Pipe schedules

A pipe schedule identifies a nominal wall-thickness series. It must be read together with the nominal pipe size and dimensional standard: Schedule 40 alone does not specify a thickness, alloy or allowable pressure. ASME B36.10-2022 standardizes dimensions of welded and seamless wrought steel pipe; ASME B36.19-2022 addresses wrought stainless steel pipe. These dimensional standards are separate from the material specification and the code used to design a piping system. ASME B36.10 scope, ASME B36.19 record

Read size and schedule together

For NPS 2, Texas Flange's steel-pipe chart lists 2.375 in OD for Schedule 40 and 80. Their nominal walls are 0.154 and 0.218 in, giving calculated nominal IDs of 2.067 and 1.939 in. Check actual wall, ovality, weld profile and any lining when a minimum usable bore matters. Texas Flange dimension table

NPS is nominal pipe size; DN is a nominal size designation used with metric practice. Neither should be treated as a measured bore. In the familiar ASME dimensional system, a given NPS retains the same nominal outside diameter as its schedule changes. More wall therefore reduces the inside diameter. Texas Flange's chart, for example, gives NPS 2 an OD of 2.375 inches for both Schedule 40 and Schedule 80. Texas Flange pipe chart

Keep three quantities distinct on a drawing or estimate:

The ID expression is geometry for a concentric circular section, not a guaranteed minimum opening. Actual wall and diameter tolerances, weld profile, lining and deposits require separate consideration. Atlas distinguishes nominal dimensional tables from tolerances contained in the applicable ASTM or API product specifications. Atlas Technical Note 12, pp. 1–3

S schedules and older wall designations

Preserve the complete designation, including the S suffix when specified. Atlas explains that the stainless S schedules have important differences from unsuffixed schedules at larger sizes. Its examples show Schedule 40, STD and 40S coinciding through NPS 10, and Schedule 80, XS and 80S through NPS 8. Those coincidences do not establish equivalence above those ranges. Stainless pipe can legitimately be ordered to a heavier unsuffixed schedule; do not silently add or remove the suffix. Atlas Technical Note 12, p. 2

STD, XS and XXS are separate wall-thickness designations, not universally interchangeable spellings for 40, 80 and 160. For a concrete comparison, Texas Flange lists NPS 12 STD wall as 0.375 inches, while NPS 12 Schedule 40 is 0.406 inches. Its NPS 12 XS and Schedule 80 entries are 0.500 and 0.688 inches respectively. Match the exact row rather than extrapolating from a small pipe size. Texas Flange STD, XS, 40 and 80 tables

Worked comparison: NPS 2

The following are nominal inch dimensions from Texas Flange's published steel-pipe chart. Wheatland's Schedule 40 fire-sprinkler product sheet independently lists the same Schedule 40 OD, wall and ID. Texas Flange, Wheatland, p. 2

Original arithmetic using those nominal dimensions:

• Schedule 80 wall is 0.218 / 0.154 = 1.416 times as thick, about 41.6% thicker, not twice as thick.
• Internal area ratio is (1.939 / 2.067)² = 0.880; Schedule 80 has about 12.0% less nominal flow area.
• For the same volumetric flow through those ideal bores, average velocity is 1 / 0.880 = 1.136 times as high, about 13.6% higher.

Schedule is not a pressure class

Select pressure capability using the specified material, product construction, design temperature, applicable design code and connection details. Do not turn a schedule number into psi, bar or a flange class. Swagelok's pipe-fitting catalog, for example, publishes pressure-rating bases by end-connection type and gives temperature limits by material; its selection guidance requires consideration of the complete system, material compatibility and adequate ratings. A fitting's rating is evidence about that identified fitting under its stated conditions. Swagelok MS-01-147, pp. 3–4 and 23

A named product can carry additional application-specific qualifications. Wheatland's cited Schedule 40 fire-sprinkler submittal lists particular ASTM types, grades and size ranges, and gives different working-pressure entries for 1–4 NPS and for 5, 6 and 8 NPS. Neither those approvals nor those ratings attach to every pipe described as Schedule 40. Verify the actual product, listing and system requirements before using them. Wheatland submittal, p. 1

Ordering and substitution

Use the schedule as one field in a complete procurement description. Record the dimensional standard and edition, NPS/DN, full wall designation or explicit nominal wall, material specification and grade, welded or seamless construction, length and end preparation. Add coating, testing, certification and project requirements where applicable. The purchase description must resolve which requirements govern when a supplier's catalog offers several combinations.

For an engineering substitution, establish both dimensional fit and service suitability. Check the nominal and permitted actual bore, joint details, remaining wall allowances and material evidence. Where the design calculation requires a minimum wall, account for the applicable manufacturing tolerance and any specified service allowance rather than treating nominal schedule wall as guaranteed remaining wall. Obtain the governing product specification's actual tolerances; an older supplier summary is a useful explanation, not a replacement for the ordered edition. Atlas Technical Note 12, pp. 1–3, Swagelok system-selection guidance

Source versions and scope

Reviewed 2026-11-01, America/Los_Angeles. ASME's public front matter identifies B36.10-2022 issued June 30, 2022, and B36.19-2022 issued May 31, 2022; the public excerpts do not supply the full dimensional tables. Atlas Technical Note 12 prints revision 8 April 2021 and refers to older ASME editions in its table notes. Wheatland prints document code WFS-121824; its URL directory date is not a revision date. Swagelok prints MS-01-147, Rev T, May 2022, despite a copyright range ending 2024. Texas Flange's live chart has no explicit technical edition. Use the contractually applicable standard, including relevant errata, for acceptance. B36.10 front matter, B36.19 front matter

Stock forms and dimensions