BSP & BSPT Pipe Thread Size Chart
By WhatSizeBoltEdited 11 min read5 source links
The BSP/BSPT dimension chart from 1/8" to 2": TPI, pitch, major and minor diameter, and tap drills for G, R, Rc and Rp — plus why a 1/2" BSP fitting will never seal onto a 1/2" NPT fitting even though both are 14 threads per inch.
A 1/2” BSP fitting and a 1/2” NPT fitting are both fourteen threads per inch. Thread one into the other and it will start, snug up, and feel almost right — right up until it either will not pull tight or will not stop leaking. They were never the same thread. Here is the full BSP/BSPT dimension chart, the G/R/Rc/Rp letters that tell them apart, and the trap in the size numbers themselves.
The four letters: G, R, Rc, Rp
BSP is really two related standards. ISO 228-1 (the old British BS 2779) covers the parallel thread, marked G. ISO 7-1 (the old BS 21) covers the taper thread: R for the external (male) taper thread, Rc for an internal (female) taper thread, and Rp for an internal parallel thread sized to mate with an R. A parallel G never seals on its own thread; a taper R always can. That distinction runs the rest of this article.
- G — external and internal parallel thread (ISO 228-1). Used in pairs: G male into G female.
- R — external taper thread (ISO 7-1). This is the thread everyone means by “BSPT.”
- Rc — internal taper thread (ISO 7-1), for an R to thread into. Common in the US and Japan; rarer in Europe.
- Rp — internal parallel thread (ISO 7-1) sized to mate with an R male. This pairing — taper male into parallel female — is the usual European “jointing thread.”
55° vs 60°: the thread form, and the 1:16 taper
BSP and BSPT share one thread form: a 55° included angle with the crest and root rounded off by a small arc, the same Whitworth profile that also defines BSW and BSF fasteners — see British thread standards: BSW, BSF & BA explained. NPT cuts a different, 60° form with the crest and root simply cut flat.
The taper itself is not what separates them. Both a BSPT R thread and an NPT thread are cut on the same rate, 1:16 — for every 16 units you move along the thread, the diameter changes by 1 unit (3/4 inch of diameter change per foot of length). What never lines up is the 5° gap in flank angle, plus the diameters themselves, covered next.
BSP / BSPT dimension chart, 1/8”–2”
Threads per inch, pitch, major and minor diameter, and tap drill for both the parallel (G) and taper (R/Rc) forms — the dimensions are identical between G and R/Rc at a given size (the taper is a change in diameter along the length, not at the ISO gauge plane the table is measured at). Pitch is shown to three decimals and is exactly 25.4 ÷ TPI.
| Size | TPI | Pitch (mm) | Major dia, mm (in) | Minor dia, mm (in) | Tap drill G (mm) | Tap drill R/Rc (mm) |
|---|---|---|---|---|---|---|
| G/R 1/8” | 28 | 0.907 | 9.73 (0.3830”) | 8.57 (0.3372”) | 8.8 | 8.6 |
| G/R 1/4” | 19 | 1.337 | 13.16 (0.5180”) | 11.45 (0.4506”) | 11.8 | 11.5 |
| G/R 3/8” | 19 | 1.337 | 16.66 (0.6560”) | 14.95 (0.5886”) | 15.3 | 15.0 |
| G/R 1/2” | 14 | 1.814 | 20.95 (0.8250”) | 18.63 (0.7335”) | 19.1 | 18.7 |
| G/R 3/4” | 14 | 1.814 | 26.44 (1.0410”) | 24.12 (0.9495”) | 24.6 | 24.2 |
| G/R 1” | 11 | 2.309 | 33.25 (1.3090”) | 30.29 (1.1926”) | 30.9 | 30.4 |
| G/R 1-1/4” | 11 | 2.309 | 41.91 (1.6500”) | 38.95 (1.5335”) | 39.5 | 39.1 |
| G/R 1-1/2” | 11 | 2.309 | 47.80 (1.8820”) | 44.84 (1.7656”) | 45.4 | 45.0 |
| G/R 2” | 11 | 2.309 | 59.61 (2.3470”) | 56.66 (2.2306”) | 57.2 | 56.8 |
Sealing method: thread seal vs bonded washer
The G/R/Rc/Rp letters are not just naming trivia — they set how the joint is meant to seal, and the two methods are not interchangeable.
