NPT is NATIONAL PIPE TAPER (AMERICAN)                  Thread form per ANSI/ASME B1.20.1 
NPS is NATIONAL PIPE STRAIGHT (PARALLEL)              Thread form per ANSI/ASME B1.20.1 
NPTF is NATIONAL PIPE TAPER FOR FUEL                      Thread form per SAE J476 
NPSM is NATIONAL PIPE STRAIGHT MECHANICAL       Thread form per ANSI/ASME B1.20.1

National Pipe threads are similar in function to BSP threads, but are not generally interchangeable.
NPTF threads (also known as Dryseal) are an improvement to NPT.
Controlled truncation of threads mean the metal-to-metal thread seal is at root and crest of threads, in addition to flanks of threads.
Use of thread sealant is recommended for NPT male and NPT female connection.

Male thread Outside Diameter (OD), and female thread Inside Diameter (ID) are measured at the first full thread near end of the fitting.

 

NPT Threads Dimensions

   NPT

NPT THREAD
SIZE

PITCH

MALE THREAD OD

FEMALE
THREAD ID

 inch

TPI

mm

inch

 mm

inch

1/8

27

 9.9

 0.39

8.4

 0.33

 1/4

18

13.2

 0.52

 11.2

0.44

3/8

18

 16.6

 0.65

14.7

0.58

1/2

14

20.6

0.81

17.8

0.70

3/4

14

26.0

1.02

23.4

0.92

1

11.5

32.5

1.28

29.5

1.16

1  1/4

11.5

41.2

1.62

38.1

1.50

1  1/2

11.5

47.3

1.86

43.9

1.73

2

11.5

59.3

2.33

56.4

2.22

2  1/2

8

71.5

2.82

69.1

2.72

3

8

87.3

3.44

84.8

3.34

   NPSM

NPSM THREAD
SIZE

PITCH

FEMALE
THREAD ID

inch

 TPI

mm

inch

1/8

27

8.6

0.34

1/4

18

11.9

0.47

3/8

18

15.0

0.59

1/2

14

19.1

0.75

3/4

14

24.6

0.97

1

11.5

30.5

1.20

1  1/4

11.5

39.4

1.55

1  1/2

11.5

45.5

1.79

2

11.5

57.4

2.26

2  1/2

8

68.8

2.71

3

8

84.6

3.33

Thread size refers to the nominal bore of the pipe.
Subtract approximately 1/4” (6 mm) from thread measurement for nominal pipe size.
Pitch is Threads Per Inch (TPI).