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NPT Threads (National Pipe Tapered )

NPT Threads

Nominal Pipe Size (inch) No. of threads per inch Outside Dia. (inch) Outside Dia. (mm) Pitch of thread Depth of thread Truncation, max. Pitch diameter at plane of hand-tight engagement Length from end of pipe to plane of hand-tight engagement Length of useful thread Length of washout thread Pitch Diameter at Beginning of External Thread Start (inch) Effective Thread, External Diameter (inch)
P h l E L1 L2
in mm in mm in mm in mm in Threads in Threads in Threads
1⁄16 " NPT 27 0.3125 7.938 0.03704 0.941 0.02963 0.753 . . . . . . 0.28118 . . . 0.16 . . . 0.2611 . . . 0.1285 3.47 0.27118 0.2875
1/8" NPT 27 0.405 10.287 0.03704 0.941 0.02963 0.753 0.0036 0.091 0.3736 9.489 0.162 4.36 0.2639 7.12 0.1285 3.47 0.36351 0.38
1/4" NPT 18 0.54 13.716 0.05556 1.411 0.04444 1.129 0.0049 0.124 0.49163 12.487 0.228 4.1 0.4018 7.23 0.1928 3.47 0.47739 0.5025
3/8" NPT 18 0.675 17.145 0.05556 1.411 0.04444 1.129 0.0049 0.124 0.62701 15.926 0.24 4.32 0.4078 7.34 0.1928 3.47 0.61201 0.6375
1/2" NPT 14 0.84 21.336 0.07143 1.814 0.05714 1.451 0.0056 0.142 0.77843 19.772 0.32 4.48 0.5337 7.47 0.2478 3.47 0.75843 0.79179
3/4" NPT 14 1.05 26.67 0.07143 1.814 0.05714 1.451 0.0056 0.142 0.98887 25.117 0.339 4.75 0.5457 7.64 0.2478 3.47 0.96768 1.00179
1" NPT 11.5 1.315 33.401 0.08696 2.209 0.06957 1.767 0.0063 0.16 1.23863 31.461 0.4 4.6 0.6828 7.85 0.3017 3.47 1.21363 1.2563
1 1/4" NPT 11.5 1.66 42.164 0.08696 2.209 0.06957 1.767 0.0063 0.16 1.58338 40.218 0.42 4.83 0.7068 8.13 0.3017 3.47 1.55713 1.6013
1 1/2" NPT 11.5 1.9 48.26 0.08696 2.209 0.06957 1.767 0.0063 0.16 1.82234 46.287 0.402 4.83 0.7235 8.32 0.3017 3.47 1.79609 1.8413
2" NPT 11.5 2.375 60.325 0.08696 2.209 0.06957 1.767 0.0063 0.16 2.29627 58.325 0.436 5.01 0.7565 8.7 0.3017 3.47 2.26902 2.3163
2 1/2" NPT 8 2.875 73.025 0.125 3.175 0.1 2.54 0.0078 0.198 2.76215 70.159 0.682 5.46 1.1375 9.1 0.4337 3.47 2.71953 2.79062
3" NPT 8 3.5 88.9 0.125 3.175 0.1 2.54 0.0078 0.198 3.3885 86.068 0.766 6.13 1.2 9.6 0.4337 3.47 3.34062 3.41562
3 1/2" NPT 8 4 101.6 0.125 3.175 0.1 2.54 0.0078 0.198 3.88881 98.776 0.821 6.57 1.25 10 0.4337 3.47 3.8375 3.91562
4" NPT 8 4.5 114.3 0.125 3.175 0.1 2.54 0.0078 0.198 4.38712 111.433 0.844 6.75 1.3 10.4 0.4337 3.47 4.33438 4.41562
5" NPT 8 5.563 141.3 0.125 3.175 0.1 2.54 0.0078 0.198 5.44929 138.412 0.937 7.5 1.4063 11.25 0.4337 3.47 5.39073 5.47862
6" NPT 8 6.625 168.275 0.125 3.175 0.1 2.54 0.0078 0.198 6.50597 165.252 0.958 7.66 1.5125 12.1 0.4337 3.47 6.44609 6.54062
8" NPT 8 8.625 219.075 0.125 3.175 0.1 2.54 0.0078 0.198 8.50003 215.901 1.63 8.5 1.7125 13.7 0.4337 3.47 8.43359 8.54062
10" NPT 8 10.75 273.05 0.125 3.175 0.1 2.54 0.0078 0.198 10.62094 296.772 1.21 9.58 1.925 15.4 0.4337 3.47 10.54531 10.66562
12" NPT 8 12.75 323.85 0.125 3.175 0.1 2.54 0.0078 0.198 12.61781 320.493 1.36 10.88 2.125 17 0.4337 3.47 12.53281 12.66562
14" NPT 8 14 355.6 0.125 3.175 0.1 2.54 0.0078 0.198 13.87262 352.365 1.562 12.5 2.25 18 0.4337 3.47 13.775 13.91562
16" NPT 8 16 406.4 0.125 3.175 0.1 2.54 0.0078 0.198 15.87575 403.244 1.812 14.5 2.45 19.6 0.4337 3.47 15.7625 15.91562
18" NPT 8 18 457.2 0.125 3.175 0.1 2.54 0.0078 0.198 17.875 454.025 2 16 2.65 21.2 0.4337 3.47 17.75 17.91562
