This document provides specifications for National Pipe Tapered Thread (NPT), including: - Dimensions for NPT threads of various nominal pipe sizes from 1/16" to 24", including threads per inch, diameters, lengths, depths, etc. - Tapping drill sizes for NPT threads with and without reamers. - Notes on reaming holes before tapping and that pipe size refers to industry designation rather than inside or outside diameter.

1. MARYLAND METRICS
THREAD DATA CHARTS
NATIONAL PIPE TAPERED THREAD (NPT)
NPT Thread - American Standard Taper Pipe Thread (ANSI/ASME B1.20.1), Taper 1:16
Thread Form Data - Basic Dimensions of American National Standard Taper Threads, NPT (National Pipe Tapered)
Nomin
al Pipe
Size
(inch)
Numb
er of
thread
s per
inch
Outside
Diamet
er
(inch)
Outside
Diamet
er (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
Diamete
r at
Beginnin
g of
External
Thread
Start
(inch)
Effectiv
e
Thread,
Externa
l
Diamet
er
(inch)
P h l E L1 L2
in mm in mm in mm in mm in
Thread
s in
Thread
s in
Thread
s
1⁄16 27 0.3125 7.938
0.0370
4
0.94
1
0.0296
3
0.75
3 0.28118 0.16
0.261
1
0.128
5 3.47 0.27118 0.28750
1/8" 27 0.405 10.287
0.0370
4
0.94
1
0.0296
3
0.75
3
0.0036
0
0.09
1 0.37360 9.489
0.16
2 4.36
0.263
9 7.12
0.128
5 3.47 0.36351 0.38000
1/4" 18 0.540 13.716
0.0555
6
1.41
1
0.0444
4
1.12
9
0.0049
0
0.12
4 0.49163 12.487
0.22
8 4.10
0.401
8 7.23
0.192
8 3.47 0.47739 0.50250
3/8" 18 0.675 17.145
0.0555
6
1.41
1
0.0444
4
1.12
9
0.0049
0
0.12
4 0.62701 15.926
0.24
0 4.32
0.047
8 7.34
0.192
8 3.47 0.61201 0.63750
1/2" 14 0.840 21.336
0.0714
3
1.81
4
0.0571
4
1.45
1
0.0056
0
0.14
2 0.77843 19.772
0.32
0 4.48
0.533
7 7.47
0.247
8 3.47 0.75843 0.79179
3/4" 14 1.050 26.670
0.0714
3
1.81
4
0.0571
4
1.45
1
0.0056
0
0.14
2 0.98887 25.117
0.33
9 4.75
0.545
7 7.64
0.247
8 3.47 0.96768 1.00179
1" 11.5 1.315 33.401
0.0869
6
2.20
9
0.0695
7
1.76
7
0.0063
0
0.16
0 1.23863 31.461
0.40
0 4.60
0.682
8 7.85
0.301
7 3.47 1.21363 1.25630
1-1/4" 11.5 1.660 42.164
0.0869
6
2.20
9
0.0695
7
1.76
7
0.0063
0
0.16
0 1.58338 40.218
0.42
0 4.83
0.706
8 8.13
0.301
7 3.47 1.55713 1.60130
1-1/2" 11.5 1.900 48.260
0.0869
6
2.20
9
0.0695
7
1.76
7
0.0063
0
0.16
0 1.82234 46.287
0.40
2 4.83
0.723
5 8.32
0.301
7 3.47 1.79609 1.84130
2" 11.5 2.375 60.325
0.0869
6
2.20
9
0.0695
7
1.76
7
0.0063
0
0.16
0 2.29627 58.325
0.43
6 5.01
0.756
5 8.70
0.301
7 3.47 2.26902 2.31630

