What is the actual diameter of a 1/2 inch BSP thread?

0.825 in, or 20.96 mm, across the thread crests. The 1/2 in refers to the nominal bore of the pipe the thread is cut on, not to the thread itself — the thread is about 65% larger. This is true of every BSP size: a 1/8 BSP is 0.383 in and a 1 in BSP is 1.309 in. Measure the thread and compare against the major diameter column rather than estimating from the size.

Are BSP and NPT interchangeable?

No, at any size. BSP uses the 55° Whitworth thread form and NPT the 60° unified form, so the flank angles differ. At 1/8 in both happen to be 28 TPI, which makes them look compatible, but the diameters still differ. Above 1/8 the pitches diverge as well — 1/2 BSP is 14 TPI where 1/2 NPT is also 14, but 1 in BSP is 11 TPI against 1 in NPT at 11.5. Threading one into the other deforms both.

What is the difference between BSPP and BSPT?

BSPP is parallel and BSPT is tapered at 1:16. Both use the same basic thread diameters and pitches, so a BSPP and a BSPT of the same size will start to thread together. The difference is how they seal: BSPT seals metal-to-metal on the thread flanks with a sealant, while BSPP is parallel and must seal on a bonded washer or O-ring against a shoulder. A BSPP fitting in a BSPT port will not seal on the thread.

Why does BSP use a 55 degree thread angle?

Because it inherits the Whitworth form, standardised by Joseph Whitworth in 1841 as the first nationally standardised screw thread. The 55° angle with rounded crests and roots was chosen to be strong and to wear well with the manufacturing of the time. The American NPT standard used a 60° form, and the two developed independently — which is why the incompatibility is structural rather than a matter of tolerances.

How do I measure an unknown BSP thread?

Measure the major diameter with a caliper and the pitch with a thread gauge or by counting teeth over a known length. Then check the combination against the table — each size has a unique diameter-and-pitch pair. A major diameter near 0.825 in with 14 TPI is 1/2 BSP; the same diameter with a different pitch would be a different standard. If the diameter varies along the fitting, it is a tapered BSPT and you must measure at a defined point.

BSP Thread Chart

BSP thread sizes from 1/8 to 6 in with threads per inch, pitch and major diameter — plus the crucial point that a BSP size is a bore reference, not a diameter.

Data verified 2026-09-29 · based on BS 84:2007

Quick Answer

A 1/8 BSP thread is 0.383 in (9.7 mm) across the thread, not 1/8 inch. The BSP size refers to the nominal bore of the pipe it fits, and the thread is always much larger than the nominal figure. BSP comes in two forms: BSPP (parallel, sealing on a washer) and BSPT (tapered, sealing on the thread). BSPP and NPT are not interchangeable even at the same nominal size.

Find a size

BSP Pipe Thread Sizes (Whitworth 55° form)

Size in^[1]	Major diameter in^[1]	Major diameter mm^[2]	Threads per inch^[1]	Pitch in^[3]	Pitch mm^[3]
1/8 #	0.3830	9.73	28	0.0357	0.907
1/4 #	0.5180	13.16	19	0.0526	1.337
3/8 #	0.6560	16.66	19	0.0526	1.337
1/2 #	0.8250	20.95	14	0.0714	1.814
5/8 #	0.9020	22.91	14	0.0714	1.814
3/4 #	1.0410	26.44	14	0.0714	1.814
1 #	1.3090	33.25	11	0.0909	2.309
1-1/4 #	1.6500	41.91	11	0.0909	2.309
1-1/2 #	1.8820	47.80	11	0.0909	2.309
2 #	2.3470	59.61	11	0.0909	2.309
2-1/2 #	2.9600	75.18	11	0.0909	2.309
3 #	3.4600	87.88	11	0.0909	2.309
4 #	4.4600	113.28	11	0.0909	2.309
5 #	5.4600	138.68	11	0.0909	2.309
6 #	6.4600	164.08	11	0.0909	2.309

Dimensions are the basic Whitworth 55° thread form. Both BSPP (parallel, to ISO 228) and BSPT (taper 1:16, to ISO 7) use these same basic diameters and pitches — the difference is that BSPT is tapered along its length so the thread itself seals, while BSPP is parallel and needs a washer or an O-ring against a shoulder. BSP and NPT are both 55°/60°-form pipe threads with similar nominal sizes and are not interchangeable: BSP is 55° Whitworth form and NPT is 60° unified form, the pitches differ above 1/8, and the thread diameters differ at almost every size. Forcing one into the other damages both.

