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BSPP, BSPT and NPT thread

Threaded connection plays an important role in pipe connection and seal where conveys liquids, gases and steam. Till today, in threaded connection world it is governed by 2 kinds — British Standard Pipe Thread (BSP) and National pipe tapered (NPT).
BSP was created by British engineer Joseph Whitworth in 1841 that are widely used in Europe, Africa and most Asian coutries.
NPT was invented several years later in 1864 by American engineer William Sellers which are applicable in American countries.

BSP

BSP includes BSPP (British Standard Pipe Parallel) and BSPT (British Standard Pipe Taper).
BSPP:
BSPP has parallel threads, namely straight threads. These threads have a constant diameter. We call it Gas (in short “G”) too.
BSPT:
BSPT has tapered threads whose diameter is not constant in these threads, it increases or decreases as the thread length advances.
British Standard Pipe Taper Thread (BSPT) has been adopted internationally as a standard scale used in plumbing fittings, and it is defined by BS EN 10226-1 and ISO 7-1. It has Whitworth thread form (rounded peaks and valleys) with thread angle 55° and tapered thread 1°47′.
BSPT
BSPT thread chart
Pipe size Thread number
in 25.4mm		 Thread pitch
(P)		 Thread height
(h)		 Diameter at gauge plane Nominal gauge length
(external thread)
Major gauge diameter
(d=D)		 Pitch
(d2=D2)		 Minor
(d1=D1)
(inch) (mm) (no.) (mm) (mm) (mm) (mm) (mm) (inch) (mm)
3/8" 17.2 19 1.337 0.856 16.662 15.806 14.95 1/4 6.4
1/2" 21.3 14 1.814 1.162 20.955 19.793 18.631 0.3214 8.2
3/4" 26.9 14 1.814 1.162 26.441 25.279 24.117 3/8 9.5
1" 33.7 11 2.309 1.479 33.249 31.770 30.291 0.4091 10.4
1-1/4" 42.4 11 2.309 1.479 41.910 40.431 38.952 1/2 12.7
1-1/2" 48.3 11 2.309 1.479 47.803 46.324 44.845 1/2 12.7
2" 60.3 11 2.309 1.479 59.614 58.125 56.656 5/8 15.9
2-1/2" 76.1 11 2.309 1.479 75.184 73.705 72.226 11/16 17.5
3" 88.9 11 2.309 1.479 87.884 86.405 84.926 13/16 20.6
4" 114.3 11 2.309 1.479 113.030 111.551 110.072 1 25.4
5" 139.7 11 2.309 1.479 138.430 136.951 135.472 1-1/8 28.6
6" 165.1 11 2.309 1.479 163.830 162.351 160.872 1-1/8 28.6

NPT

National Pipe Thread Taper (NPT) is a US standard for tapered threads used to join pipes and fittings, and it is defined by ANSI/ASME standard B1.20.1. It has a Sellers thread form (flattened peaks and valleys) with thread angle 60° and tapered thread 1°47′.
NPT thread chart
Pipe size Thread number Thread pitch Thread height Hand-tight engagement Effective thread
D No. P h Length Threads Diameter Length Threads Diameter
(inch) (mm) (no. / inch) (inch) inch (L1, inch) (no.) (E1, inch) (L2, inch) (no.) (E3, inch)
3/8" 17.1 18 0.05556 0.04444 0.240 4.32 0.62701 0.4078 7.34 0.63750
1/2" 21.3 14 0.07143 0.05714 0.320 4.48 0.77843 0.5337 7.47 0.79178
3/4" 26.7 14 0.07143 0.05714 0.339 4.75 0.98887 0.5457 7.64 1.00178
1" 33.4 11.5 0.08696 0.06957 0.400 4.60 1.23863 0.6828 7.85 1.25631
1-1/4" 42.2 11.5 0.08696 0.06957 0.420 4.83 1.58338 0.7068 8.13 1.60131
1-1/2" 48.3 11.5 0.08696 0.06957 0.402 4.83 1.82234 0.7235 8.32 1.84131
2" 60.3 11.5 0.08696 0.06957 0.436 5.01 2.29627 0.7565 8.7 2.31630
2-1/2" 73.0 8 0.12500 0.10000 0.682 5.46 2.76215 1.1375 9.1 2.79063
3" 88.9 8 0.12500 0.10000 0.766 6.13 3.3885 1.2000 9.6 3.41563
4" 114.3 8 0.12500 0.10000 0.844 6.75 4.38712 1.3000 10.4 4.41563
5" 141.3 8 0.12500 0.10000 0.937 0.94 5.44929 1.4063 11.25 5.47863
6" 168.3 8 0.12500 0.10000 0.958 7.66 6.50597 1.5125 12.1 6.54063
Note:
L1 = length of hand-tight engagement between internal and external threads.
E1 = basic pitch diameter at plane of hand-tight engagement length (L1), external thread, and large end of internal thread.
L2 = length of effective thread, external thread. Equal to L1 plus wrench make-up threads.
E2 = basic pitch diameter at plane of effective thread length (L2), external thread.
E3 = basic pitch diameter at plane of wrench-tight engagement.

Could BSPP connect with BSPT?

To remember this rule: a tapered male can fit a tapered or parallel female.
That is to say, a BSPT (tapered) male thread can fit both a BSPT (tapered) and a BSPP (parallel) female since the taper of the male will fit equally into a cylindrical thread. When using a tapered tap on a parallel female, sealing material is recommended to guarantee the tightness of the joint. But a BSPP (parallel) male thread will only fit into a cylindrical (BSPP) female thread, as it will be impossible to fit into a tapered recess.

Could BSPT connect with NPT?

Obviously not! After learning the charts we know they have different tooth angle and pitch. To know the difference between them will help you to choose correct pipe fittings connection in plumbing system. If any more questions that confuses you in thread connection, please drop us a message.
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