Pipe Schedules and Wall Thickness: A Practical Guide to ASME B36.10 and B36.19

"Four inch Schedule 40" is the most common line on a pipe requisition and one of the most misunderstood. The four inches is not a diameter you can measure, the schedule is not a thickness, and the same schedule number means different walls in carbon steel and stainless steel at the larger sizes. This guide explains the dimensional system behind ASME B36.10M and B36.19M so that a buyer can read a schedule chart, check a delivery and spot a specification error before it reaches site.

NPS, DN and outside diameter: the fixed dimension

Nominal pipe size (NPS) is a label, not a measurement. For NPS 12 and smaller the actual outside diameter is numerically larger than the nominal size; for NPS 14 and larger the outside diameter equals the nominal size in inches. DN is the metric designator of the same label, so NPS 4 and DN 100 are the same pipe. The outside diameter is the one dimension that never changes with schedule, because fittings, flanges and threading tools are made to it.

Because the OD is fixed, a heavier schedule always means a smaller bore. This is the point that catches people out in hydraulic calculations and when matching pipe to lined or socket-type fittings.

What a schedule number is, and is not

A schedule is a wall-thickness designation that B36.10M lists for each nominal size. The numbered schedules are 5, 10, 20, 30, 40, 60, 80, 100, 120, 140 and 160, alongside three older names, STD (standard), XS (extra strong) and XXS (double extra strong). The schedule number does not correspond directly to a pressure rating; two pipes of the same schedule but different sizes have different pressure capacities, because the wall-to-diameter ratio differs. Schedule is a way of ordering a wall thickness, and the allowable pressure is then calculated from that wall, the material allowable stress, the weld joint factor and the corrosion allowance.

Not every schedule exists at every size. Schedules 20, 30 and 60 appear only from NPS 8 upwards, Schedule 5 is a thin wall common in stainless and rare in carbon steel, and the heavy schedules 120 to 160 are made in a limited size range. Always check the chart for the size you want before writing a schedule that the mill does not roll.

STD, XS and XXS: the legacy designations

STD, XS and XXS predate the schedule numbers and are still in daily use. Their relationship to the numbered schedules is simple in small sizes and diverges in large ones.

STD equals Schedule 40 from NPS 1/2 through NPS 10. At NPS 12 they part company: STD is 0.375 in (9.53 mm) while Schedule 40 is 0.406 in (10.31 mm), and for NPS 14 and above STD stays at 0.375 in while Schedule 40 continues to increase. XS equals Schedule 80 through the small and medium sizes (the chart shows them equal up to NPS 8) and likewise separates at the larger sizes, with XS fixed at 0.500 in; refer to the standard for the exact crossover size. XXS is a separate heavy wall and does not coincide with Schedule 80 or Schedule 160 at any size.

The practical rule for a buyer: in NPS 10 and below, "STD" and "Sch 40" on a requisition are the same pipe and so are "XS" and "Sch 80". From NPS 12 upwards, write the one the designer actually intends, and if the drawing mixes them, ask.

B36.10 carbon steel wall thicknesses

The table below gives the walls a Riyadh MEP or industrial contractor will meet most often. Values are in millimetres with the inch figure from the standard in brackets.

Two observations. At NPS 8 the XXS wall (22.23 mm) is thinner than Schedule 160 (23.01 mm), a reminder that XXS is not "the heaviest". And the jump from Schedule 40 to Schedule 80 is roughly 40 to 60 % more wall across these sizes, which is why "upgrade to Sch 80" is such an expensive sentence on a large-bore system.

For NPS 12 and above the chart must be read column by column, because STD, Sch 40, XS and Sch 80 are all different values; for example NPS 12 Schedule 80 is 17.48 mm, far heavier than NPS 12 XS. Refer to the standard for those sizes rather than extrapolating from the table.

B36.19 stainless schedules and where they diverge

ASME B36.19M covers welded and seamless austenitic stainless steel pipe and uses four schedules with an S suffix: 5S, 10S, 40S and 80S. The suffix exists precisely because the walls are not always the same as the carbon steel schedules of the same number. B36.10M covers carbon, alloy and ferritic stainless steel and, for austenitic stainless, is also used when a wall outside the four S schedules is wanted.

