Dimensions
Cross-check: the standard plain-end weight formula W = 10.6802 x t x (OD - t) gives 0.57 lb/ft here, 0.4% from the published 0.57 lb/ft. The published figure is the one printed above.
NPS 3/8 schedule 40 in numbers
NPS 3/8 schedule 40 pipe measures 0.675 in (17.14 mm) across the outside with a 0.091 in wall, so the bore is 0.493 in (12.52 mm). That bore gives a flow area of 0.1909 in² and holds 0.009916 US gallons per foot (0.1232 litres per metre). The outside diameter is 7.4 times the wall, and the steel itself has a cross-section of 0.1670 in² and weighs 0.57 lb per foot plain end. One trade size up, NPS 1/2 in the same schedule carries 1.59 times the flow area.
Schedule 30 is the next lighter wall at NPS 3/8: 0.073 in, 0.018 in thinner, which opens the bore to 0.529 in and adds 15% to the flow area, for 18% less weight per foot.
Schedule 80 is the next heavier wall at NPS 3/8: 0.126 in, 0.035 in thicker, which narrows the bore to 0.423 in and cuts 26% from the flow area, for 30% more weight per foot.
Next trade size up in this schedule: NPS 1/2 schedule 40, bore 0.622 in.
Worked out from the dimensions
Plain geometry of a hollow circle with OD = 0.675 in and ID = 0.493 in: flow area π/4·ID², metal area π/4·(OD² − ID²), moment of inertia π/64·(OD⁴ − ID⁴), section modulus I ÷ (OD/2), radius of gyration √(I ÷ metal area), capacity flow area × 12 ÷ 231 (231 in³ to the US gallon). Nominal dimensions, so mill tolerance and corrosion allowance are not included.
Why 3/8 inch pipe is not 3/8 inches
Nominal Pipe Size is a label, not a measurement. NPS 3/8 pipe has an outside diameter of 0.675 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 0.091 inches, so the bore comes out at 0.493 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.
Other schedules in NPS 3/8
Same outside diameter of 0.675 inches throughout - only the wall and the bore move.
Frequently asked questions
What is the wall thickness of 3/8 inch schedule 40 pipe?
0.091 inches (2.31 mm). The outside diameter is 0.675 inches (17.14 mm) whatever the schedule, so this wall leaves an inside diameter of 0.493 inches.
What is the inside diameter of 3/8 inch schedule 40 pipe?
0.493 inches (12.52 mm). Inside diameter is outside diameter minus twice the wall: 0.675 - 2 x 0.091 = 0.493 inches.
Is 3/8 inch pipe actually 3/8 inches?
No. NPS 3/8 is a trade size, not a measurement. The pipe measures 0.675 inches across the outside and 0.493 inches across the bore in this schedule. Only the outside diameter is fixed across schedules.
How much does 3/8 inch schedule 40 pipe weigh?
0.57 pounds per foot plain end (0.84 kg per metre), for carbon steel. The figure excludes coatings, linings and fittings.
Is schedule 40 the same as STD or 40S at this size?
Yes, at NPS 3/8. STD and 40S are the same 0.091 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.
Not published here
3 schedules at NPS 3/8 appear in only one of the published tables cross-checked for this section, so they are named rather than printed: 160, XXS, 5S. Everything shown above carries at least two independent sources.
Where this table stops
This table runs from NPS 1/8 to NPS 24 and stops there. ASME B36.10M itself continues to NPS 80; those larger sizes are not carried here, so a size above NPS 24 is missing rather than undefined.
Where these figures come from
Dimensions are as defined by ASME B36.10M (welded and seamless wrought steel pipe). This site is an independent reference and is not affiliated with, endorsed by or approved by any standards body.
Every value on this page was cross-checked against 4 independent published sources: