The following table chart defines Straight Pipe Threads for Mechanical Joints, NPSM and NPSL ANSI/ASME B1.20.1, American Pipe Threads Standard.
Straight pipe threads are designated NPSC = National Pipe Straight Coupling.
Also generically referred to as "IPS Threads" or "Iron Straight Threads".
Note that straight pipe threads, unlike tapered pipe thread do not provide any sealing function. Typically, a design that utilizes a straight pipe thread will incorporate a sealing features, like an oring to facilitate fluid sealing.
All chart data given in inches
|
Nominal Pipe Size
|
Threads per Inch
|
External Thread
|
Internal Thread
|
|||||||
|
Allowance
|
Major Diameter
|
Pitch Diameter
|
Minor Diameter
|
Pitch Diameter
|
||||||
|
Max. aa
|
Min.
|
Max.
|
Min.
|
Min. aa
|
Max.
|
Min.
|
Max.
|
|||
|
Free-fitting Mechanical Joints for Fixtures—NPSM
|
||||||||||
|
1/8
|
27
|
0.0011
|
0.397
|
0.390
|
0.3725
|
0.3689
|
0.358
|
0.364
|
0.3736
|
0.3783
|
|---|---|---|---|---|---|---|---|---|---|---|
|
1/4
|
18
|
0.0013
|
0.526
|
0.517
|
0.4903
|
0.4859
|
0.468
|
0.481
|
0.4916
|
0.4974
|
|
3/8
|
18
|
0.0014
|
0.662
|
0.653
|
0.6256
|
0.6211
|
0.603
|
0.612
|
0.6270
|
0.6329
|
|
1/2
|
14
|
0.0015
|
0.823
|
0.813
|
0.7769
|
0.7718
|
0.747
|
0.759
|
0.7784
|
0.7851
|
|
3/4
|
14
|
0.0016
|
1.034
|
1.024
|
0.9873
|
0.9820
|
0.958
|
0.970
|
0.9889
|
0.9958
|
|
1
|
11 1/2
|
0.0017
|
1.293
|
1.281
|
1.2369
|
1.2311
|
1.201
|
1.211
|
1.2386
|
1.2462
|
|
1 1/4
|
11 1/2
|
0.0018
|
1.638
|
1.626
|
1.5816
|
1.5756
|
1.546
|
1.555
|
1.5834
|
1.5912
|
|
1 1/2
|
11 1/2
|
0.0018
|
1.877
|
1.865
|
1.8205
|
1.8144
|
1.785
|
1.794
|
1.8223
|
1.8302
|
|
2
|
11 1/2
|
0.0019
|
2.351
|
2.339
|
2.2944
|
2.2882
|
2.259
|
2.268
|
2.2963
|
2.3044
|
|
2 1/2
|
8
|
0.0022
|
2.841
|
2.826
|
2.7600
|
2.7526
|
2.708
|
2.727
|
2.7622
|
2.7720
|
|
3
|
8
|
0.0023
|
3.467
|
3.452
|
3.3862
|
3.3786
|
3.334
|
3.353
|
3.3885
|
3.3984
|
|
3 1/2
|
8
|
0.0023
|
3.968
|
3.953
|
3.8865
|
3.8788
|
3.835
|
3.848
|
3.8888
|
3.8988
|
|
4
|
8
|
0.0023
|
4.466
|
4.451
|
4.3848
|
4.3771
|
4.333
|
4.346
|
4.3871
|
4.3971
|
|
5
|
8
|
0.0024
|
5.528
|
5.513
|
5.4469
|
5.4390
|
5.395
|
5.408
|
5.4493
|
5.4598
|
|
6
|
8
|
0.0024
|
6.585
|
6.570
|
6.5036
|
6.4955
|
6.452
|
6.464
|
6.5060
|
6.5165
|
|
Loose-fitting Mechanical Joints for Locknut Connections—NPSL
|
||||||||||
|
1/8
|
27
|
...
|
0.409
|
...
|
0.3840
|
0.3805
|
0.362
|
...