Up to NPS 8, 40S and 80S match Schedule 40 and 80 to within rounding. The divergence starts at NPS 10 for 80S: carbon steel Schedule 80 at NPS 10 is 15.09 mm, while 80S is 12.70 mm. At NPS 12, 40S is 9.53 mm against Schedule 40 at 10.31 mm, and 80S stays at 12.70 mm against Schedule 80 at 17.48 mm. At NPS 14, 40S is 9.53 mm against Schedule 40 at 11.13 mm, and 10S differs from Schedule 10 from NPS 14 through NPS 22.

The implication for procurement is direct. A stainless requisition that says "Sch 40" for NPS 12 pipe is ambiguous: a stainless mill will read it as 40S and supply 9.53 mm, a designer who ran the calculation on B36.10 expected 10.31 mm. Write the S suffix when you mean B36.19M, and write "to ASME B36.10M" explicitly when you need a carbon-steel wall in stainless.

Schedules 5S and 10S are thin walls intended for welded, low-pressure and corrosion-driven service where stainless is chosen for its resistance rather than its strength; they are not suitable for threading.

From schedule to pressure: how to pick a wall

The designer's sequence is: calculate the minimum wall from design pressure, allowable stress and weld joint factor; add corrosion, erosion and threading allowances; divide by 0.875 to allow for the permitted 12.5 % under-tolerance on mill wall; then choose the next schedule up. The buyer's sequence is shorter but should respect the same logic.

First, confirm which standard governs: B36.10M for carbon and alloy steel, B36.19M for austenitic stainless unless the line class says otherwise. Second, check that the schedule exists at that size. Third, be aware that changing manufacturing type changes the answer: under ASME B31.3 an ERW pipe needs roughly 15 % more calculated wall than seamless for the same pressure, so a substitution from seamless to ERW at the same schedule may need the engineer's confirmation on thin-wall lines. Fourth, remember the tolerances when checking a delivery: wall may be 12.5 % under nominal at any point, and weight per metre may be 6.5 % over or 3.5 % under the theoretical value for A106 pipe, so a single ultrasonic reading slightly below nominal is not a rejection.

Finally, state everything on the enquiry: NPS, schedule (with S suffix where relevant), the dimensional standard, the material specification and grade, seamless or welded, and end finish. "4 inch Sch 40 A106 Gr B seamless, B36.10M, plain ends" leaves nothing to interpretation; "4 inch Sch 40 pipe" leaves almost everything.

Key takeaways

• NPS and DN are labels; outside diameter is fixed per size and a heavier schedule reduces the bore.
• STD equals Schedule 40 through NPS 10 and XS equals Schedule 80 in the small and medium sizes; from NPS 12 upwards they diverge and must be specified explicitly.
• B36.19M stainless schedules 5S, 10S, 40S and 80S match the carbon steel walls only up to about NPS 8; at NPS 10 to 14 the S schedules are thinner, in some cases by several millimetres.
• Schedule is not a pressure rating; pressure capacity depends on size, material, weld factor and allowances.
• Specify dimensional standard, schedule with suffix, material grade and manufacturing type together, and allow for the 12.5 % wall tolerance when inspecting.

Frequently asked questions

Is Schedule 40 the same as standard wall?

Yes for NPS 1/2 to NPS 10. At NPS 12, STD is 9.53 mm and Schedule 40 is 10.31 mm, and above that STD remains at 9.53 mm while Schedule 40 keeps increasing. Write the designation the drawing uses.

Why is my NPS 12 stainless pipe thinner than the carbon steel pipe of the same schedule?

Because stainless pipe is normally supplied to ASME B36.19M, where 40S at NPS 12 is 9.53 mm and 80S is 12.70 mm, against 10.31 mm and 17.48 mm for Schedule 40 and 80 in B36.10M. If the design needs the heavier wall, the requisition must say B36.10M.

Does a higher schedule number always mean a higher pressure rating?

For the same size and material, yes, because the wall is thicker. Across sizes, no: NPS 2 Schedule 40 has a far higher pressure capacity than NPS 12 Schedule 40 because its wall is a much larger fraction of its diameter.

How Al Safsaaf can help

Al Safsaaf Trading & Contracting Company stocks and supplies carbon steel pipe and stainless steel pipe for MEP and industrial projects, and can confirm the wall thickness and dimensional standard behind any schedule on your drawing. Send the size, schedule, material and quantity through our contact page for a checked quotation.