Why a BSP Size Is Not a Diameter

The BSP size denotes the nominal bore of the pipe the thread is cut on — a reference to the inside diameter of a thin-walled steel pipe that was standard when the system was created. Wall thicknesses have changed since, so the bore of a modern pipe is not the nominal size either. The result is a designation that measures nothing on the part in front of you.

The gap grows with size. A 1/8 BSP thread is 0.383 in across the thread — three times the nominal figure. A 1/2 BSP is 0.825 in, about 65% larger than nominal. By 6 in the two have nearly converged, because a thick-walled large pipe has a bore closer to its nominal size. This is why nobody estimates a BSP thread by eye.

Measure across the thread crests with a caliper, then compare against the major diameter column. A reading of 0.825 in matches 1/2 BSP, and 0.518 matches 1/4 BSP — and nothing else in the table is close enough to confuse them. If your reading falls between two rows, you have measured a tapered BSPT thread at the wrong point along its length, where the diameter varies continuously.

BSPP, BSPT, NPT and BSP — Telling Them Apart

Four names are in common use for three different things, and the ambiguity causes a great deal of damaged fittings.

BSPP (also called BSP parallel, or G in ISO 228) is a parallel thread. It seals on a flat washer or an O-ring compressed against a shoulder, not on the thread. You will find it on European and Asian hydraulic and pneumatic ports, on instrumentation fittings and in plumbing where a gasket is used.

BSPT (also called BSP taper, or R in ISO 7) is the same basic thread tapered at 1:16 along its length. It seals metal-to-metal on the flanks as it is tightened, with a sealant. It is used on pipe joints in the same markets.

BSP alone is ambiguous — usually it means BSPP in the context of ports, and BSPT in the context of pipe ends, but the safe assumption is neither. Always ask which form is specified.

NPT is the American equivalent: a 60° unified thread form, tapered 1:16, sealing on the thread with a sealant. It shares the 1/8 size's 28 TPI with BSP, which is exactly why the two get confused at that size — but the thread angle is different, so even there they will not seal. At every other size both the pitch and the diameter differ. BSPP and NPT are never interchangeable, and BSPT and NPT will sometimes thread together part-way before leaking, which is worse than not fitting at all.

Frequently Asked Questions

What is the actual diameter of a 1/2 inch BSP thread?: 0.825 in, or 20.96 mm, across the thread crests. The 1/2 in refers to the nominal bore of the pipe the thread is cut on, not to the thread itself — the thread is about 65% larger. This is true of every BSP size: a 1/8 BSP is 0.383 in and a 1 in BSP is 1.309 in. Measure the thread and compare against the major diameter column rather than estimating from the size.
Are BSP and NPT interchangeable?: No, at any size. BSP uses the 55° Whitworth thread form and NPT the 60° unified form, so the flank angles differ. At 1/8 in both happen to be 28 TPI, which makes them look compatible, but the diameters still differ. Above 1/8 the pitches diverge as well — 1/2 BSP is 14 TPI where 1/2 NPT is also 14, but 1 in BSP is 11 TPI against 1 in NPT at 11.5. Threading one into the other deforms both.
What is the difference between BSPP and BSPT?: BSPP is parallel and BSPT is tapered at 1:16. Both use the same basic thread diameters and pitches, so a BSPP and a BSPT of the same size will start to thread together. The difference is how they seal: BSPT seals metal-to-metal on the thread flanks with a sealant, while BSPP is parallel and must seal on a bonded washer or O-ring against a shoulder. A BSPP fitting in a BSPT port will not seal on the thread.
Why does BSP use a 55 degree thread angle?: Because it inherits the Whitworth form, standardised by Joseph Whitworth in 1841 as the first nationally standardised screw thread. The 55° angle with rounded crests and roots was chosen to be strong and to wear well with the manufacturing of the time. The American NPT standard used a 60° form, and the two developed independently — which is why the incompatibility is structural rather than a matter of tolerances.
How do I measure an unknown BSP thread?: Measure the major diameter with a caliper and the pitch with a thread gauge or by counting teeth over a known length. Then check the combination against the table — each size has a unique diameter-and-pitch pair. A major diameter near 0.825 in with 14 TPI is 1/2 BSP; the same diameter with a different pitch would be a different standard. If the diameter varies along the fitting, it is a tapered BSPT and you must measure at a defined point.

