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I went through this when I was very young and it is confusing.

Here is the same as below but the website which is easier to figure out.

Its two different worlds...I hope this helps.

http://www.plumbingstore.com/pipethreadsizing.html


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


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.






thats absolutely awesome! you spent way more time explaining this and I would have. Two thumbs up.