NPT Explained | Basic Power Industries FAQs
National Pipe Threads (NPT) Explained
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 |
Column 6 |
OD
Fraction Inch
(quick reference only) |
OD Actual
(inches) |
Pipe Thread
Size |
Normal engagement
for tight joint
(dimension "C") |
Threads
per Inch |
Circumference
(inches) |
5/16 |
0.3125 |
1/16 |
0.2611 |
27 |
0.9817 |
13/32 |
0.405 |
1/8 |
0.2639 |
27 |
1.272 |
35/64 |
0.540 |
1/4 |
0.4018 |
18 |
1.696 |
43/64 |
0.675 |
3/8 |
0.4078 |
18 |
2.121 |
27/32 |
0.840 |
1/2 |
0.5337 |
14 |
2.639 |
1-3/64 |
1.050 |
3/4 |
0.5457 |
14 |
3.299 |
1-5/16 |
1.315 |
1 |
0.6828 |
11-1/2 |
4.131 |
1-21/32 |
1.660 |
1-1/4 |
0.7068 |
11-1/2 |
5.215 |
1-29/32 |
1.900 |
1-1/2 |
0.7235 |
11-1/2 |
5.969 |
2-3/8 |
2.375 |
2 |
0.7565 |
11-1/2 |
7.461 |
2-7/8 |
2.875 |
2-1/2 |
1.1375 |
8 |
9.032 |
3-1/2 |
3.5 |
3 |
1.2000 |
8 |
10.995 |
4 |
4.0 |
3-1/2 |
1.2500 |
8 |
12.566 |
4-1/2 |
4.5 |
4 |
1.3000 |
8 |
14.137 |
5-5/8 |
5.563 |
5 |
1.4063 |
8 |
17.476 |
6-5/8 |
6.625 |
6 |
1.5125 |
8 |
20.812 |
8-5/8 |
8.625 |
8 |
1.7125 |
8 |
27.095 |
10-3/4 |
10.75 |
10 |
1.9250 |
8 |
33.771 |
12-3/4 |
12.75 |
12 |
2.1250 |
8 |
40.054 |