Pipe Dimensions: NPS, DN, Schedules, Lengths and Pipe Ends (ASME B36.10 Guide)

A pipe cannot be ordered, fitted or stress-checked from a tape measurement alone. Two pipes that look identical on site can differ by several millimetres in wall thickness, and an “NPS 6” pipe is not 6 inches across anywhere. This guide explains how pipe sizes are designated, how schedules set wall thickness, how small-bore and large-bore lines are treated differently, which mill lengths you can buy, and how each pipe end is prepared and joined. All tables can be filtered, exported to CSV/JSON or printed as a branded PDF for site use.

Key terminology in pipe sizing

Nominal Pipe Size (NPS)

NPS is the North American size designator for pipe. It is a label, not a measurement. When the system was created, the number roughly matched the bore of the heaviest-used wall of the day; as materials improved and walls got thinner, the outside diameter was frozen so that fittings stayed interchangeable, and the bore was allowed to change. The result is the rule every piping engineer learns early:

• NPS 1/8 to NPS 12: the actual OD is larger than the NPS number. NPS 2 has an OD of 2.375 in (60.3 mm); NPS 12 has an OD of 12.75 in (323.8 mm).
• NPS 14 and larger: the OD in inches equals the NPS number. NPS 24 has an OD of exactly 24.000 in (610 mm).

Nominal Diameter (DN)

DN is the metric designator defined in ISO 6708. Like NPS, it is a reference number used to match pipe, fittings, flanges and valves, not a diameter you can measure. For NPS 4 and above, DN is normally 25 × NPS (NPS 8 = DN 200). Below NPS 4 the pairs are fixed by convention (NPS 1/2 = DN 15, NPS 2 = DN 50, NPS 3 = DN 80).

Outside diameter (OD), wall thickness (WT) and inside diameter (ID)

For any NPS the OD stays the same whatever the schedule. Changing the schedule only moves the bore inward. That is why the same weld-neck flange bore or socket-weld fitting must be matched to the wall thickness, while the outside fit-up stays constant.

NPS to DN to OD conversion chart

The table below lists the ASME outside diameter for each NPS and the nearest European (EN 10220 / ISO 4200 series 1) outside diameter. Rows flagged ⚠ are where ASME and EN pipe do not share the same OD, so ASME fittings will not fit EN pipe without a transition. This mismatch is a common cause of rejected material on mixed-standard projects.

Pipe is specified in two equivalent ways: as OD × WT (for example Ø 88.9 × 5.49 mm), which is common on European and Asian drawings, or as NPS × schedule (for example NPS 3 Sch 40), which is common in ASME-based specifications. Both describe the same pipe. Because API 5L line pipe and many EN products are ordered by OD and wall, it is good practice to show both on line lists and MTO sheets.

Pipe schedules and wall thickness

The schedule number is a dimensionless designator for nominal wall thickness. It was originally linked to the ratio Sch ≈ 1000 × P / S (internal pressure over allowable stress), but today it is simply a lookup key into the ASME tables. Two families are in use:

• Carbon and alloy steel, ASME B36.10-2022: Sch 10, 20, 30, 40, 60, 80, 100, 120, 140, 160 and the weight classes STD (standard), XS (extra strong) and XXS (double extra strong).
• Stainless steel, ASME B36.19-2022: Sch 5S, 10S, 40S and 80S. The “S” suffix matters: 40S equals Sch 40 only up to NPS 10, and 80S equals Sch 80 only up to NPS 8.

A higher schedule gives a thicker wall, a smaller bore, more weight and more pressure capacity at the same OD. Pressure capacity is not fixed by schedule alone; it depends on material allowable stress, temperature, corrosion allowance and joint efficiency, so wall thickness must be confirmed by calculation to ASME B31.3 (process), B31.1 (power) or B31.4/B31.8 (pipelines). See our pipe sizes and pipe schedule guide for the pressure-design side.

