NPT Thread Dimensions Calculator was developed to show basic dimensions of general purpose NPT thread (inch) according to ANSI/ASME B1.20.1-1983 standard.

Some important notes from the ANSI/ASME B1.20.1-1983 standard:

- For left hand threads, include LH to the designation. (Example: 1/4 - 18 NPT - LH)

- For pressure-tight joints, it is intended that taper pipe threads be made up wrench-tight with a sealant. To prevent galling on stainless steels and similar materials, the sealant usually contains a lubricant.

- For pressure-tight joints which may be used without sealant (Dryseal Compounds), check ANSI B1.20.3 standard.

- Thread taper angle = 1/16 taper = 1 deg. 47 min = 0.75 in./ft .

- The angle between the sides of the thread is 60 deg.

- Thread Designation: The nominal pipe size, number of threads per inch and the thread series symbol in sequence. Ex: 3/8 - 18 NPT

- Standard does not include an allowance for plating. If plating is desired, it may be necessary to modify the threads since the same final gauging requirements must be applied for plated and unplated parts.

American National Standard Taper Pipe Thread (NPT)
American National Standard Taper Pipe Thread (NPT)


Parameter Value
Nominal Pipe Size

Notes: If the use of reamer selection is "yes" for taper pipe thread, then ream the hole before tapping with a reamer having a taper of 3/4 inch per foot.


Value Symbol Parameter Unit
Nominal Pipe Size - --- ---
Outer Diameter of Pipe D --- inch
Threads per inch n --- ---
Pitch of Thread P --- inch
Pitch Diameter at Beginning
 of External Thread
E0 ---
Hand-tight Engagement
 Length a
L1a ---
--- thread
Hand-tight Engagement
 Diameter b
E1b --- inch
External Effective
Thread Length c
L2c ---
--- thread
External Effective
Thread Diameter
E2 --- inch
Length L1 plane to L2 plane
(External Thread)
L2-L1 ---
--- thread
Wrench Makeup Length
for Internal Threadf
L3f --- inch
--- thread
Wrench Makeup
E3f --- inch
Vanish Thread V ---
--- thread
Overall External
Thread Length
L4 --- inch
Nominal Complete
External Thread Lengthd
L5d ---
Nominal Complete
External Thread Diameterd
E5d ---
Basic Height of Thread h ---
Maximum Height of Thread hmax ---
Minimum Height of Thread hmin ---
Increase in Diameter / Thread - ---
Basic Minor Diameter
at Small End of Pipe e
K0e ---
Maximum Thread Truncation  fmax ---
Minimum Thread Truncation fmin ---
Height of Sharp V Thread ---
Tolerance - Taper on Pitch Line (3/4 in./ft) Max. --- in/ft
Min. ---
Tolerance - Lead in Length of Effective Threads - --- in/in
Tolerance - 60 Degree Angle of Threads - --- deg

Note: The dimensions of the NPT Thread are given to four or five decimal places. While this implies a greater degree of precision than is ordinarily achieved, these dimensions are the basis of gage dimensions and are so expressed for the purpose of eliminating errors in computations.
Notea: Also length of thin ring gage and length from gauging notch to small end of plug gage.
Noteb: Also pitch diameter at gauging notch (hand-tight plane).
Notec: Also length of plug gage.
Noted: L5 from the end of the pipe determines the plane beyond which the thread form is imperfect at the crest. The next two threads are perfect at the root. At this plane the cone formed by the crests of the thread intersects the cylinder forming the external surface of the pipe. L5 = L2 − 2p.
Notee: Given as information for use in selecting tap drills. See Drill Sizes for NPT Pipe Threads Calculator.
Notef: MIL—P—7105 gives the wrench makeup as three threads for 3 in. and smaller. The E3 dimensions are then as follows for MIL—P—7105: Size 2 1⁄2 in., 2.69609 and size 3 in., 3.31719.

- The maximum allowable deviation in the commercial product is one turn large or small from gages made to the basic dimensions.

NPT Pipe Threads Manufacturing Tolerances
NPT Pipe Threads Manufacturing Tolerances


NPT: National Pipe Thread Taper


Link Usage
NPT Thread Chart Tabulated form for NPT thread calculator

  • ANSI/ASME B1.20.1 - 1983 Pipe Threads, General Purpose (Inch)
  • Oberg, E. , Jones ,F.D. , Horton H.L. , Ryffel H.H., (2016) . Machinery's Handbook . 30th edition.  Industrial Press Inc.
  • Oberg, E. , Jones ,F.D. , Horton H.L. , Ryffel H.H., (2012) . Machinery's Handbook . 29th edition.  Industrial Press Inc. , pp 1956 - 1965