By Peter Winne
Common pipe-related acronyms
| NPT | National (American) Pipe Thread Tapered |
| NPTF | National (American) Pipe Thread Tapered variation for Fuel |
| NPS | Two different usages. re: thread size: National (American) Pipe Straight (parallel thread instead of taper) re: pipe diameter Nominal Pipe Size |
| NPSF | National (American) Pipe Straight (parallel thread instead of taper) Fuel * References internal female thread. Commonly mated with male NPTF although not ideal w/ male taper to straight female |
| BSPT | British Standard Pipe Thread Tapered |
| BSPP | British Standard Pipe Thread Parallel |
| IPS | Iron Pipe Straight |
| IPT | Iron Pipe Thread |
| TPI | Threads Per Inch |
| WOG | Water Oil Gas |
| SCH | Schedule |
| UNF | Unified Fine Thread Series |
Important Points for Pipe Specs
- Nominal Pipe Size (NPS) is NEVER the actual measurement of either the OD (Outside diameter) or ID (Inside Diameter) of the threaded male or female fitting. Therefore, simply measuring the width of the pipe will not suffice. Reference measurements to a table, or try mating with a known pipe size before ordering.
- Tapered Pipe Thread is found on ends of pipe (male), Nipples, Fittings (i.e., unions, couplings, elbows, tees, etc.). Note: Sharply angled threads are very critical to the joint being tight, with no leaks. Thread sealant or tape is typically required to complete the seal.
- Straight Pipe Threads are Found on thru-hulls, seacocks, adapters, used to facilitate lock nuts; needs gaskets or O-rings to seal.
National Pipe Thread Tapered (NPT) is a U.S. standard for tapered threads used to join pipes and fittings. ANSI/ASME standard B1.20.1 covers threads of 60-degree form with flat crests and roots in sizes from 1/16 inch to 24 inch Nominal Pipe Size [NPS] (this standard also covers various parallel ('straight') threads). Commonly-used sizes are ⅛, ¼, ⅜, ½, ¾, 1, 1¼, 1½, and 2 inch, appearing on pipe and fittings by most U.S. suppliers. Smaller sizes than those listed are occasionally used for compressed air. Larger sizes are used less frequently because other methods of joining are more practical at 3 inches and above in most applications.
Nominal Pipe Size is loosely related to the inside diameter of schedule 40 pipe. Because of the pipe wall thickness, the actual diameter of the threads is larger than the NPS, considerably so for small NPS. Other schedules of pipe have different wall thickness but the OD (outer diameter) and thread profile remain the same, so the inside diameter of the pipe is therefore different from the nominal diameter.
Threaded pipes can provide an effective seal for pipes transporting liquids, gases, steam, and hydraulic fluid. These threads are now used in materials other than steel and brass, including PVC, nylon, bronze and cast iron.
The taper on NPT threads allows them to form a seal when torqued as the flanks of the threads compress against each other, as opposed to parallel/straight thread fittings or compression fittings in which the threads merely hold the pieces together and do not provide the seal. However, a clearance remains between the crests and roots of the threads, resulting in a leakage around this spiral. This means that NPT fittings must be made leak-free with the aid of thread seal tape or a thread sealant compound. The use of tape or sealant will also help to limit corrosion on the threads, which otherwise can make future disassembly nearly impossible.
There is also a semi-compatible variant called National Pipe Thread Tapered Fine (NPTF) also called Dryseal American National Standard Taper Pipe Thread, defined by ANSI B1.20.3, designed to provide a more leak-free seal without the use of Teflon tape or other sealant compound. NPTF threads are the same basic shape, but with crest and root heights adjusted for an interference fit, eliminating the spiral leakage path.
Sometimes NPT threads are referred to as MPT ('Male Pipe Thread'), MNPT, or NPT(M) for male (external) threads; and FPT ('Female Pipe Thread'), FNPT, or NPT(F) for female (internal) threads. An equivalent designation is MIP (Male iron pipe) and FIP (Female iron pipe). Also, the terms NPS and NPSM are sometimes used to designate a parallel, instead of taper, thread (this should not be confused with NPS meaning Nominal Pipe Size).
NPT Pipe Thread Sizes
| Nominal NPT Size | Outer Diameter inch (mm) | Threads per inch | Pitch of Thread inch (mm) |
| 1/16" | 0.3125 in (7.94 mm) | 27 | 0.03704 in (0.94082 mm) |
| ⅛" | 0.405 in (10.29 mm) | 27 | 0.03704 in (0.94082 mm) |
| ¼" | 0.540 in (13.72 mm) | 18 | 0.05556 in (1.41122 mm) |
| ⅜" | 0.675 in (17.15 mm) | 18 | 0.05556 in (1.41122 mm) |
| ½" | 0.840 in (21.34 mm) | 14 | 0.07143 in (1.81432 mm) |
| ¾" | 1.050 in (26.67 mm) | 14 | 0.07143 in (1.81432 mm) |
| 1" | 1.315 in (33.40 mm) | 11½ | 0.08696 in (2.20878 mm) |
| 1¼" | 1.660 in (42.16 mm) | 11½ | 0.08696 in (2.20878 mm) |
| 1½" | 1.900 in (48.26 mm) | 11½ | 0.08696 in (2.20878 mm) |
| 2" | 2.375 in (60.33 mm) | 11½ | 0.08696 in (2.20878 mm) |
| 2½" | 2.875 in (73.02 mm) | 8 | 0.12500 in (3.175 mm) |
| 3" | 3.500 in (88.90 mm) | 8 | 0.12500 in (3.175 mm) |
| 4" | 4.500 in (114.30 mm) | 8 | 0.12500 in (3.175 mm) |
| 5" | 5.563 in (141.30 mm) | 8 | 0.12500 in (3.175 mm) |
| 6" | 6.625 in (168.27 mm) | 8 | 0.12500 in (3.175 mm) |
| 10" | 10.750 in (273.05 mm) | 8 | 0.12500 in (3.175 mm) |
| 12" | 12.750 in (323.85 mm) | 8 | 0.12500 in (3.175 mm) |
| 14" OD | 14 in (355.60 mm) | 8 | 0.12500 in (3.175 mm) |
| 16" OD | 16 in (406.40 mm) | 8 | 0.12500 in (3.175 mm) |
| 18" OD | 18 in (457.20 mm) | 8 | 0.12500 in (3.175 mm) |
| 20" OD | 20 in (508.00 mm) | 8 | 0.12500 in (3.175 mm) |
| 24" OD | 24 in (609.60 mm) | 8 | 0.12500 in (3.175 mm) |
British Standard Pipe (BSP) threads are used commonly in many countries outside the U.S. The most common BSP threaded fittings are tapered with pressure tight joints achieved by the threads. While similar in appearance to National Pipe Taper (NPT) threaded fittings, the two styles are not interchangeable.
BSP and NPT threads are not compatible due to the differences in their thread forms, and not just the fact that most diametrical sizes have a different pitch. BSP threads have a 55° included angle and have rounded peaks and valleys (this is a Whitworth thread form); NPT threads have a 60° included angle and have flattened peaks and valleys (this is a Sellers thread form).
Note: BSP fittings are NOT compatible with metric pipe thread.