This table provides dimensions of pipe threads for various standard sizes.
The threads per inch and outside diameter of the pipe for each standard pipe size is provided. This data allows for proper identification of pipe sizes.
Pipe Dimensions ------ -------- ------- Pipe Threads Outside Size per Inch Dia. ------ -------- ------- 1/8" 27 0.405" 1/4" 18 0.540" 3/8" 18 0.675" 1/2" 14 0.840" 3/4" 14 1.050" 1" 11-1/2 1.315" 1-1/4" 11-1/2 1.660" 1-1/2" 11-1/2 1.900" 2" 11-1/2 2.375" 2-1/2" 8 2.875" 3" 8 3.500" 3-1/2" 8 4.000" 4" 8 4.500" 5" 8 5.563" 6" 8 6.625" 8" 8 8.625" 10" 8 10.750" 12" 8 12.750" ------ -------- -------
Two common pipe thread sizes exist, the tapered National Pipe Thread (NPT) and 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 Dryseal 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.