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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.

Definition of pipe thread acronyms
NPT	National Pipe Thread
FPT	Female Pipe Thread
MPT	Male Pipe Thread
PTF	SAE short tape pipe thread
NPTF	American National taper pipe thread for dryseal pressure-tight joints
NPSM	American National straight pipe thread for mechanical joints
NPSI	American National straight intermediate pipe thread

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