- Jointing thread (taper): an R male threaded into an Rc or Rp female. The taper wedges the flanks together as it is driven home, and that interference is the seal — helped along by PTFE tape or a liquid sealant on the threads. This is the BSPT (and NPT) way: the thread is the seal.
- Longscrew thread (parallel): a G male threaded into a G female. Two parallel threads cannot wedge tight, so they are not pressure-tight on the thread at all — sealing comes from a soft compressible washer or O-ring squeezed between the end face of the male thread and the socket or nipple face, clamped up with a backnut. No tape on a BSPP joint helps the seal; it only helps the threads turn.
Compression fittings, hydraulic adapters with a captured O-ring, and immersion-heater bosses are the classic G/BSPP-with-washer case. Identifying which kind of joint is in front of you before you reach for the PTFE tape is exactly what how to identify a hydraulic fitting walks through.
BSP vs NPT: why matching TPI still will not fit
Only two of the nine sizes in this chart even share a threads-per-inch count with their NPT counterpart — and those two, 1/2” and 3/4”, are exactly where the mix-up happens. Everywhere else the TPI itself is different (28 vs 27 at 1/8”, 19 vs 18 at 1/4” and 3/8”, 11 vs 11.5 from 1” up), so a fitting simply will not spin onto the wrong standard far enough to fool anyone.
| Size | TPI, BSP vs NPT | Flank angle | Major dia (BSP) vs OD (NPT) | BSP short of NPT by |
|---|---|---|---|---|
| 1/8” | 28 vs 27 (different) | 55° vs 60° | 9.73 mm vs 10.29 mm | 0.56 mm |
| 1/4” | 19 vs 18 (different) | 55° vs 60° | 13.16 mm vs 13.72 mm | 0.56 mm |
| 3/8” | 19 vs 18 (different) | 55° vs 60° | 16.66 mm vs 17.14 mm | 0.48 mm |
| 1/2” | 14 vs 14 (same) | 55° vs 60° | 20.95 mm vs 21.34 mm | 0.38 mm |
| 3/4” | 14 vs 14 (same) | 55° vs 60° | 26.44 mm vs 26.67 mm | 0.23 mm |
| 1” | 11 vs 11.5 (different) | 55° vs 60° | 33.25 mm vs 33.40 mm | 0.15 mm |
| 1-1/4” | 11 vs 11.5 (different) | 55° vs 60° | 41.91 mm vs 42.16 mm | 0.25 mm |
| 1-1/2” | 11 vs 11.5 (different) | 55° vs 60° | 47.80 mm vs 48.26 mm | 0.46 mm |
| 2” | 11 vs 11.5 (different) | 55° vs 60° | 59.61 mm vs 60.33 mm | 0.71 mm |
Take 1/2”: both are 14 TPI, so a 1/2” BSPT nipple will physically start into a 1/2” NPT socket. But BSP’s major diameter at that size is 20.955 mm against NPT’s 21.336 mm — a 0.38 mm gap — and the flank angle is 55° against 60°. The thread starts, then binds or strips before the taper has drawn up tight, and the flanks never carry full contact even if it does snug down. Engineering ToolBox’s own NPT reference puts it plainly: “ISO threads have 55° taper angle versus 60° for NPT… ISO and NPT threads should not be mixed.”
The nominal-bore trap
BSP size numbers are not thread diameters. The Wikipedia summary of the standard is exact on this: the size “was originally based on the inner diameter… of a steel tube for which the thread was intended,” back when pipe walls were thick and the bore was close to the number stamped on the size. Modern thin-wall pipe broke that link, and the standard itself now just treats the number as a label — “the listed diameter size is the major outer diameter of the external thread” is not what the number 3/4 means in millimetres.
Worked example: pull a fitting off the shelf and its threaded end measures 26.4 mm across the crests — more than a full inch (25.4 mm). The instinct is to call it “1 inch,” because 1 inch is exactly what it is over. It is not. That is G3/4 — the fourth row in the chart above, listed at “3/4” because that was the bore of the pipe it once threaded onto, not because it measures 3/4 inch anywhere on the part. Read the size off the chart by TPI and diameter together, never off a tape measure alone.