Free pipe dimension calculator

Select a size and schedule to get ID, minimum wall, steel mass, water-filled mass and internal volume. Use the override box for non-standard walls (e.g. API 5L line pipe ordered by OD × WT).

Manufacturing tolerances on OD and wall

Nominal dimensions are not what arrives on site. ASTM product specifications allow a band around the nominal OD and an under-tolerance on wall thickness. Fit-up, bevel matching and pressure design all have to respect these limits.

Small-bore vs. large-bore pipes

Most piping specifications split the line list at NPS 2 (DN 50). The split is not about pressure; it changes how the pipe is joined, how it is supported and how it is analysed.

• Small-bore pipe (NPS 2 and below): vents, drains, instrument connections, sample points and utility branches. These are often treated as non-critical and handled by standard details, but they are the single biggest source of fatigue leaks in process plants. A short Sch 80 branch on a vibrating header with an unbraced valve on the end is a classic failure point.
• Large-bore pipe (above NPS 2): the main process and utility lines. These are butt-welded and flanged, and hot, large or critical lines go through formal flexibility and stress analysis.

Standard pipe lengths

Steel pipe is supplied in random lengths because mills cut each tube to the length the process produces, within a band. The two common categories are single random length (SRL), nominally about 6 m (20 ft), and double random length (DRL), nominally about 12 m (40 ft). Ordering DRL for long runs reduces the number of field welds, NDT and fit-up hours; SRL is easier to transport, handle in pipe racks and fit inside modules.

Common types of pipe ends

End preparation decides how a pipe is joined, how strong the joint is, whether it can be dismantled and what inspection it needs. The end type is part of the pipe description on every MTO and purchase order (PE, BE, TE, grooved, flanged), so getting it wrong means rework at site.

Plain ends (PE)

A plain end is a square cut at 90° to the pipe axis with burrs removed. It is the standard end for small-bore pipe joined with socket-weld fittings, slip-on flanges or mechanical couplings, all of which use fillet welds or clamps rather than full-penetration welds. Fit-up is easy and no weld metal enters the bore, but the small gap at the socket can trap liquid, so many specifications forbid socket welds in severe erosive, crevice-corrosive or radioactive service.

Bevelled ends (BE)

Bevelled ends, also called weld ends or butt-weld ends, are prepared to ASME B16.25-2022. The standard preparation for walls above about 3 mm and up to 22 mm is a 37.5° ± 2.5° bevel with a 1.6 ± 0.8 mm (1/16 ± 1/32 in) root face. Walls thicker than 22 mm get a compound bevel to save weld metal. Very thin walls may be square cut or lightly chamfered. Butt welds give the strongest, most leak-tight joint and are standard for large-bore and hazardous service; their drawbacks are the welding and NDT effort and a small internal weld bead that affects pigging and very clean services.

Threaded ends (TE)

Threaded pipe carries a tapered external thread, in North America to ASME B1.20.1 (NPT), with a taper of 1 in 16 on diameter and a 60° thread form. The taper wedges the joint tight, usually with PTFE tape or thread sealant. Threaded joints need no welding, which helps where hot work is restricted, and are common in utility, air and water lines up to about NPS 2. Because the thread removes part of the wall and creates a notch, B31.3 restricts threaded joints in severe cyclic conditions and requires a sufficiently heavy wall (typically Sch 80 for small sizes). Avoid them for toxic, flammable or high-pressure fluids unless seal-welded and permitted by the pipe class. ISO 7-1/BSPT threads are not interchangeable with NPT.

Grooved ends

A groove is roll-formed or machined near the pipe end to suit a gasketed coupling housing, typically to AWWA C606. Bolting the housing compresses the gasket and locks into the groove, giving a joint that installs quickly, absorbs small movement and comes apart without cutting. Grooved systems are widely used for fire protection, HVAC, cooling water and mining. Pressure and temperature limits come from the coupling manufacturer and the gasket elastomer.