2. 2-1/2" 8 2.875 73.025
0.1250
0
3.17
5
0.1000
0
2.54
0
0.0078
0
0.19
8 2.76215 70.159
0.68
2 5.46
1.137
5 9.10
0.433
7 3.47 2.71953 2.79062
3" 8 3.500 88.900
0.1250
0
3.17
5
0.1000
0
2.54
0
0.0078
0
0.19
8 3.38850 86.068
0.76
6 6.13
1.200
0 9.60
0.433
7 3.47 3.34062 3.41562
3-1/2" 8 4
101.60
0
0.1250
0
3.17
5
0.1000
0
2.54
0
0.0078
0
0.19
8 3.88881 98.776
0.82
1 6.57
1.250
0 10.00
0.433
7 3.47 3.83750 3.91562
4" 8 4.5
114.30
0
0.1250
0
3.17
5
0.1000
0
2.54
0
0.0078
0
0.19
8 4.38712
111.43
3
0.84
4 6.75
1.300
0 10.40
0.433
7 3.47 4.33438 4.41562
5" 8 5.563
141.30
0
0.1250
0
3.17
5
0.1000
0
2.54
0
0.0078
0
0.19
8 5.44929
138.41
2
0.93
7 7.50
1.406
3 11.25
0.433
7 3.47 5.39073 5.47862
6" 8 6.625
168.27
5
0.1250
0
3.17
5
0.1000
0
2.54
0
0.0078
0
0.19
8 6.50597
165.25
2
0.95
8 7.66
1.512
5 12.10
0.433
7 3.47 6.44609 6.54062
8" 8 8.625
219.07
5
0.1250
0
3.17
5
0.1000
0
2.54
0
0.0078
0
0.19
8 8.50003
215.90
1
1.63
0 8.50
1.712
5 13.70
0.433
7 3.47 8.43359 8.54062
10" 8 10.750
273.05
0
0.1250
0
3.17
5
0.1000
0
2.54
0
0.0078
0
0.19
8
10.6209
4
296.77
2
1.21
0 9.58
1.925
0 15.40
0.433
7 3.47
10.5453
1
10.6656
2
12" 8 12.750
323.85
0
0.1250
0
3.17
5
0.1000
0
2.54
0
0.0078
0
0.19
8
12.6178
1
320.49
3
1.36
0 10.88
2.125
0 17.00
0.433
7 3.47
12.5328
1
12.6656
2
14" 8 14
355.60
0
0.1250
0
3.17
5
0.1000
0
2.54
0
0.0078
0
0.19
8
13.8726
2
352.36
5
1.56
2 12.50
2.250
0 18.00
0.433
7 3.47
13.7750
0
13.9156
2
16" 8 16
406.40
0
0.1250
0
3.17
5
0.1000
0
2.54
0
0.0078
0
0.19
8
15.8757
5
403.24
4
1.81
2 14.50
2.450
0 19.60
0.433
7 3.47
15.7625
0
15.9156
2
18" 8 18
457.20
0
0.1250
0
3.17
5
0.1000
0
2.54
0
0.0078
0
0.19
8
17.8750
0
454.02
5
2.00
0 16.00
2.650
0 21.20
0.433
7 3.47
17.7500
0
17.9156
2
20" 8 20
508.00
0
0.1250
0
3.17
5
0.1000
0
2.54
0
0.0078
0
0.19
8
19.8703
1
504.70
6
2.12
5 17.00
2.850
0 22.80
0.433
7 3.47
19.7375
0
19.9156
2
24" 8 24
609.60
0
0.1250
0
3.17
5
0.1000
0
2.54
0
0.0078
0
0.19
8
23.6809
4
606.06
6
2.37
5 19.00
3.250
0 26.00
0.433
7 3.47
23.7125
0
23.9156
2
Nominal
Pipe Size
(inch)
Number of
threads per
inch
TappingDrills: Taper NPT with Reamer * TappingDrills: 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 27 15/64" 0.2344 5.9531 C 0.242 6.1468 1/4" 0.25 6.35
1/8" 27 21/64" 0.3281 8.3344 Q 0.332 8.4328 11/32" 0.3438 8.7313
1/4" 18 27/64" 0.4219 10.7156 7/16" 0.4375 11.1125 7/16" 0.4375 11.1125
3/8" 18 9/16" 0.5625 14.2875 37/64" 0.5781 14.6844 19/32" 0.5938 15.0813
1/2" 14 11/16" 0.6875 17.4625 45/64" 0.7031 17.8594 23/32" 0.7188 18.2563
3/4" 14 57/64" 0.8906 22.6219 29/32" 0.9063 23.0188 15/16" 0.9375 23.8125
1" 11.5 1-1/8" 1.125 28.575 1-9/32" 1.2813 32.5438 1-5/32" 1.1563 29.3688
1-1/4" 11.5 1-15/32" 1.4688 37.3063 1-31/64" 1.4844 37.7031 1-1/2" 1.5 38.1
1-1/2" 11.5 1-23/32" 1.7188 43.6563 1-23/32" 1.7188 43.6563 1-3/4" 1.75 44.45
2" 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" 8 2-25/32" 2.7813 70.6438 2-5/8" 2.625 66.675 2-5/8" 2.625 66.675
3" 8 3-9/32" 3.2813 83.3438 3-1/4" 3.25 82.55 3-1/4" 3.25 82.55

3. 3-1/2" 8 3-3/4" 3.75 95.25 3-3/4" 3.75 95.25 3-3/4" 3.75 95.25
4" 8 4-1/4" 4.25 107.95 4-1/4" 4.25 107.95 4-1/4" 4.25 107.95
5" 8 5-1/4" 5.25 133.35 5-9/32" 5.2813 134.1438 5-5/16" 5.3125 134.9375
6" 8 6-1/4" 6.25 158.75 6-11/32" 6.3438 161.1313 6-3/8" 6.375 161.925
Notes:
* Ream the hole before tapping with a reamer having a taper of 3/4 inch per foot. Pipe Size
is the accepted industrydesignation and does notrefer to either the inside or the outside
diameter (ID or OD) of a pipe or a fitting. Dimensions in Inches exceptwhere stated
E= Pitch diameter at hand-tightplane.This is also the pitch diameter atthe gauge plane.
**'= Truncation from point of thread triangle to flat (not shown in diagram). Minimum = 0.033P for all pitches.See table for
maximum.
Tolerances:
When using L1 gauges to check threads,the thread is within permissible tolerance ifthe 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 ofthreads per inch
NPT = symbol for the thread series and
form (i.e., National (American)Standard Pipe,Taper)
Note- Basic dimensions are given to four or five decimal places to eliminate errors when
calculating gauge dimensions,theydo not implya greater degree of precision than is normally
obtainable.
L1= Length of normal hand-tightengagement.This is also the L1 gauge length.
(Longer thread engagementmaybe 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 proportionallysmaller.)
L2= Effective Length of thread