Pipe schedules · NPS 1-1/4
ASME B36.10M (welded and seamless wrought steel pipe) · DN 32
1-1/4 inch Schedule 80 pipe
Outside diameter 1.660" (42.16 mm), wall 0.191" (4.85 mm), inside diameter 1.278" (32.46 mm), 3.00 lb per foot.
Dimensions
Cross-check: the standard plain-end weight formula W = 10.6802 x t x (OD - t) gives 3.00 lb/ft here, 0.1% from the published 3.00 lb/ft. The published figure is the one printed above.
NPS 1-1/4 schedule 80 in numbers
NPS 1-1/4 schedule 80 pipe measures 1.660 in (42.16 mm) across the outside with a 0.191 in wall, so the bore is 1.278 in (32.46 mm). That bore gives a flow area of 1.283 in² and holds 0.06664 US gallons per foot (0.8276 litres per metre). The outside diameter is 8.7 times the wall, and the steel itself has a cross-section of 0.8815 in² and weighs 3.00 lb per foot plain end. One trade size up, NPS 1-1/2 in the same schedule carries 1.38 times the flow area.
Schedule 40 is the next lighter wall at NPS 1-1/4: 0.140 in, 0.051 in thinner, which opens the bore to 1.380 in and adds 17% to the flow area, for 24% less weight per foot.
Schedule 160 is the next heavier wall at NPS 1-1/4: 0.250 in, 0.059 in thicker, which narrows the bore to 1.160 in and cuts 18% from the flow area, for 26% more weight per foot.
Next trade size up in this schedule: NPS 1-1/2 schedule 80, bore 1.500 in.
Worked out from the dimensions
Plain geometry of a hollow circle with OD = 1.660 in and ID = 1.278 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 1-1/4 inch pipe is not 1-1/4 inches
Nominal Pipe Size is a label, not a measurement. NPS 1-1/4 pipe has an outside diameter of 1.660 inches, and it keeps that outside diameter in every schedule made. What the schedule number changes is the wall: at schedule 80 the wall is 0.191 inches, so the bore comes out at 1.278 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 1-1/4
Same outside diameter of 1.660 inches throughout - only the wall and the bore move.
Frequently asked questions
What is the wall thickness of 1-1/4 inch schedule 80 pipe?
0.191 inches (4.85 mm). The outside diameter is 1.660 inches (42.16 mm) whatever the schedule, so this wall leaves an inside diameter of 1.278 inches.
What is the inside diameter of 1-1/4 inch schedule 80 pipe?
1.278 inches (32.46 mm). Inside diameter is outside diameter minus twice the wall: 1.660 - 2 x 0.191 = 1.278 inches.
Is 1-1/4 inch pipe actually 1-1/4 inches?
No. NPS 1-1/4 is a trade size, not a measurement. The pipe measures 1.660 inches across the outside and 1.278 inches across the bore in this schedule. Only the outside diameter is fixed across schedules.
How much does 1-1/4 inch schedule 80 pipe weigh?
3.00 pounds per foot plain end (4.47 kg per metre), for carbon steel. The figure excludes coatings, linings and fittings.
Is schedule 80 the same as XS or 80S at this size?
Yes, at NPS 1-1/4. XS and 80S are the same 0.191 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:
Nearby sizes
Schedule 80 in other trade sizes