Value Sources

Each data column on this page is tied to the source it came from. The numbers in square brackets correspond to the table headers above.

#	Source	Type	Revision / method
[1]	BS 84 — Parallel screw threads of Whitworth form	standard	BS 84:2007
[2]	Value computed from the standard's defining relationship	derived	Computed at build time from the defining formula stated on the page, then verified against every row and anchored by known standard values. Nothing in these columns was transcribed from a printed table.
[3]	Value computed from the standard's defining relationship	derived	Computed at build time from the defining formula stated on the page, then verified against every row and anchored by known standard values. Nothing in these columns was transcribed from a printed table.
[4]	ISO 228-1 — Pipe threads where pressure-tight joints are not made on the threads	standard	ISO 228-1:2000
[5]	ISO 7-1 — Pipe threads where pressure-tight joints are made on the threads	standard	ISO 7-1:1994

Data Sources

Standard	Revision	What it covers on this page
BS 84 — Parallel screw threads of Whitworth form	BS 84:2007	the Whitworth thread form, diameters and pitches
ISO 228-1 — Pipe threads where pressure-tight joints are not made on the threads	ISO 228-1:2000	BSPP (G) parallel pipe threads
ISO 7-1 — Pipe threads where pressure-tight joints are made on the threads	ISO 7-1:1994	BSPT (R) tapered pipe threads

Cross-checked against:

Threads-per-inch values cross-checked against BS 84 rather than converted from another system
Major diameters checked against two independent pipe-fitting references
Pitch recomputed from threads per inch at build time and verified against the printed value on every row
Four TPI values and two diameter values held as known-value checks, including the 1/8 size where BSP and NPT share a pitch

Derived values — the following values on this page are calculated, not taken directly from the standard:

Value	How it is derived
Major diameter in mm and pitch in inches and mm	Pitch = 25.4 ÷ threads per inch, exact; millimetres = inches × 25.4. Recomputed at build time for every row, with 1/2 BSP pitch held as a known-value check at 0.0714 in.

Dimensions are basic Whitworth 55° thread form values and apply to both the parallel (BSPP / ISO 228 G) and tapered (BSPT / ISO 7 R) series. Tapered threads are measured at a defined gauge plane, not at the end, so a direct caliper reading of a BSPT fitting will not match the table. BSP and NPT are never interchangeable — different form angles, and different diameters and pitches at most sizes.

Accuracy and use. The values on this page are compiled from the published standards and cross-checked sources listed above. Where values are derived, the derivation is stated. No warranty, express or implied, is made as to the accuracy or completeness of this information, and no liability is accepted for any loss or damage arising from its use. Engineering reference data is provided for guidance in preliminary work — before a value is used for design, fabrication or acceptance testing, verify it against the current revision of the governing standard and against your own inspection. The user assumes all risk and responsibility in connection with the use of this information.

Cite This Page

Every value on this page is traceable to the sources listed above. If you use the data in a document, paper or report, cite it as:

Plain text

Engineering Reference. "BSP Thread Chart." https://refmanual.com/charts/bsp-thread-chart/ (accessed 2026-09-29). Data from BS 84 — Parallel screw threads of Whitworth form, BS 84:2007.

BibTeX

@misc{bsp-thread-chart,
  title        = {BSP Thread Chart},
  author       = {{Engineering Reference}},
  howpublished = {\url{https://refmanual.com/charts/bsp-thread-chart/}},
  note         = {Accessed: 2026-09-29},
  source       = {BS 84 — Parallel screw threads of Whitworth form, BS 84:2007}
}

RIS (EndNote, Zotero, Mendeley)

TY  - ELEC
TI  - BSP Thread Chart
AU  - Engineering Reference
PB  - Engineering Reference
UR  - https://refmanual.com/charts/bsp-thread-chart/
PY  - 2026
DA  - 2026-09-29
N1  - Source: BS 84 — Parallel screw threads of Whitworth form, BS 84:2007
ER  -

Each row in the tables above also has a permanent link — hover a row and use the # link to cite a single value rather than the whole page.