Pipe Size


Update: August 16, 2003
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A common mistake for determining pipe size is measuring the physical outside pipe diameter (O.D.).
This method is wrong for common pipe sizes.


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 5 will be your nominal pipe thread size.

Column 1 Column 2 Column 3 Column 4 Column 5 Column 6 Column 7
OD Fraction Inch
(for reference only)
Actual (decimal) Inch. Schedule 40 ID Fraction (decimal)
(for reference only)
Schedule 80 ID Fraction (decimal)
(for reference only)
Pipe Size Normal engagement for tight joint
(dimension "C")
Threads per Inch 
5/16 0.3125 1/16 0.2611 27
13/32 0.405 0.269 0.215 1/8 0.2639 27
35/64 0.540 0.364 0.302 1/4 0.4018 18
43/64 0.675 0.493 0.423 3/8 0.4078 18
27/32 0.840 0.622 0.546 1/2 0.5337 14
1-3/64 1.050 0.824 0.724 3/4 0.5457 14
1-5/16 1.315 1.049 0.957 1 0.6828 11-1/2
1-21/32 1.660 1.380 1.278 1-1/4 0.7068 11-1/2
1-29/32 1.900 1.610 1.500 1-1/2 0.7235 11-1/2
2-3/8 2.375 2.067 1.939 2 0.7565 11-1/2
2-7/8 2.875 2.469 2.323 2-1/2 1.1375 8
3-1/2 3.5 3.069 2.900 3 1.2000 8
4 4.0 3.548 3.364 3-1/2 1.2500 8
4-1/2 4.5 4.026 3.826 4 1.3000 8
6-5/8 6.625 6.065 6
8-5/8 8.625 7.981 8

Table values are in inches
O.D. = Outside Diameter
I.D. = Inside Diameter


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 Teflon 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.