October 28, 2026
A pipe nipple is a short length of pipe with male threads on one or both ends, used to connect two fittings, extend a run, or provide a transition. Carbon steel nipples are workhorses of plumbing, fire protection, oil & gas utilities, and OEM skids. Yet "size" is the single most misunderstood aspect of specifying them: the nominal label rarely matches a measured dimension. This reference decodes carbon steel pipe nipple sizes so your bills of material are unambiguous.
A nipple is cut from carbon steel pipe (typically ASTM A53 or A106 for the pipe body) and threaded at both ends (double male), one end (male-female "hose" nipple), or, in barrel form, with threads running its full length. The central, unthreaded section is the "plain" or "body" length. Nipples are produced as welded (ERW) or seamless, and supplied black (self-colour) or galvanized.
The first trap: NPS is a nominal designation, not a diameter. A "1/2-inch" nipple has an outside diameter (OD) of 0.840 in. (21.3 mm). The OD is standardized and does not change with wall thickness — only the inside diameter (ID) changes as the schedule increases. The reference for these dimensions is ASME B36.10M.
| NPS (in.) | Outside Diameter (in.) | Outside Diameter (mm) |
|---|---|---|
| 1/8 | 0.405 | 10.3 |
| 1/4 | 0.540 | 13.7 |
| 3/8 | 0.675 | 17.1 |
| 1/2 | 0.840 | 21.3 |
| 3/4 | 1.050 | 26.7 |
| 1 | 1.315 | 33.4 |
| 1-1/4 | 1.660 | 42.2 |
| 1-1/2 | 1.900 | 48.3 |
| 2 | 2.375 | 60.3 |
| 2-1/2 | 2.875 | 73.0 |
| 3 | 3.500 | 88.9 |
| 4 | 4.500 | 114.3 |
| 6 | 6.625 | 168.3 |
| 8 | 8.625 | 219.1 |
Rule of thumb: Always order and measure by NPS for fitting compatibility, but design clearances and wrench access using the actual OD from the table above.
The schedule defines wall thickness for a given NPS and therefore sets the inside diameter and pressure rating. Common schedules for nipples:
Because the OD is fixed, a thicker schedule reduces the ID and raises the pressure capability. The pipe material grade (e.g., ASTM A53 Grade B, A106 Grade B) and the selected schedule together determine the allowable working pressure, which must be confirmed against the system design code.
For NPT threads, the length of the threaded portion is not arbitrary. ASME B1.20.1 fixes the effective (L1) and overall (L2) thread lengths per size. The two mating parts must screw together until the full L1 is engaged to form a proper taper seal. Representative values:
These figures illustrate ordering only; always use the current ASME B1.20.1 table for fabrication and inspection.
A nipple's "length" is normally its overall length including the threaded ends. Common length descriptors:
ASTM A733 governs welded and seamless carbon steel (and austenitic stainless) pipe nipples, including length conventions and end configurations. Many suppliers also cut and thread to custom overall lengths — specify the exact dimension and thread standard on the order.
Nominal Pipe Size (NPS) is a named size, not a measured one. A 1/2 in. NPS pipe has an outside diameter of 0.840 in. (21.3 mm). This historical convention is defined in ASME B36.10M.
Schedule is the wall-thickness series. For a given NPS, Schedule 40 (STD) defines a specific wall thickness and inside diameter; Schedule 80 (XS) is thicker and rated for higher pressure. The outside diameter stays the same across schedules.
A close nipple is the shortest standard nipple: the two male threaded ends meet with essentially no unthreaded middle section. It is used where two fittings must be joined almost back-to-back.
For NPT threads, the effective thread length (L1) and overall thread length (L2) are fixed by ASME B1.20.1 and increase with size — roughly 0.47 in. for 1/2 in. NPS up to about 1.75 in. for 2 in. NPS. The two mating parts must engage to the full L1 for a sound seal.
Yes. While standard lengths follow ASTM A733 increments, many suppliers cut and thread nipples to custom lengths (e.g., 30 mm to 3000 mm) for skid-mounted packages and retrofit work. Specify the exact overall length and thread standard on the order.
Carbon steel pipe nipple sizing rests on three ideas: NPS is nominal (use the OD table for real dimensions), schedule sets the wall and pressure rating, and thread length follows ASME B1.20.1. By quoting NPS, schedule, exact length, and thread standard on every line item — and anchoring the spec to ASTM A733, ASME B36.10M, and ASME B1.20.1 — you eliminate the ambiguity that causes mismatched fittings and line rework.