Flanged ends

Flanged pipe has a weld-neck, slip-on or integral flange so that the joint can be bolted with a gasket. Flanges are dimensioned to ASME B16.5 up to NPS 24 and ASME B16.47 from NPS 26 to 60. They allow easy dismantling at equipment, valves and spool breaks and avoid site welding where materials or fire risk make welding difficult. Each flanged joint is a potential leak path, so bolt load, gasket selection and external bending moments must be checked.

Socket and spigot (bell) ends

One end is enlarged into a socket (bell) and the other left as a spigot. They are standard in ductile iron (ISO 2531, AWWA C111 push-on joints), PVC and GRP water and drainage lines. Push-on joints tolerate a small angular deflection, typically 3° to 5° depending on size and manufacturer, but are not restrained against end thrust, so bends and tees need thrust blocks or restrained joints.

Buttress (flared) ends

Borosilicate glass process pipe (ISO 3587) uses flared or conical buttress ends joined with backing flanges and a gasket. The joint suits corrosive, low-pressure service in chemical and pharmaceutical plants but must be protected from impact, thermal shock and over-tightening.

How to specify a pipe correctly (6 steps)

1. Fix the size. State NPS and DN (e.g. NPS 6 / DN 150) and confirm the OD standard: ASME B36.10/B36.19 or EN 10220.
2. Fix the wall. Give schedule or wall in mm (Sch 40 = 7.11 mm). Check that the nominal wall minus 12.5 % mill tolerance and corrosion allowance still meets the B31.3 pressure-design thickness.
3. Fix the material. Name specification, grade and manufacturing route: e.g. ASTM A106 Gr B seamless, ASTM A312 TP316L welded, API 5L X52 PSL2.
4. Fix the ends. State PE, BE (to ASME B16.25), TE (NPT to ASME B1.20.1) or grooved, matching the joint table in the pipe class.
5. Fix the length. Order SRL, DRL or fixed cut lengths with tolerance, and add a cutting allowance to the take-off.
6. Fix inspection and documents. Call up hydrotest or NDE, marking, end caps, and EN 10204 3.1 mill certificates.

Common pipe dimension mistakes to avoid

• Ordering by NPS only: An NPS 6 pipe can be 3.40 mm (10S) or 21.95 mm (XXS). Always state schedule or wall.
• Mixing ASME and EN pipe: DN 65 and DN 125 have different ODs; ASME fittings will not butt up to EN pipe.
• Using nominal wall in pressure design: Deduct 12.5 % mill tolerance plus corrosion and thread allowances first.
• Assuming STD = Sch 40 everywhere: True only to NPS 10. A wrong assumption at NPS 12+ mismatches flange bores and bevels.
• Threading thin stainless pipe: 5S and 10S walls are too thin to thread; use socket-weld, butt-weld or grooved joints.
• Bottoming socket welds: Leave about 1.6 mm gap to avoid shrinkage cracking.

Frequently asked questions

Quiz: test your knowledge of pipe dimensions

Standards referenced

• ASME B36.10-2022, Welded and Seamless Wrought Steel Pipe
• ASME B36.19-2022, Welded and Seamless Wrought Stainless Steel Pipe
• ASME B16.25-2022, Buttwelding Ends
• ASME B31.3-2024, Process Piping
• ASME B16.5-2025 Pipe Flanges and Flanged Fittings; ASME B16.47 Large Diameter Steel Flanges; ASME B16.11 Forged Fittings, Socket-Welding and Threaded
• ASME B1.20.1, Pipe Threads, General Purpose (Inch)
• ASTM A53/A53M, A106/A106M, A312/A312M, A530/A530M; API Spec 5L
• ISO 6708 (DN), EN 10220 / ISO 4200 (metric OD series), ISO 2531, ISO 3587, AWWA C606, AWWA C111
• Energy Institute, Guidelines for the Avoidance of Vibration Induced Fatigue Failure in Process Pipework (AVIFF)

Dimensional data in this article are reproduced as engineering summaries for education and quick reference. For design, procurement and inspection always use the current licensed edition of each standard named in your contract.