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What a schedule actually is

A pipe schedule is a designation for wall thickness within a given nominal pipe size (NPS). It is not a pressure rating and not a thickness in itself. The same schedule number gives a different wall for each NPS, so the designation must always be read together with the size.

The outside diameter of a given NPS stays fixed, so a heavier schedule reduces the inside diameter. This is why NPS and schedule together define the flow area, weight and pressure capacity of a pipe.

The two governing standards

Two ASME standards cover dimensions for the schedules most buyers meet:

• ASME B36.10M - welded and seamless wrought steel pipe, generally used for carbon and alloy steel. It contains the schedules 10, 20, 30, 40, 60, 80, 100, 120, 140 and 160, as well as the weight classes STD, XS and XXS.
• ASME B36.19M - welded and seamless stainless steel pipe. It introduces the schedules with the suffix S: 5S, 10S, 40S and 80S. In this standard, STD and XS correspond to 40S and 80S for the sizes where those designations overlap.

Do not assume that every schedule exists for every size. Each standard lists the available combinations in its dimension tables, and those tables are the only reliable source for wall thickness values.

Schedule and weight-class designations

Where STD, XS and the schedules coincide

STD, XS and XXS predate the schedule system. For smaller sizes, STD has the same wall as SCH 40 and XS the same as SCH 80. For larger sizes this equality no longer holds, and the weight-class wall can differ from the schedule wall. For this reason, state the designation exactly as the project specifies it and do not treat the terms as interchangeable without checking the table.

XXS has no matching schedule number in the standard; it is its own designation. Note also that SCH 160 and XXS are not the same wall, even if they are close in some sizes.

Schedule is not pressure rating

Allowable pressure depends on the wall thickness, outside diameter, material allowable stress at design temperature, manufacturing tolerance and corrosion allowance. Design codes such as ASME B31.3 (process piping) or ASME B31.1 (power piping) provide the calculation method; in Europe, EN 13480 applies to metallic industrial piping. Use the calculated minimum thickness to select the schedule, not the other way round.

Tolerances and ordering details

ASME B36.10M wall thicknesses are nominal. Pipe made to ASTM product specifications, such as ASTM A106, ASTM A53/A53M, ASTM A312/A312M or ASTM A358/A358M, normally permits an under-tolerance on wall, and the permitted value is given in the product specification itself, not in B36.10M. Check that standard for the exact allowance and for how it relates to your minimum-wall requirement.

Outside North America, other systems exist. EN 10220 (dimensions and masses per unit length for welded and seamless pipes) and ISO 1127 for stainless steel pipe use outside diameter and wall thickness in millimetres instead of schedule numbers. GOST 8732 and GOST 8734 (seamless pipe) and DIN 2448 are further examples where wall is stated directly. AS/NZS 1163 and AS 1074 are used in Australia and New Zealand with their own size and wall designations. When converting between systems, match the real outside diameter and wall rather than the name.

Buyer checklist

• State NPS (or DN and outside diameter) together with the schedule or weight class.
• Name the dimensional standard: ASME B36.10M or ASME B36.19M, or the equivalent EN, ISO or GOST standard.
• Name the product specification, grade and whether the pipe is seamless or welded.
• Confirm whether the wall is nominal or a minimum wall.
• Check wall tolerance in the product specification.
• Verify the schedule exists for the NPS in the dimension table.
• Match fittings and flanges bore (for example butt-weld ends) to the pipe schedule.
• Specify end finish, such as plain or bevelled ends, and any required testing or certificate type.