| HOW TO MEASURE AND TELL WHAT THE SIZE OF A NPT THREAD-NATIONAL PIPE THREAD IS. |
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) OD Actual
(Inches) 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
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