Pipe Threads ___

ASME B1.20.1 (NPT/API)

NPS Number of Threads Perinch 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 vanish(or washout) thread
P H L E L1 L2 L2
in mm in mm in mm in mm in Threads in Threads in Threads
BASIC DIMENSIONS OF AMERICAN NATIONAL STANDARD TAPER THREADS, NPT (ASME B1.20.1)
1/8” 27 0.03704 0.941 0.02963 0.753 0.00360 0.091 0.37360 9.489 0.162 4.36 0.2639 7.12 0.1285 3.47
1/4” 18 0.05556 1.411 0.04444 1.129 0.00490 0.124 0.49163 12.487 0.228 4.10 0.4018 7.23 0.1928 3.47
3/8” 18 0.05556 1.411 0.04444 1.129 0.00490 0.124 0.62701 15.926 0.240 4.32 0.0478 7.34 0.1928 3.47
1/2” 14 0.07143 1.814 0.05714 1.451 0.00560 0.142 0.77843 19.772 0.320 4.48 0.5337 7.47 0.2478 3.47
3/4” 14 0.07143 1.814 0.05714 1.451 0.00560 0.142 0.98887 25.117 0.339 4.75 0.5457 7.64 0.2478 3.47
1” 11.5 0.08696 2.209 0.06957 1.767 0.00630 0.160 1.23863 31.461 0.400 4.60 0.6828 7.85 0.3017 3.47
1-1/4” 11.5 0.08696 2.209 0.06957 1.767 0.00630 0.160 1.58338 40.218 0.420 4.83 0.7068 8.13 0.3017 3.47
1-1/2” 11.5 0.08696 2.209 0.06957 1.767 0.00630 0.160 1.82234 46.287 0.402 4.83 0.7235 8.32 0.3017 3.47
2” 11.5 0.08696 2.209 0.06957 1.767 0.00630 0.160 2.29627 58.325 0.436 5.01 0.7565 8.70 0.3017 3.47
2-1/2” 8 0.12500 3.175 0.10000 2.540 0.00780 0.198 2.76215 70.159 0.682 5.46 1.1375 9.10 0.4337 3.47
3” 8 0.12500 3.175 0.10000 2.540 0.00780 0.198 3.38850 86.068 0.766 6.13 1.2000 9.60 0.4337 3.47
3-1/2” 8 0.12500 3.175 0.10000 2.540 0.00780 0.198 3.88881 98.776 0.821 6.57 1.2500 10.00 0.4337 3.47
4” 8 0.12500 3.175 0.10000 2.540 0.00780 0.198 4.38712 111.433 0.844 6.75 1.3000 10.40 0.4337 3.47
5” 8 0.12500 3.175 0.10000 2.540 0.00780 0.198 5.44929 138.412 0.937 7.50 1.4063 11.25 0.4337 3.47
6” 8 0.12500 3.175 0.10000 2.540 0.00780 0.198 6.50597 165.252 0.958 7.66 1.5125 12.10 0.4337 3.47
8” 8 0.12500 3.175 0.10000 2.540 0.00780 0.198 8.50003 215.901 1.630 8.50 1.7125 13.70 0.4337 3.47
10” 8 0.12500 3.175 0.10000 2.540 0.00780 0.198 10.62094 296.772 1.210 9.58 1.9250 15.40 0.4337 3.47
12” 8 0.12500 3.175 0.10000 2.540 0.00780 0.198 12.61781 320.493 1.360 10.88 2.1250 17.00 0.4337 3.47
14” 8 0.12500 3.175 0.10000 2.540 0.00780 0.198 13.87262 352.365 1.562 12.50 2.2500 18.00 0.4337 3.47
16” 8 0.12500 3.175 0.10000 2.540 0.00780 0.198 15.87575 403.244 1.812 14.50 2.4500 19.60 0.4337 3.47
18” 8 0.12500 3.175 0.10000 2.540 0.00780 0.198 17.87500 454.025 2.000 16.00 2.6500 21.20 0.4337 3.47
20” 8 0.12500 3.175 0.10000 2.540 0.00780 0.198 19.87031 504.706 2.125 17.00 2.8500 22.80 0.4337 3.47
24” 8 0.12500 3.175 0.10000 2.540 0.00780 0.198 23.68094 606.066 2.375 19.00 3.2500 26.00 0.4337 3.47
E = Pitch diameter at hand-tight plane. This is also the pitch diameter at the gauge plane.
L2 = Effective length of thread = Truncation from point of thread triangle to flat (not shown in diagram). Minimum = 0.033P for all pitches. See table for maximum.
NPT = symbol for the thread series and form (i.e., National (American) Standard Pipe, Taper)
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.)

Example designation:3/8 – 18 NPT

where

3/8 = nominal pipe size

18 = number of threads per inch

Tolerances 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.
Note: 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 precisions
than is normally obtainable. Metric dimensions, where shown, are calculated from the inch values and rounded.