Why 3/4 inch pipe is not 3/4 inches
Nominal Pipe Size is a label, not a measurement. NPS 3/4 pipe has an outside diameter of 1.050 inches, and it keeps that outside diameter in every schedule made. What the schedule number changes is the wall: at schedule 40 the wall is 0.113 inches, so the bore comes out at 0.824 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 3/4 inch schedule 40 pipe?
0.113 inches (2.87 mm). The outside diameter is 1.050 inches (26.67 mm) whatever the schedule, so this wall leaves an inside diameter of 0.824 inches.
What is the inside diameter of 3/4 inch schedule 40 pipe?
0.824 inches (20.93 mm). Inside diameter is outside diameter minus twice the wall: 1.050 - 2 x 0.113 = 0.824 inches.
Is 3/4 inch pipe actually 3/4 inches?
No. NPS 3/4 is a trade size, not a measurement. The pipe measures 1.050 inches across the outside and 0.824 inches across the bore in this schedule. Only the outside diameter is fixed across schedules.
How much does 3/4 inch schedule 40 pipe weigh?
1.13 pounds per foot plain end (1.69 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 40 the same as STD at this size?
Yes, at NPS 3/4. STD is the same 0.113 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.