|
0.3863
|
0.3898
|
|
1/4
|
18
|
...
|
0.541
|
...
|
0.5038
|
0.4986
|
0.470
|
...
|
0.5073
|
0.5125
|
|
3/8
|
18
|
...
|
0.678
|
...
|
0.6409
|
0.6357
|
0.607
|
...
|
0.6444
|
0.6496
|
|
1/2
|
14
|
...
|
0.844
|
...
|
0.7963
|
0.7896
|
0.753
|
...
|
0.8008
|
0.8075
|
|
3/4
|
14
|
...
|
1.054
|
...
|
1.0067
|
1.0000
|
0.964
|
...
|
1.0112
|
1.0179
|
|
1
|
11 1/2
|
...
|
1.318
|
...
|
1.2604
|
1.2523
|
1.208
|
...
|
1.2658
|
1.2739
|
|
1 1/4
|
11 1/2
|
...
|
1.663
|
...
|
1.6051
|
1.5970
|
1.553
|
...
|
1.6106
|
1.6187
|
|
1 1/2
|
11 1/2
|
...
|
1.902
|
...
|
1.8441
|
1.8360
|
1.792
|
...
|
1.8495
|
1.8576
|
|
2
|
11 1/2
|
...
|
2.376
|
...
|
2.3180
|
2.3099
|
2.265
|
...
|
2.3234
|
2.3315
|
|
2 1/2
|
8
|
...
|
2.877
|
...
|
2.7934
|
2.7817
|
2.718
|
...
|
2.8012
|
2.8129
|
|
3
|
8
|
...
|
3.503
|
...
|
3.4198
|
3.4081
|
3.344
|
...
|
3.4276
|
3.4393
|
|
3 1/2
|
8
|
...
|
4.003
|
...
|
3.9201
|
3.9084
|
3.845
|
...
|
3.9279
|
3.9396
|
|
4
|
8
|
...
|
4.502
|
...
|
4.4184
|
4.4067
|
4.343
|
...
|
4.4262
|
4.4379
|
|
5
|
8
|
...
|
5.564
|
...
|
5.4805
|
5.4688
|
5.405
|
...
|
5.4884
|
5.5001
|
|
6
|
8
|
...
|
6.620
|
...
|
6.5372
|
6.5255
|
6.462
|
...
|
6.5450
|
6.5567
|
|
8
|
8
|
...
|
8.615
|
...
|
8.5313
|
8.5196
|
8.456
|
...
|
8.5391
|
8.5508
|
|
10
|
8
|
...
|
10.735
|
...
|
10.6522
|
10.6405
|
10.577
|
...
|
10.6600
|
10.6717
|
|
12
|
8
|
...
|
12.732
|
...
|
12.6491
|
12.6374
|
12.574
|
...
|
12.6569
|
12.6686
|
For ANSI Standard Straight Pipe Thread the major and the minor diameters of the internal thread and the minor diameter of the external thread vary with the pitch diameter. The major diameter of the external thread is usually determined by the diameter of the pipe. These theoretical diameters result from adding the depth of the truncated thread (0.666025 x p) to the maximum pitch diameters, and it should be understood that commercial pipe will not always have these maximum major diameters.
Notes for Free-fitting Fixture Threads:
The minor diameters of external threads and major diameters of internal threads are those as produced by commercial straight pipe dies and commercial ground straight pipe taps.
The major diameter of the external thread has been calculated on the basis of a truncation of 0.10825p, and the minor diameter of the internal thread has been calculated on the basis of a truncation of 0.21651p, to provide no interference at crest and root when product is gaged with gages made in accordance with the Standard.
Notes for Loose-fitting Locknut Threads:
The locknut thread is established on the basis of retaining the greatest possible amount of metal thickness between the bottom of the thread and the inside of the pipe. In order that a locknut may fit loosely on the externally threaded part, an allowance equal to the "increase in pitch diameter per turn" is provided, with a tolerance of 1 1/2 turns for both external and internal threads.