20" NPT 8 20 508 0.125 3.175 0.1 2.54 0.0078 0.198 19.87031 504.706 2.125 17 2.85 22.8 0.4337 3.47 19.7375 19.91562
24" NPT 8 24 609.6 0.125 3.175 0.1 2.54 0.0078 0.198 23.68094 606.066 2.375 19 3.25 26 0.4337 3.47 23.7125 23.91562
Nominal Pipe Size (inch) Number of threads per inch Tapping Drills: Taper NPT with Reamer * Tapping Drills: Taper NPT without Reamer Tapping Drills: Straight NPS
Drill Size Equivalents (in) Equivalents (mm) Drill Size Equivalents (in) Equivalents (mm) Drill Size Equivalents (in) Equivalents (mm)
1⁄16 " NPT 27 15/64 0.2344 5.9531 C 0.242 6.1468 1/4 0.25 6.35
1/8" NPT 27 21/64 0.3281 8.3344 Q 0.332 8.4328 11/32 0.3438 8.7313
1/4" NPT 18 27/64 0.4219 10.7156 7/16 0.4375 11.1125 7/16 0.4375 11.1125
3/8" NPT 18 9/16 0.5625 14.2875 37/64 0.5781 14.6844 19/32 0.5938 15.0813
1/2" NPT 14 11/16 0.6875 17.4625 45/64 0.7031 17.8594 23/32 0.7188 18.2563
3/4" NPT 14 57/64 0.8906 22.6219 29/32 0.9063 23.0188 15/16 0.9375 23.8125
1" NPT 11.5 1-1/2008 1.125 28.575 1-9/32 1.2813 32.5438 1-5/32 1.1563 29.3688
1 1/4" NPT 11.5 1-15/32 1.4688 37.3063 1-31/64 1.4844 37.7031 1-1/2002 1.5 38.1
1 1/2" NPT 11.5 1-23/32 1.7188 43.6563 1-23/32 1.7188 43.6563 1-3/2004 1.75 44.45
2" NPT 11.5 2-5/32 2.1563 54.7688 2-3/16 2.1875 55.5625 2-3/16 2.1875 55.5625
2 1/2" NPT 8 2-25/32 2.7813 70.6438 2-5/2008 2.625 66.675 2-5/2008 2.625 66.675
3" NPT 8 3-9/32 3.2813 83.3438 3-1/2004 3.25 82.55 3-1/2004 3.25 82.55
3 1/2" NPT 8 3-3/2004 3.75 95.25 3-3/2004 3.75 95.25 3-3/2004 3.75 95.25
4" NPT 8 4-1/2004 4.25 107.95 4-1/2004 4.25 107.95 4-1/2004 4.25 107.95
5" NPT 8 5-1/2004 5.25 133.35 5-9/32 5.2813 134.1438 5-5/16 5.3125 134.9375
6" NPT 8 6-1/2004 6.25 158.75 6-11/32 6.3438 161.1313 6-3/2008 6.375 161.925
  • *Ream the hole before tapping with a reamer having a taper of 3/4 inch per foot. Pipe Size is the accepted industry designation and does not refer to either the inside or the outside diameter (ID or OD) of a pipe or a fitting. Dimensions in Inches except where stated
  • E = Pitch diameter at hand-tight plane. This is also the pitch diameter at the gauge plane.
  • **' = Truncation from point of thread triangle to flat (not shown in diagram). Minimum = 0.033P for all pitches. See table for maximum.
  • When using L1 gauges to check threads, the thread is within permissible tolerance if the ring gauge face, or plug gauge notch, is +/- 1 turn from being flush with the end of the thread.
  • Example designation:
  • 3/8 - 18 NPT
  • where:
    • 3/8 = nominal pipe size
    • 18 = number of threads per inch
    • NPT = symbol for the thread series and form (i.e., National (American)Standard Pipe, Taper) Basic dimensions are given to four or five decimal places to eliminate errors when calculating gauge dimensions, they do not imply a greater degree of precision than is normally obtainable.
    • National Pipe Tapered

  • L1= Length of normal hand-tight engagement. This is also the L1 gauge length. (Longer thread engagement may be used in special applications, such as flanges for high pressure use. In such cases the pitch diameter, E, remains as specified and the diameter at the end of the pipe is proportionally smaller.)
  • L2= Effective Length of thread