BSP Thread Chart (British Standard Pipe, G)

Parallel BSP threads from G1/16 to G4, with threads per inch, basic diameters and tap drills from ISO 228-1. BSP is the pipe thread of Europe, Asia, Australia and most of the world outside North America.

Basic sizes per ISO 228-1. * G1-3/8 is not in ISO 228-1; values from manufacturers’ tables.
Thread	TPI	Pitch (mm)	Major Ø (mm)	Pitch Ø (mm)	Minor Ø (mm)	Major Ø (in)	Tap drill (mm)
G1/16	28	0.907	7.723	7.142	6.561	0.3041	6.8
G1/8	28	0.907	9.728	9.147	8.566	0.3830	8.8
G1/4	19	1.337	13.157	12.301	11.445	0.5180	11.8
G3/8	19	1.337	16.662	15.806	14.950	0.6560	15.25
G1/2	14	1.814	20.955	19.793	18.632	0.8250	19
G5/8	14	1.814	22.911	21.749	20.588	0.9020	21
G3/4	14	1.814	26.441	25.279	24.118	1.0410	24.5
G7/8	14	1.814	30.201	29.039	27.878	1.1890	28.25
G1	11	2.309	33.249	31.770	30.292	1.3090	30.75
G1-1/8	11	2.309	37.897	36.418	34.940	1.4920	35.5
G1-1/4	11	2.309	41.910	40.431	38.953	1.6500	39.5
G1-3/8 *	11	2.309	44.323	42.844	41.366	1.7450	41.75
G1-1/2	11	2.309	47.803	46.324	44.846	1.8820	45.25
G1-3/4	11	2.309	53.746	52.267	50.789	2.1160	51
G2	11	2.309	59.614	58.135	56.657	2.3470	57
G2-1/4	11	2.309	65.710	64.231	62.753	2.5870	63.3
G2-1/2	11	2.309	75.184	73.705	72.227	2.9600	72.8
G2-3/4	11	2.309	81.534	80.055	78.577	3.2100	79.1
G3	11	2.309	87.884	86.405	84.927	3.4600	85.5
G3-1/2	11	2.309	100.330	98.851	97.373	3.9500	97.7
G4	11	2.309	113.030	111.551	110.073	4.4500	110.4

The Whitworth form

G1/2 as an example. Every BSP thread has the same proportions: height 0.640 × pitch, radius 0.137 × pitch.

BSP threads use the Whitworth profile: flanks at 55° with the crests and roots rounded to a radius. Only four pitches cover the whole range: 28 threads per inch for G1/16 and G1/8, 19 for G1/4 and G3/8, 14 from G1/2 to G7/8, and 11 from G1 upward.

G, R, Rc and Rp

G is the parallel thread of ISO 228-1, also called BSPP. It does not seal on the thread; the joint is made with a washer or O-ring against a face.

R (male taper), Rc (female taper) and Rp (female parallel) are the threads of ISO 7-1 and EN 10226, also called BSPT. They have the same pitch and form as G and the same diameters at the gauge plane, but they seal on the thread and need a sealant.

A letter after the size of an external G thread gives its tolerance class: G1/2 A is the tighter class, G1/2 B the looser one. Internal G threads have one class and carry no letter.

NPT vs BSP · Identify a thread from measurements

Common questions

What is the tap drill for G1/4?

The tap drill for G1/4 (1/4 BSP parallel) is 11.8 mm. The basic minor diameter is 11.445 mm, and the tolerance on it lies entirely above that size.

What is the tap drill for G1/2?

The tap drill for G1/2 is 19 mm, against a basic minor diameter of 18.631 mm.

Is BSP the same as NPT?

No. BSP has 55° flanks with rounded crests, NPT has 60° flanks with flat crests, and only the 1/2 and 3/4 sizes share the same 14 threads per inch. They do not seal together.

Other charts and tools

Sources: ISO 228-1 Table 1 (dimensions), ISO 7-1 (taper threads) and tap manufacturers’ drill charts. How this site checks its data. Last reviewed 29 September 2026.