedules.
How much does 12 inch schedule 120 pipe weigh?
125.61 pounds per foot plain end (186.92 kg per metre), for carbon steel. Stainless of the same dimensions runs a little lighter, and the figure excludes coatings, linings and fittings.
Is schedule 120 the same as XXS at this size?
Yes, at NPS 12. XXS is the same 1.000 inch wall here. That coincidence is per size and does not hold everywhere: standard wall and schedule 40 part company above NPS 10, and extra strong and schedule 80 part company above NPS 8.