For npt pipe thread fittings, ASME B1.20.1 is the governing document for the dimensions listed below. Mechanical and performance requirements for this part family are set by its own standards, not by the bolt and screw property-class documents.
ASME B1.20.1 — Scope
ASME B1.20.1 specifies the dimensional requirements for npt pipe thread fittings. Material, surface condition and any performance rating are set by separate documents specific to this part family — ISO 898-1 and ISO 898-2, which govern bolt and nut property classes, do not apply here.
Tolerance & Product Grades
Tolerances for npt pipe thread fittings are given as limits on the individual dimensions in the table rather than as a fastener product grade. Where a general-tolerance class is cited on the drawing instead, it applies to the dimensions not otherwise toleranced.
Dimensions Specified by ASME B1.20.1
| Size | Nominal size (in) | Major diameter (mm) | Pitch (mm) | Threads per inch (TPI) | Taper | Reference standard | Notes |
|---|---|---|---|---|---|---|---|
| 1/16 NPT | 1/16 | 7.938 | 0.941 | 27 | 1:16 | ASME B1.20.1 | Tapered pipe thread, self-sealing |
| 1/8 NPT | 1/8 | 10.287 | 0.941 | 27 | 1:16 | ASME B1.20.1 | Tapered pipe thread, self-sealing |
| 1/4 NPT | 1/4 | 13.716 | 1.411 | 18 | 1:16 | ASME B1.20.1 | Tapered pipe thread, self-sealing |
| 3/8 NPT | 3/8 | 17.145 | 1.411 | 18 | 1:16 | ASME B1.20.1 | Tapered pipe thread, self-sealing |
| 1/2 NPT | 1/2 | 21.336 | 1.814 | 14 | 1:16 | ASME B1.20.1 | Tapered pipe thread, self-sealing |
| 3/4 NPT | 3/4 | 26.67 | 1.814 | 14 | 1:16 | ASME B1.20.1 | Tapered pipe thread, self-sealing |
| 1 NPT | 1 | 33.401 | 2.209 | 11.5 | 1:16 | ASME B1.20.1 | Tapered pipe thread, self-sealing |
| 1-1/4 NPT | 1-1/4 | 42.164 | 2.209 | 11.5 | 1:16 | ASME B1.20.1 | Tapered pipe thread, self-sealing |
| 1-1/2 NPT | 1-1/2 | 48.26 | 2.209 | 11.5 | 1:16 | ASME B1.20.1 | Tapered pipe thread, self-sealing |
| 2 NPT | 2 | 60.325 | 2.209 | 11.5 | 1:16 | ASME B1.20.1 | Tapered pipe thread, self-sealing |
| 2-1/2 NPT | 2-1/2 | 73.025 | 3.175 | 8 | 1:16 | ASME B1.20.1 | Tapered pipe thread, self-sealing |
| 3 NPT | 3 | 88.9 | 3.175 | 8 | 1:16 | ASME B1.20.1 | Tapered pipe thread, self-sealing |
| 3-1/2 NPT | 3-1/2 | 101.6 | 3.175 | 8 | 1:16 | ASME B1.20.1 | Tapered pipe thread, self-sealing |
| 4 NPT | 4 | 114.3 | 3.175 | 8 | 1:16 | ASME B1.20.1 | Tapered pipe thread, self-sealing |
| 5 NPT | 5 | 141.3 | 3.175 | 8 | 1:16 | ASME B1.20.1 | Tapered pipe thread, self-sealing |
| 6 NPT | 6 | 168.275 | 3.175 | 8 | 1:16 | ASME B1.20.1 | Tapered pipe thread, self-sealing |
Equivalent Standards
Where direct equivalents exist (ISO ↔ DIN, ISO ↔ ASME), they are listed on the parent NPT Pipe Thread Fittings page. Always confirm against the dimensions on this page, since some "equivalent" standards differ in tolerances and head geometry.
When to Specify ASME B1.20.1
ASME B1.20.1 is the inch-based reference; for metric programs choose the ISO equivalent instead.
