Dimensions
Cross-check: the standard plain-end weight formula W = 10.6802 x t x (OD - t) gives 70.59 lb/ft here, 0.1% from the published 70.65 lb/ft. The published figure is the one printed above.
NPS 18 standard wall (std) in numbers
NPS 18 standard wall (std) pipe measures 18.000 in (457.20 mm) across the outside with a 0.375 in wall, so the bore is 17.250 in (438.14 mm). That bore gives a flow area of 233.7 in² and holds 12.14 US gallons per foot (150.8 litres per metre). The outside diameter is 48.0 times the wall, and the steel itself has a cross-section of 20.76 in² and weighs 70.65 lb per foot plain end. One trade size up, NPS 20 in the same schedule carries 1.25 times the flow area.
Schedule 20 is the next lighter wall at NPS 18: 0.312 in, 0.063 in thinner, which opens the bore to 17.376 in and adds 1.5% to the flow area, for 17% less weight per foot.
Schedule 30 is the next heavier wall at NPS 18: 0.438 in, 0.063 in thicker, which narrows the bore to 17.124 in and cuts 1.5% from the flow area, for 16% more weight per foot.
Next trade size up in this schedule: NPS 20 standard wall (std), bore 19.250 in.
Worked out from the dimensions
Plain geometry of a hollow circle with OD = 18.000 in and ID = 17.250 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 18 inch pipe is not 18 inches
Nominal Pipe Size is a label, not a measurement. NPS 18 pipe has an outside diameter of 18.000 inches, and it keeps that outside diameter in every schedule made. What the schedule number changes is the wall: at standard wall (std) the wall is 0.375 inches, so the bore comes out at 17.250 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 18
Same outside diameter of 18.000 inches throughout - only the wall and the bore move.
Frequently asked questions
What is the wall thickness of 18 inch standard wall (std) pipe?
0.375 inches (9.53 mm). The outside diameter is 18.000 inches (457.20 mm) whatever the schedule, so this wall leaves an inside diameter of 17.250 inches.
What is the inside diameter of 18 inch standard wall (std) pipe?
17.250 inches (438.14 mm). Inside diameter is outside diameter minus twice the wall: 18.000 - 2 x 0.375 = 17.250 inches.
Is 18 inch pipe actually 18 inches?
No. NPS 18 is a trade size, not a measurement. The pipe measures 18.000 inches across the outside and 17.250 inches across the bore in this schedule. Only the outside diameter is fixed across schedules.
How much does 18 inch standard wall (std) pipe weigh?
70.65 pounds per foot plain end (105.17 kg per metre), for carbon steel. The figure excludes coatings, linings and fittings.
Is standard wall (std) the same as 40S at this size?
Yes, at NPS 18. 40S is the same 0.375 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.
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 5 independent published sources: