Pipe Thread Chart — BSP (G/R), NPT and Metric: Dimensions, TPI and Identification
Dimensional reference compiled by cBallast. 3-page PDF; the full content is on this page.
Compiled by
cBallast
Subject
Pipe threads and fittings
Document type
Dimensional reference
Reference
2026 edition
Issued
2026-10-04
Pages
3
Format
Web page and PDF
Dimension table for BSPP (G), BSPT (R), NPT and metric fine threads used on fittings, glands and sensors: major diameter, threads per inch or pitch, tap drill, how to tell them apart with calipers, and which pairs seal together.
The three families
BSPP / G (ISO 228-1) is a parallel British pipe thread that seals on a gasket, O-ring or bonded washer under the fitting shoulder — never in the thread itself. BSPT / R (ISO 7-1) is the tapered version that seals in the thread with sealant or PTFE tape; Rc is the tapered female, Rp the parallel female that mates with R male. NPT (ASME B1.20.1) is the American tapered thread with a 60° flank angle against the 55° of the British forms — the two taper families are not interchangeable even where diameters look close. Metric fine threads (M12 × 1.5, M20 × 1.5 …) are parallel and dominate cable glands and sensor bosses.
Dimension table
Nominal
G/R major Ø (mm)
BSP TPI
NPT major Ø (mm)
NPT TPI
G tap drill (mm)
1/8"
9.73
28
10.24
27
8.8
1/4"
13.16
19
13.62
18
11.8
3/8"
16.66
19
17.05
18
15.25
1/2"
20.96
14
21.22
14
19.0
3/4"
26.44
14
26.57
14
24.5
1"
33.25
11
33.23
11.5
30.75
1 1/4"
41.91
11
41.98
11.5
39.5
1 1/2"
47.80
11
48.05
11.5
45.25
2"
59.61
11
60.09
11.5
57.0
Note the classic trap at 1/2" and 3/4": G and NPT major diameters differ by well under a millimetre but the flank angles differ (55° vs 60°), so a forced assembly leaks under vibration. At 1" the TPI differ (11 vs 11.5) — a thread gauge settles it instantly.
Identification in four steps
Parallel or tapered? Lay a straight edge on the crests — a taper is visible over 3–4 threads.
Measure the major diameter with calipers on a male thread and find the closest row above; pipe threads are named for the bore of the original steel pipe, not the thread OD.
Count TPI with a pitch gauge: 19 or 14 says British; 18 says NPT; a metric pitch (1.5 mm) says gland thread.
Check the seat. A flat face, O-ring groove or 30° cone means the joint seals on the face and the thread is parallel G; bare tapered threads mean R or NPT with sealant.
A 1/2" BSP (G 1/2) male thread measures 20.96 mm across the crests with 14 threads per inch.
Are BSP and NPT threads interchangeable?
No. BSP uses a 55° flank angle and NPT 60°, and the TPI differs at several sizes; a mixed joint may screw together but will not seal reliably.
How does a G (BSPP) thread seal?
On the face: a bonded washer, gasket or O-ring under the fitting shoulder carries the seal, while the parallel thread only provides clamping.
What thread is on a standard cable gland?
European glands use metric fine threads — most commonly M16 × 1.5, M20 × 1.5, M25 × 1.5 — parallel, sealed by a washer or O-ring at the entry face.
How do I tell NPT from BSPT quickly?
Count threads per inch: 18 TPI at 1/4"–3/8" or 11.5 TPI at 1" and above means NPT; 19 or 11 TPI means the British taper. Both are tapered, so only the pitch and flank angle separate them.