ASME B36.10M · Pipe schedules explained

How steel pipe schedules work

NPS vs actual OD, schedule numbers vs STD/XS/XXS, why the wall sets ID, weight, and pressure, and how to calculate weight per foot yourself — with worked 2" Sch 40 and Sch 80 examples.

Where to get the dimensions

Carbon and alloy steel pipe dimensions are standardized in ASME B36.10M (stainless: B36.19M), published by ASME. In practice, the fastest source is your pipe supplier's or mill's catalog and the mill test report that ships with the pipe. Your piping spec or line list names the schedule for each service.

Pressure ratings depend on the material, temperature, corrosion allowance, and the governing code (for example ASME B31.1, B31.3, or B31.9) — not on the schedule alone. Not engineering advice.

The terms on a pipe spec, decoded

Term	What it means
NPS (nominal pipe size)	The trade size. For NPS 1/8 through 12, it is a name, not a measurement. From NPS 14 up, the OD equals the NPS.
OD (outside diameter)	Fixed for a given NPS regardless of schedule — that is what lets Sch 40 and Sch 80 share fittings, threads, and hangers.
Wall thickness (t)	What the schedule actually controls. More wall = more weight, smaller ID, more pressure and corrosion allowance.
ID (inside diameter)	OD − 2 × wall. Matters for flow, velocity, and fitting internal alignment on welded joints.
Schedule numbers	Sch 5, 10, 20, 30, 40, 60, 80, 100, 120, 140, 160. A higher number means a thicker wall at the same NPS, but the same schedule number is a different wall at different sizes.
STD / XS / XXS	Older "weight class" names: standard, extra strong, double extra strong. They overlap with Sch 40 and Sch 80 in smaller sizes and diverge in large sizes.
"S" suffix (5S, 10S, 40S, 80S)	Stainless steel schedules from B36.19M. Many match the carbon-steel walls, but not all — do not assume.

Worked examples — NPS 2

Example	Input	Steps	Result
A. 2" Sch 40	NPS 2: OD 2.375 in, wall 0.154 in.	ID = 2.375 − 2(0.154) = 2.067 in. Weight = 10.69 × (2.375 − 0.154) × 0.154 = 3.66 lb/ft.	ID 2.067 in · ≈ 3.65 lb/ft
B. 2" Sch 80	NPS 2: same OD 2.375 in, wall 0.218 in.	ID = 2.375 − 2(0.218) = 1.939 in. Weight = 10.69 × (2.375 − 0.218) × 0.218 = 5.03 lb/ft.	ID 1.939 in · ≈ 5.02 lb/ft
C. Takeoff	380 ft of 2" Sch 40.	380 × 3.65 lb/ft = 1,387 lb.	≈ 0.69 tons (plain end, empty)
D. Flooded weight	2" Sch 40 full of water (for hanger loads).	Water area = π × 2.067² ÷ 4 = 3.36 in². Water ≈ 0.433 lb/ft per in² → 3.36 × 0.433 = 1.45 lb/ft.	≈ 3.65 + 1.45 = 5.1 lb/ft

NPS 2 values shown to illustrate the math only. 10.69 = steel density × π converted to inch-feet units; published weights may differ in the last digit from rounding. Get other sizes from your supplier catalog or the standard.

Common field mistakes

Mistake	What to do instead
Measuring the pipe and calling it the size	A pipe that measures 2-3/8" across is NPS 2. Measure OD with a tape or calipers and match it to the nominal size.
Assuming Sch 40 = STD at every size	True in small and mid sizes only. On large bore, order exactly what the line list calls for.
Treating schedule as a pressure rating	Allowable pressure depends on material, temperature, joint type (threading removes wall), and the piping code. The engineer or spec sets it.
Mixing carbon and stainless schedules	A 10S stainless wall is not always the same as Sch 10 carbon steel. Check the spec for each material.
Threading thin-wall pipe	Threading cuts into the wall. Light-wall schedules may not have enough wall left to thread — use grooved, welded, or pressed joints as specified.

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Reading pipe sizes and schedules

Steel pipe is identified by nominal pipe size (NPS) and a schedule. For a given NPS the outside diameter is fixed — the schedule only changes the wall thickness, and with it the inside diameter, weight, and pressure capacity. That is why 2" Sch 80 threads into the same fittings as 2" Sch 40 but has a smaller bore.

You do not need a table to get the numbers that matter for takeoff and hanger loads. With the OD and wall from the supplier catalog, ID = OD − 2t and plain-end carbon-steel weight ≈ 10.69 × (OD − t) × t lb/ft.

For weld-process piping, traceability matters as much as dimensions. Field PM's weld mapping tracks every joint, welder, and NDE result against the spool drawings. See also how NPT pipe threads work and how copper tube sizes work.

FAQ

What is the difference between Schedule 40 and Schedule 80 pipe?+

At a given nominal pipe size the outside diameter is the same, so both fit the same fittings and threads. Schedule 80 has a thicker wall — smaller bore, more weight per foot, and a higher pressure capacity. For NPS 2, Sch 40 has a 0.154" wall and Sch 80 a 0.218" wall on the same 2.375" OD.

Does NPS equal the actual pipe diameter?+

No. For NPS 1/8 through 12 the nominal size is a trade name that is close to, but not equal to, either diameter — a "2-inch" pipe is 2.375" OD. From NPS 14 up, the OD equals the nominal size in inches. The inside diameter always depends on the schedule.

Is Schedule 40 the same as STD?+

In the smaller and mid sizes, Schedule 40 and STD (standard weight) have the same wall, and Schedule 80 and XS (extra strong) match. They split apart in larger sizes, where STD and XS stay at a fixed wall while schedule walls keep growing. Always order by the designation on the drawings or line list.

How do I calculate steel pipe weight per foot?+

For plain-end carbon steel pipe: weight (lb/ft) ≈ 10.69 × (OD − wall) × wall, with OD and wall in inches. The 10.69 constant comes from steel density (about 490 lb/ft³) and π. For 2" Sch 40: 10.69 × (2.375 − 0.154) × 0.154 ≈ 3.66 lb/ft.

Where do I find dimensions for other sizes?+

From the pipe manufacturer or distributor catalog, the mill test report for the heat you received, your piping spec, or ASME B36.10M (welded and seamless wrought steel pipe) and B36.19M (stainless). Most mills publish free dimension and weight data.

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