| Nominal Pipe Size (inch) | Number of threads per inch | Outside Diameter (inch) | Fitting Allowance |
| 1?16 | 27 | 0.3125 | 0.2611 |
| 1/8″ | 27 | 0.405 | 0.2639 |
| 1/4″ | 18 | 0.54 | 0.4018 |
| 3/8″ | 18 | 0.675 | 0.4078 |
| 1/2″ | 14 | 0.84 | 0.5337 |
| 3/4″ | 14 | 1.05 | 0.5457 |
| 1″ | 11.5 | 1.315 | 0.6828 |
| 1-1/4″ | 11.5 | 1.66 | 0.7068 |
| 1-1/2″ | 11.5 | 1.9 | 0.7235 |
| 2″ | 11.5 | 2.375 | 0.7565 |
| 2-1/2″ | 8 | 2.875 | 1.1375 |
| 3″ | 8 | 3.5 | 1.2 |
| 3-1/2″ | 8 | 4 | 1.25 |
| 4″ | 8 | 4.5 | 1.3 |
| 5″ | 8 | 5.563 | 1.4063 |
| 6″ | 8 | 6.625 | 1.5125 |
| 8″ | 8 | 8.625 | 1.7125 |
| 10″ | 8 | 10.75 | 1.925 |
| 12″ | 8 | 12.75 | 2.125 |
| 14″ | 8 | 14 | 2.25 |
| 16″ | 8 | 16 | 2.45 |
| 18″ | 8 | 18 | 2.65 |
| 20″ | 8 | 20 | 2.85 |
| 24″ | 8 | 24 | 3.25 |
- Reference ANSI/ASME B1.20.1-1983 (R1992)
- Angle between sides of thread is 60 degrees.
- Thread taper, is 3⁄4 inch per foot.
- All units are in inches.
All pipe thread types used in plumbing are specified by the American National Standard for pipe as accredited by the American National Standards Institute (ANSI).
National Pipe Threads (NPT) have tapered threads. These are the most common threads used for general purposes. NPT threads are used for joining and sealing pipe to fittings in low pressure air or liquids and also mechanical applications. The tapered thread is 3/4″ over one foot of length. Tapered threads are deeper at the end of the pipe and are increasingly shallower the further they are from the end of the pipe. The taper on the pipe only allows the pipe to screw inside the fitting until it is forced to stop because of the taper. The distance the pipe can be screwed into the fitting is specified by the ANSI standard. After tightening with a wrench the threads may have slight spaces between the pipe and fitting which could cause a leak so a pipe sealant must be used to ensure any gaps are filled.
The Dry-seal thread (NPTF) also have tapered threads. NPTF threads are used when the application is such that pipe sealing compounds may fail due to higher heat or pressure than normal NPT threads can withstand. The threads are designed to seal mechanically by slightly, but sufficiently, crushing the threads when tightened with a wrench. This allows for joining the pipe and fitting without sealants.
The NPT and NPTF threads are interchangeable with sealants such as PTFE tape. None of the other thread standards are interchangeable. Female NPT threads can be designated as “FPT” and male NPT threads can be designated as “MPT.”
National Standard Free-Fitting Straight Mechanical Pipe Threads (NPSM) have straight threads which are only used for joining. A washer or gasket is needed to seal this type of threaded connection.
Three less common threads exist, the Garden Hose Thread (GHT), Fire Hose Coupling (NST) and British Standard Taper Pipe Thread (BSPT).
| Definitions of Pipe Thread Acronyms | |
|---|---|
| NPT | National Pipe Thread (tapered) |
| FPT | Female Pipe Thread (NPT) |
| MPT | Male Pipe Thread (NPT) |
| IPS | Iron pipe size (NPT) |
| PTF | SAE short taper pipe thread – dryseal |
| NPTF | American National taper pipe thread for dryseal pressure-tight joints |
| NPSM | American National straight pipe thread for mechanical joints |
| NPSI | American National straight intermediate pipe thread |
| GHT | Garden Hose Thread |
| NST | Fire Hose Thread |
| BSPT | British Standard taper pipe thread (metric dimensions) |