BSP vs NPT

Threads

BSP

BSP thread form stands for British Standard Pipe and is common in Australia and the commonwealth countries. It is based on trade size rather than actual diameter which can lead to some confusion when measuring ports.

There are two types of BSP threads;
- BSPP - Female & male thread are both (also known as G)
- BSPT - Female thread is parallel and the male thread is tapered (also know as R/Rp) (the female thread can also be tapered it is then Rc, these are fairly rare to find). Within BSPT it is also common to call the female thread BSPP (parallel) and the male BSPT (tapered) even though they are both technically a BSPT thread form (the female would be parallel and the male would be tapered).

Both threads have the same pitch, angle (55 degrees) and shape (rounded peaks and valleys).

The below table gives the major and minor diameter for each BSP Trade Thread Size. The minor diameter may be ever so slightly smaller than what is in the table depending upon where it has been manufactured to in the thread gauge.

Trade Size	Threads per inch	Pitch		Major Diameter		Minor Diameter		Gage Length
Trade Size	Threads per inch	Inch	mm	Inch	mm	Inch	Mm	Inch	mm
1/8	28	0.0357	0.907	0.383	9.728	0.3372	8.565	0.1563	3.97
1/4	19	0.0526	1.337	0.518	13.157	0.4506	11.445	0.2367	6.012
3/8	19	0.0526	1.337	0.656	16.662	0.5886	14.95	0.25	6.35
1/2	14	0.0714	1.814	0.825	20.955	0.7336	18.633	0.3214	8.164
3/4	14	0.0714	1.814	1.041	26.441	0.9496	24.12	0.375	9.525
1	11	0.0909	2.309	1.309	33.249	1.1926	30.292	0.4091	10.391
1 ¼	11	0.0909	2.309	1.65	41.91	1.5336	38.953	0.5	12.7
1 ½	11	0.0909	2.309	1.882	47.803	1.7656	44.846	0.5	12.7
2	11	0.0909	2.309	2.347	59.614	2.2306	56.657	0.625	15.875
2 ½	11	0.0909	2.309	2.96	75.184	2.8436	72.227	0.6875	17.463
3	11	0.0909	2.309	3.46	87.884	3.3436	84.927	0.8125	20.638
4	11	0.0909	2.309	4.45	113.03	4.3336	110.073

NPT

NPT stands for National Pipe Thread and is an American standard thread. It may also be reffered to as MPT , MNPT or NPT (M) for male external threads and FPT, FNPT or NPT(F) for female interal threads. A thread sealant must always be used to achieve a leak free seal (except for NPTF). It is also based on Trade Size rather than actual diameter (similar to BSP in this regard).

Both threads have the same pitch, angle (60 degrees) and shape (flat peaks and valleys).

The below table gives the Threads Per Inch, Pithc and Major Diameter for NPT Threads.

Trade Size	Threads per inch	Pitch		Major Diameter (O.D)
Trade Size	Threads per inch	Inch	mm	Inch	mm
1/8	27	0.03704	0.94082	0.405	10.29
1/4	18	0.05556	1.41122	0.54	13.72
3/8	18	0.05556	1.41122	0.675	17.15
1/2	14	0.07143	1.81432	0.84	21.34
3/4	14	0.07143	1.81432	1.05	26.67
1	11 ½	0.08696	2.20878	1.315	33.4
1 ¼	11 ½	0.08696	2.20878	1.66	42.16
1 ½	11 ½	0.08696	2.20878	1.9	48.26
2	11 ½	0.08696	2.20878	2.375	60.33
2 ½	8	0.125	3.175	2.875	73.03
3	8	0.125	3.175	3.5	88.9
4	8	0.125	3.175	4.5	114.3

BSP vs NPT

NPT threads are common in the United States and a few other countries, BSP threads (also know as the R subset according to ISO 7) are widely used in many other countries.

BSPT -British Standard Pipe Taper
BSPP -British Standard Pipe Parallel (G)
NPT -National Pipe Taper
NPS -National Pipe Straight
R - External taper (ISO7)
Rp - Internal Parallel (ISO7/1)
Rc - Internal Taper (ISO 7)
Rs - External Parallel

While the actual specified outside diameters of American National Pipe differ slightly from those of British Standard Pipe, either thread may reliably be cut onto a pipe of its respective trade size. BSPT equivalent is NPT and BSPP’s equivalent is NPS.

Never swap threads if it is a high pressure application.

NPT/NPS and BSP threads are not compatible due to the differences in their thread forms, and not just the fact that most sizes have a different pitch. NPT/NPS threads have a 60° angle and have flattened peaks and valleys (Sellers thread form) where as BSP threads have a 55° angle and have rounded peaks and valleys (Whitworth thread form).

NPT and BSP thread pitches (threads per inch) are listed below. To determine pitch, use a thread gauge or count the number of threads that fall into a 1" span.

Trade Size	Pitch (Threads per Inch)
Trade Size	NPT/NPS	BSP
1/8	27	28
1/4	18	19
3/8	18	19
1/2	14	14
¾	14	14
1	11 ½	11
1 ¼	11 ½	11
1 ½	11 ½	11
2	11 ½	11
2 ½	8	11
3	8	11
3 ½	8	11
4	8	11
5	8	11
6	8	11

How to Measure Threads

To identify the trade size of the thread, first it must be determined whether the thread is tapered or parallel. This is done by measuring the first, fourth/fifth and last full thread. If the diameter increases from first thread to the last thread on a male thread or decreases from first to last thread on a female thread, then the thread is classified as a tapered thread (such as BSPT). If the diameter stays the same from the first to last thread then the thread is classified as a parallel thread (BSPP).

Once it is known whether the thread is parallel or tapered, it can be measured to determine it’s trade size. This trade size will not directly represent what is measured, ie. ½" BSP does not equal ½” measured.

For a parallel thread, any thread can be used to measure for its trade size, and for a tapered thread the fourth or fifth full thread is used. This measurement can then be referenced against a thread table to match it to its trade size.

Note: Dimensions may differ slightly between measurement and dimensions of the trade sizes due to variances in tolerances in measuring and manufacturing.

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