BSP Thread Size Chart

BSP (British Standard Pipe) threads have a 55° Whitworth profile with rounded crests and roots. The family has two branches: parallel BSPP (G, ISO 228-1) and tapered BSPT (R, Rc, Rp; ISO 7-1). In older British standards they appear as BS 2779 and BS 21. The chart gives the major and minor diameters, pitch and threads per inch for sizes from 1/8" to 2".

How to read the chart

• Nominal size comes from the pipe size convention and is not the thread diameter: a G 1/2 thread has a major diameter of 20.955 mm, not 12.7 mm.
• The designation column shows the notations used for the same size. G is a parallel thread; R denotes male tapered, Rc female tapered and Rp female parallel.
• TPI and pitch are two ways of writing the same information: pitch = 25.4 / TPI. 1/8" is 28 TPI; 1/4" and 3/8" are 19 TPI; 1/2" to 7/8" are 14 TPI; 1" and above are 11 TPI.
• Major diameter is the crest-to-crest diameter of a male thread and minor diameter that of a female thread. The typical reading columns give the value to expect with a caliper.
• Tap drill sizes are for G (parallel) threads only and are approximate; an Rc tapered thread needs a smaller drill and a taper reamer. Check the value against your workshop standard.

BSPP vs BSPT

On a G parallel thread, the thread only holds the joint; the seal is made by a bonded seal, an O-ring port (ISO 1179 / DIN 3852) or the 60° cone on hose ends. On an R tapered thread the seal is made on the thread itself, and PTFE tape or liquid thread sealant is needed. In ISO 7-1, a tapered R male end mates with a tapered Rc or parallel Rp female thread. Rp has the same nominal dimensions as G, but a different tolerance and purpose. The 5/8" and 7/8" sizes are defined only in ISO 228-1 (G); they have no tapered equivalents.

Measuring tips

On a parallel thread you can measure the diameter at any full thread. On a tapered thread the diameter depends on where you measure: measure at the 4th–5th full thread, and confirm the taper by measuring at the first and last full thread; with a 1:16 taper the diameter changes by 1 mm over 16 mm of length. Sizes that share the same TPI, such as 1/4" and 3/8", are told apart by diameter; pitch alone is not enough.

Commonly confused sizes

• 1/2" and 3/4" BSP vs NPT: Both are 14 TPI; the diameter difference is 0.38 mm at 1/2" and 0.23 mm at 3/4". What tells them apart is the 55° / 60° profile and the crest shape.
• G 1/4 vs 1/2-20 UNF: 13.157 mm and 19 TPI against 12.7 mm and 20 TPI; a diameter difference of 0.46 mm.
• G 5/8 vs 7/8-14 UNF: Both 14 TPI; major diameters of 22.911 and 22.225 mm, profile angles of 55° and 60°.
• G 1, 1-5/16-12 UN and M33×2: Major diameters of 33.249, 33.338 and 33.0 mm; pitches of 2.309, 2.117 and 2.0 mm. A pitch gauge is essential in this group.

On stainless steel BSP connections

On AISI 316/316L threads, forcing and dry assembly can cause galling; the thread should run in freely by hand for a few turns before it is tightened with a wrench. On tapered threads, apply sealing tape leaving the first thread clear. Do not use tape on parallel threads; the seal is made by the washer seal or O-ring. Choose the seal and O-ring material to suit the fluid and temperature.