This is exactly the trap the site’s Whitworth coverage warns about on the fastener side — see the Whitworth spanner size chart for the bolt-diameter version of the same “the number is not the measurement” problem, and British thread standards (BSW, BSF, BA) for telling all three families apart on a bench with no chart in hand. For any thread that is not a pipe thread at all, start at thread pitch before assuming BSP.
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Thread pitch gauge, 55° Whitworth and 60° metric
A leaf gauge cut to the real 55° Whitworth form settles which of the nine sizes above you are holding in about ten seconds — faster and more reliable than a tape measure across a rounded crest.
BSP taper/parallel tap and die set
Separate taps for the G (parallel) and R/Rc (taper) forms in this chart — do not try to cut a taper thread with a parallel tap or the two tap-drill columns above will both be wrong for the job in front of you.
Frequently asked questions
Is BSP the same as BSPT?
No. BSP (or BSPP) is the parallel thread, ISO 228-1, marked G. BSPT is the taper thread, ISO 7-1, marked R (male) or Rc (female). They share the same 55° Whitworth thread form and, at a given size, the same major/minor diameter and TPI — the difference is only whether the diameter changes along the length.
Can I thread a BSP fitting into an NPT fitting?
Not reliably. Even at the sizes where the threads-per-inch match (1/2” and up), the 55° vs 60° flank angle and a few tenths of a millimetre of diameter difference mean the flanks do not carry full contact. It may start and even snug up, then leak or gall. Use a BSP-to-NPT adapter fitting instead of forcing the two threads together.
What do the letters G, R, Rc and Rp mean?
G is an external or internal parallel thread (ISO 228-1). R is an external taper thread, Rc an internal taper thread, and Rp an internal parallel thread sized for an R, all under ISO 7-1. R threaded into Rc or Rp is a self-sealing taper joint; G into G needs a separate washer.
Why does a BSP fitting need a sealing washer if NPT does not?
Because a lot of BSP fittings are parallel (G) on both sides, and two parallel threads cannot wedge tight against each other. The seal comes from a bonded washer or O-ring clamped by a backnut instead of from the thread. NPT (and BSPT’s R/Rc/Rp) are taper threads, which do seal on the thread itself, usually helped by PTFE tape or a liquid sealant.
Why is a 3/4” BSP fitting more than an inch across?
Because the size number was never a thread measurement. It is a holdover from when BSP sizes named the bore of the steel tube the thread was cut for, and pipe walls have gotten thinner since. G3/4”’s major diameter is 26.44 mm — over 25.4 mm, a full inch — and it is still called 3/4.
What tap drill do I use for a BSP hole?
It depends which thread you are cutting. For a parallel G thread use the G column in the chart above (roughly 80% engagement); for a taper R or Rc thread use the R/Rc column (roughly 95%). The two are different sizes at every row — using the wrong one either strips the tap or leaves too little thread to seal.
Keep going
Bottom line: BSP, BSPT and NPT can look close enough on a shelf to swap by accident, but "close" is not a fit spec. Match the letter (G to G, R to Rc/Rp, or a proper BSP-to-NPT adapter) before you match the tape-measure reading, and check the thread pitch tool if the part in your hand is not a pipe thread at all.
Sources
- Wikipedia: British Standard Pipe — thread form (55°, rounded crest/root, 1:16 taper), the G/R/Rc/Rp/Rs designations, jointing vs longscrew sealing methods, the ISO 228 / ISO 7 dimension table, and the nominal-bore history.
- British Metrics: BSP/BSPT thread chart — independent cross-check of TPI, major and minor diameter for every G row.
- TorqBolt: BSPT thread dimensions — independent cross-check of pitch, major and minor diameter for every R row.
- Wikipedia: National pipe thread — the ASME B1.20.1 NPT table (TPI, pipe OD), 60° flat thread form, 1:16 taper rate.
- Engineering ToolBox: ANSI B1.20.1 NPT — independent cross-check of TPI and pipe OD, and the source for the “ISO threads have 55°… NPT should not be mixed” quote used above.