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NPT and BSP Pipe Thread TPI and Outside Diameter Reference

NPT and BSP threads look similar and are routinely confused at the counter. This table lists TPI, pitch and outside diameter side by side, so you can identify a fitting or set up a threading cycle correctly.

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Nominal sizeThread typeThreads per inchPitch (mm)Major OD (in)Major OD (mm)Form
1/16
NPT
27
0.941
0.3125
7.94
Tapered 1:16, 60 deg
1/8
NPT
27
0.941
0.405
10.29
Tapered 1:16, 60 deg
1/4
NPT
18
1.411
0.54
13.72
Tapered 1:16, 60 deg
3/8
NPT
18
1.411
0.675
17.15
Tapered 1:16, 60 deg
1/2
NPT
14
1.814
0.84
21.34
Tapered 1:16, 60 deg
3/4
NPT
14
1.814
1.05
26.67
Tapered 1:16, 60 deg
1
NPT
11.5
2.209
1.315
33.4
Tapered 1:16, 60 deg
1-1/4
NPT
11.5
2.209
1.66
42.16
Tapered 1:16, 60 deg
1-1/2
NPT
11.5
2.209
1.9
48.26
Tapered 1:16, 60 deg
2
NPT
11.5
2.209
2.375
60.33
Tapered 1:16, 60 deg
G 1/8
BSPP (ISO 228)
28
0.907
0.383
9.728
Parallel, 55 deg Whitworth
G 1/4
BSPP (ISO 228)
19
1.337
0.518
13.157
Parallel, 55 deg Whitworth
G 3/8
BSPP (ISO 228)
19
1.337
0.656
16.662
Parallel, 55 deg Whitworth
G 1/2
BSPP (ISO 228)
14
1.814
0.825
20.955
Parallel, 55 deg Whitworth
G 3/4
BSPP (ISO 228)
14
1.814
1.041
26.441
Parallel, 55 deg Whitworth
G 1
BSPP (ISO 228)
11
2.309
1.309
33.249
Parallel, 55 deg Whitworth
G 1-1/4
BSPP (ISO 228)
11
2.309
1.65
41.91
Parallel, 55 deg Whitworth
G 1-1/2
BSPP (ISO 228)
11
2.309
1.882
47.803
Parallel, 55 deg Whitworth
G 2
BSPP (ISO 228)
11
2.309
2.347
59.614
Parallel, 55 deg Whitworth

Pipe thread nominal sizes refer to the pipe bore, not the thread diameter, which is why 1/2 NPT has an OD of 0.840 in. NPT outside diameter is the pipe OD; thread major diameter varies along the taper. BSPT (R and Rc) is tapered like NPT but has the 55 degree form and uses the same TPI values as the BSPP rows shown. NPT and BSP do not seal together reliably even when the TPI matches.

Structure fitting attributes in your ERP

Optional: we can show how to model thread standard and size in your item master.

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How to use the table

Search for the nominal size, for example 1/2, and compare the NPT row with the G 1/2 BSPP row. Both use 14 threads per inch, but the NPT form is a 60 degree thread on a 1:16 taper, while BSPP is a 55 degree Whitworth thread that is parallel. The OD columns tell you which one you are holding with a caliper: 0.840 in for 1/2 NPT versus 20.955 mm for G 1/2. Export to Excel to build a fitting identification card for the shop.

  • Count threads over one inch with a thread pitch gauge.
  • Measure the major OD with calipers on a clean male thread.
  • Check whether the thread tapers by feel and by gauge.

The standards behind the numbers

NPT is defined by ASME B1.20.1, with a 1 in 16 taper on diameter, which is 3/4 in per foot, and a thread angle of 60 degrees. BSPP threads are defined by ISO 228-1 and have a parallel form at 55 degrees. BSPT (taper) is defined in ISO 7-1. NPTF dry-seal threads (ASME B1.20.3) have a different crest and root truncation to seal without tape. Always confirm the thread standard on the drawing before choosing the tap or insert.

Programming and cutting notes

For turning, a pipe thread is a normal threading cycle with a taper value, so you need the TPI converted to lead (25.4 divided by TPI). For a 1/2 NPT lead is 1.814 mm, and the taper is 1:16 on diameter, or about 1.79 degrees per side. Tapered holes are normally cut with a pipe tap, and a tap for NPT will not cut a good G thread. Use the correct thread mill or insert profile, 60 degree for NPT and 55 degree for BSP.

Common mistakes

The classic mistake is screwing an NPT male into a BSPP female. At 1/2 in the TPI matches, the thread angles differ by five degrees, and the fitting seems to start but will leak or damage the female thread. Another mistake is quoting a pipe thread by the thread diameter instead of the nominal pipe size. In an ERP item master, always keep the standard (NPT, BSPP, BSPT, NPTF) as a separate attribute from the size.

Frequently Asked Questions

How many threads per inch is 1/2 NPT?

Half inch NPT has 14 threads per inch, a pitch of 1.814 mm. The outside diameter of the pipe it fits is 0.840 inches. 3/4 NPT also uses 14 TPI.

Can I screw NPT into BSP?

Not reliably. Even where TPI matches, such as 1/2 inch at 14 TPI, the thread angle is 60 degrees for NPT and 55 degrees for BSP, and NPT tapers while BSPP does not. It may start and then leak or gall.

What is the taper on NPT?

NPT tapers at 1 in 16 measured on the diameter, which is 3/4 inch per foot. That is about 1.79 degrees per side. The taper is what creates the sealing interference as the threads tighten.

Why is 1/2 NPT not 0.5 inch in diameter?

Nominal pipe size refers to the original inside diameter of standard pipe. The outside diameter of 1/2 inch pipe is 0.840 in, and the thread major diameter is based on that, not on the nominal size.

If fittings and ports are buried in item descriptions in your ERP, Netray can help structure thread type and size attributes so purchasing stops ordering the wrong standard.

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