BSP Thread Chart
Basic dimensions for BSP parallel threads (BSPP, ISO 228-1, "G" designation), from G 1/16 to G 6 — threads per inch, pitch, major, pitch and minor diameter.
BSP (British Standard Pipe) is a family of 55° Whitworth-form threads used worldwide, outside North America, for hydraulic and pneumatic fittings, pipework and general plumbing. The table below covers the parallel series (BSPP, "G") — a fastening thread that does not seal on the thread itself. Sealing is done with a bonded washer, O-ring, or a flat-face gasket compressed between shoulder faces.
Its tapered sibling, BSPT (external "R", internal "Rc", ISO 7-1), seals by thread interference instead — similar in principle to NPT, but with the same 55° Whitworth angle as BSPP. A taper male "R" is just as often paired with an internal parallel port ("Rp") as with a taper female one ("Rc") — see the glossary below. Neither BSPP nor BSPT is compatible with NPT: mixing them is a common, costly mistake on imported or exported hydraulic equipment, since a fitting can start threading in before it fails to seal or galls.
Quick reminder: the number in a BSP designation (e.g. G 1/2) is a legacy nominal size, not the actual thread diameter — G 1/2 measures nearly 21 mm across, not 12.7 mm.
| Size | TPI | Pitch (mm) | Major Ø (mm) | Pitch Ø (mm) | Minor Ø (mm) |
|---|---|---|---|---|---|
| G 1/16 | 28 | 0.907 | 7.723 | 7.142 | 6.561 |
| G 1/8 | 28 | 0.907 | 9.728 | 9.147 | 8.566 |
| G 1/4 | 19 | 1.337 | 13.157 | 12.301 | 11.445 |
| G 3/8 | 19 | 1.337 | 16.662 | 15.806 | 14.950 |
| G 1/2 | 14 | 1.814 | 20.955 | 19.793 | 18.631 |
| G 5/8 | 14 | 1.814 | 22.911 | 21.749 | 20.587 |
| G 3/4 | 14 | 1.814 | 26.441 | 25.279 | 24.117 |
| G 7/8 | 14 | 1.814 | 30.201 | 29.039 | 27.877 |
| G 1 | 11 | 2.309 | 33.249 | 31.770 | 30.291 |
| G 1 1/8 | 11 | 2.309 | 37.897 | 36.418 | 34.939 |
| G 1 1/4 | 11 | 2.309 | 41.910 | 40.431 | 38.952 |
| G 1 1/2 | 11 | 2.309 | 47.803 | 46.324 | 44.845 |
| G 1 3/4 | 11 | 2.309 | 53.746 | 52.267 | 50.788 |
| G 2 | 11 | 2.309 | 59.614 | 58.135 | 56.656 |
| G 2 1/4 | 11 | 2.309 | 65.710 | 64.231 | 62.752 |
| G 2 1/2 | 11 | 2.309 | 75.184 | 73.705 | 72.226 |
| G 2 3/4 | 11 | 2.309 | 81.534 | 80.055 | 78.576 |
| G 3 | 11 | 2.309 | 87.884 | 86.405 | 84.926 |
| G 3 1/2 | 11 | 2.309 | 100.330 | 98.851 | 97.372 |
| G 4 | 11 | 2.309 | 113.030 | 111.551 | 110.072 |
| G 4 1/2 | 11 | 2.309 | 125.730 | 124.251 | 122.772 |
| G 5 | 11 | 2.309 | 138.430 | 136.951 | 135.472 |
| G 5 1/2 | 11 | 2.309 | 151.130 | 149.651 | 148.172 |
| G 6 | 11 | 2.309 | 163.830 | 162.351 | 160.872 |
No thread size matches your search.
Values follow ISO 228-1 basic dimensions for the BSPP (parallel) series. Data provided for reference — always verify against the current standard for critical or safety-related applications, and never substitute BSP for NPT or vice versa.
Understanding the columns
- G designation
- Marking for BSPP (parallel) threads per ISO 228-1, e.g. "G 1/2" — used for both male and female parallel BSP threads. The number is a legacy nominal size, not the real diameter.
- BSPP vs BSPT
- BSPP = British Standard Pipe Parallel (ISO 228-1): constant diameter, seals via O-ring, bonded washer or face. BSPT = British Standard Pipe Taper (ISO 7-1): diameter decreases along the thread, seals via thread interference with sealant — functionally like NPT.
- Rc / Rp
- Designations under ISO 7-1: Rc is an internal (female) taper thread; Rp is an internal (female) parallel thread designed to mate with an external taper (R) thread while keeping a straight female port.
- Whitworth form
- The 55° symmetrical V thread angle — versus 60° for NPT and UN threads — with rounded (radiused) crests and roots, used by both BSPP and BSPT.
- Pitch diameter
- The diameter at which the thread groove and ridge widths are equal. Used to define thread fit and check gauging.
Frequently asked questions
Is BSP thread compatible with NPT?
No — never mix them. BSP uses a 55° Whitworth thread angle while NPT uses a 60° angle, and their pitches (threads per inch) differ per nominal size. BSPP also seals on a flat face or O-ring, not on the thread flanks like NPT does. A BSP fitting can start threading into an NPT port (or vice versa), but it won't seal correctly and can gall or strip the threads under torque.
How do I tell BSPP and BSPT fittings apart visually?
BSPP (G) male threads keep a constant, parallel diameter along their length and usually end in a flat face or a captive O-ring/bonded washer at the shoulder. BSPT (R) male threads visibly taper — narrower at the tip — with no face-seal feature, relying on the taper itself plus sealant to seal.
Can a BSPP male fitting be used in a BSPT (or Rc) female port?
Not reliably. Even when the nominal size and threads-per-inch line up, one thread is parallel and the other tapered, so engagement and sealing behaviour differ. A parallel male in a tapered female generally won't seal without a matched face-seal design, such as a BSPP male with a bonded washer threaded into a parallel (Rp) female port.
Why doesn't "G 1/2" measure 1/2 inch (12.7 mm)?
The number in a BSP designation is a legacy nominal pipe size, inherited from old Whitworth pipe standards and based loosely on the historical bore of the pipe the fitting joined — not the thread's actual diameter. G 1/2 has a 20.955 mm major diameter. Always check the mm dimensions in this table, or measure with a thread gauge, rather than trusting the nominal fraction.
References & further reading
- ISO 228-1:2000 — Pipe threads where pressure-tight joints are not made on the threads — the standard this chart's parallel (BSPP) dimensions are built from.
- British Standard Pipe — Wikipedia — background on BSPP vs BSPT, history and designation system.
- Swagelok — Basic Pipe Threads Explained — a practical comparison of BSP, NPT and other pipe thread families.
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