Threads

Pipe threads: BSP (G, R, Rp, Rc) vs NPT

BSP pipe threads use the 55° Whitworth form, either parallel (G, ISO 228-1) or taper (R, Rp, Rc, ISO 7-1 and EN 10226), while NPT uses a 60° form tapered 1 in 16 to ASME B1.20.1. The two systems differ in thread angle and, at most sizes, threads per inch, so BSP and NPT fittings are not interchangeable.

Names by standard

Standards in each region

Region	System	Designation	Norm	Match
USA	National pipe thread	1/2-14 NPT60° taper thread sealed with tape or compound. NPTF (Dryseal) is ASME B1.20.3; NPS/NPSM are straight versions.	ASME B1.20.1	Same material
UK	BS EN / BS EN ISO	G1/2, R1/2, Rp1/2, Rc1/2Formerly BS 2779 (parallel, replaced by BS EN ISO 228-1) and BS 21 (taper, replaced by BS EN 10226). Trade names BSPP and BSPT are still used.	BS EN ISO 228-1 (parallel), BS EN 10226-1 and -2 (taper)	Same material
EU	ISO / EN	G1/2, R1/2, Rp1/2, Rc1/2EN 10226-1 covers taper external R with parallel internal Rp; EN 10226-2 covers taper external R with taper internal Rc.	ISO 228-1, ISO 7-1, EN 10226-1 and -2	Same material
Indien	IS	R1/2, Rp1/2, Rc1/2 (IS 554); G1/2 (IS 2643)IS 554 is identical to ISO 7-1 (threads that seal on the thread). IS 2643 covers parallel threads that do not seal on the thread, aligned with ISO 228-1.	IS 554, IS 2643	Same material
Japan	JIS	G1/2, R1/2, Rp1/2, Rc1/2Based on ISO 228-1 and ISO 7-1. Older Japanese marks PF (parallel), PT (taper) and PS (parallel internal) are still seen.	JIS B 0202 (parallel), JIS B 0203 (taper)	Same material

Thread form

Geometrie

BSP thread angle: 55°, rounded crests and roots (Whitworth form)
NPT thread angle: 60°, flat crests and roots
Taper (R, Rc, NPT): 1 in 16 on diameter (62.5 mm per metre)
BSP parallel: G: no taper; seals with a gasket, washer or O-ring, not on the thread
BSP taper: R external taper; Rc internal taper; Rp internal parallel, used with R to seal on the thread
Größe: Nominal size refers to pipe bore, not thread diameter (1/2 BSP is about 20.96 mm major diameter)
Pitch expressed as: Threads per inch

Size chart

Common sizes

Nominal size	BSP major diameter (mm)	NPT pipe OD (mm)	BSP TPI	NPT TPI
1/8	9.728	10.287	28	27
1/4	13.157	13.716	19	18
3/8	16.662	17.145	19	18
1/2	20.955	21.336	14	14
3/4	26.441	26.670	14	14
1	33.249	33.401	11	11.5
1 1/4	41.910	42.164	11	11.5
1 1/2	47.803	48.260	11	11.5
2	59.614	60.325	11	11.5

BSP major diameter is the basic (gauge-plane) diameter, the same for G and R threads. NPT column is the actual outside diameter of the pipe (0.405–2.375 in). BSP threads are 55°, NPT threads 60°.

In practice

Uses and compatibility

Where it is used

Water, gas, steam and air piping and fittings
Hydraulic and pneumatic ports (G parallel threads with bonded seals or O-rings)
Valves, pumps, gauges and instrument connections
Conduit and threaded pipe nipples
NPT in North American plumbing, oil and gas, and process equipment; BSP in the UK, Europe, India, Japan and most of Asia

What interchanges

BSP and NPT are not interchangeable. The thread angles differ (55° vs 60°), crest and root shapes differ, and at 1/8, 1/4, 3/8 and 1 in and above the TPI differs.
At 1/2 and 3/4 both have 14 TPI, so a BSP fitting may engage an NPT port for a few turns, but the flanks do not seat and the joint is likely to leak under pressure. Use an adapter.
BSP parallel (G) threads do not seal on the thread; they need a gasket, bonded washer or O-ring. Do not rely on thread tape in a G joint.
A taper R external thread is designed to seal in either an Rp (parallel) or Rc (taper) internal thread, as set out in EN 10226-1 and -2.
NPT and NPTF (Dryseal) have the same pitches and can be assembled together, but only matching NPTF pairs are designed to seal without sealant.
BSP and BSW share the 55° form but not diameters: a 1/4 BSP thread is about 13.2 mm, not 1/4 in.

Fragen und Antworten

Common questions

What is the difference between BSP and NPT?

BSP uses a 55° thread angle with rounded crests; NPT uses 60° with flat crests. Most sizes also have different threads per inch, for example 1/4 BSP has 19 TPI and 1/4 NPT has 18.

Can I connect a BSP fitting to an NPT thread?

Not reliably. Even at 1/2 and 3/4, where both have 14 TPI, the different thread angles prevent a pressure-tight seat. Use a BSP-to-NPT adapter.

What do G, R, Rp and Rc mean on pipe threads?

G is a parallel thread that seals with a gasket or O-ring (ISO 228-1). R is an external taper thread, Rc an internal taper thread and Rp an internal parallel thread; R, Rc and Rp seal on the thread (ISO 7-1, EN 10226).

Why is a 1/2 inch pipe thread about 21 mm in diameter?

Pipe thread sizes are named after the nominal bore of the pipe, not the thread diameter. The thread is cut on the outside of the pipe wall, so 1/2 BSP measures about 20.96 mm and 1/2 NPT pipe about 21.3 mm across.

Which pipe thread standard does India use?

India uses BSP-form threads: IS 554 for taper threads that seal on the thread (identical to ISO 7-1) and IS 2643 for parallel threads that do not.

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References

Sources

British Standard Pipe (Wikipedia) — BSP TPI, major diameters, G/R/Rp/Rc designations, 1:16 taper.
National pipe thread (Wikipedia) — NPT TPI, pipe OD, 60° angle, ASME B1.20.1 and B1.20.3.
Roymech: Pipe threads (BS 21, BS 2779, EN 10226, ISO 228) — Supersession of BS 21 and BS 2779.
EN 10226-1:2004 Pipe threads where pressure-tight joints are made on the threads
BIS: IS 554 Pipe threads where pressure-tight joints are made on the threads
InfraLens: IS 2643 Pipe threads where pressure-tight joints are not made on the threads
JIS screw thread standards list (JIS B 0202, B 0203)

Values summarize published limits and typical data. The governing standard and the current edition control; check them before specifying.