|
Male threads:
Measure the outside diameter of the large portion of the thread at
"A"; Find figure nearest this dimension in column 1 or 2 of
chart. The dimension in column 3 will be your nominal pipe thread size. |
|
Female Threads:
Measure top diameter of thread at "B"; Find figure nearest this
dimension in column 1 or 2 of chart. The dimension in column 3 will
be your nominal pipe thread size. |
| Column
1 |
Column
2 |
Column 3 |
Column
4 |
Column
5 |
|
OD Fraction Inch
(for quick reference only) |
Actual
(decimal) Inch. |
Pipe Thread Size |
Normal
engagement for tight joint (dimension "C") |
Threads
per Inch |
| 5/16 |
0.3125 |
1/16 |
0.2611 |
27 |
| 13/32 |
0.405 |
1/8 |
0.2639 |
27 |
| 35/64 |
0.540 |
1/4 |
0.4018 |
18 |
| 43/64 |
0.675 |
3/8 |
0.4078 |
18 |
| 27/32 |
0.840 |
1/2 |
0.5337 |
14 |
| 1-3/64 |
1.050 |
3/4 |
0.5457 |
14 |
| 1-5/16 |
1.315 |
1 |
0.6828 |
11-1/2 |
| 1-21/32 |
1.660 |
1-1/4 |
0.7068 |
11-1/2 |
| 1-29/32 |
1.900 |
1-1/2 |
0.7235 |
11-1/2 |
| 2-3/8 |
2.375 |
2 |
0.7565 |
11-1/2 |
| 2-7/8 |
2.875 |
2-1/2 |
1.1375 |
8 |
| 3-1/2 |
3.5 |
3 |
1.2000 |
8 |
| 4 |
4.0 |
3-1/2 |
1.2500 |
8 |
| 4-1/2 |
4.5 |
4 |
1.3000 |
8 |
|
Two common pipe
thread sizes exist, the tapered National Pipe Thread (NPT) and the
straight National Standard Free-Fitting Straight Mechanical Pipe Thread (NPSM).
The tapered threads are for joining and sealing, the straight threads are
only for joining. The Dry-seal thread (NPTF) allows for joining without
sealants. Three less common threads exist, the Garden Hose Thread (GHT),
Fire Hose Coupling (NST) and British Standard Taper Pipe Thread (BSPT).
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."
Pipe sizes do not refer to any physical dimensions. The outside diameter of
each pipe or fitting must be measured and compared to the table for size
identification. For example, a 3/4" NPT pipe thread has an outside
diameter of 1.050 inches. Each thread size has a defined number of threads
per inch (TPI). The 3/4" NPT pipe thread has 14 threads per inch.
Both the TPI and OD of the thread are required for positive identification
of thread size because several sizes have the same TPI. |
