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BSP thread chart: sizes and inch to mm conversion
BSP, also known as the gas thread, is the European standard for fittings and valves. Its size is given in inches, but that figure is not the diameter you measure: a "1/2 inch" fitting measures 20.96 mm with a caliper, not 12.7 mm. The chart below gives the real measurement for each size, male and female. Further down: how to tell BSP from American NPT, and the special case of IBC tanks.
BSP thread size chart
Measure the outside diameter of a male thread, or the inside diameter of a female thread, then read across. The "threads per inch" column is your cross-check: count the threads over 25.4 mm, it is the most reliable way to separate two neighbouring sizes.
| Size in inches | Male: outside Ø | Female: inside Ø | Pitch | Threads per inch |
|---|---|---|---|---|
| 1/8" | 9.73 mm | 8.57 mm | 0.91 mm | 28 |
| 1/4" | 13.16 mm | 11.45 mm | 1.34 mm | 19 |
| 3/8" | 16.66 mm | 14.95 mm | 1.34 mm | 19 |
| 1/2" | 20.96 mm | 18.63 mm | 1.81 mm | 14 |
| 3/4" | 26.41 mm | 24.12 mm | 1.81 mm | 14 |
| 1" | 33.25 mm | 30.29 mm | 2.31 mm | 11 |
| 1"1/4 | 41.60 mm | 39.30 mm | 2.31 mm | 11 |
| 1"1/2 | 47.80 mm | 44.85 mm | 2.31 mm | 11 |
| 2" | 59.61 mm | 56.66 mm | 2.31 mm | 11 |
| 3" | 87.88 mm | 84.93 mm | 2.31 mm | 11 |
| 4" | 113.03 mm | 110.07 mm | 2.31 mm | 11 |
These values follow ISO 228 (formerly BS 2779). A male and a female of the same size always go together: the 1/2" male at 20.96 mm enters the 1/2" female at 18.63 mm, the difference being the depth of the thread.
Why 1/2 inch is not 12.7 mm
The inch size of a BSP thread does not describe the thread itself: it describes the inside diameter of the pipe the fitting was designed for, back when pipes were steel with thick walls. A pipe with a half-inch bore had an outside diameter of about 21 mm, and that outside was what got threaded. The name stuck, and so did the measurement.
So remember the practical rule: you do not convert inches into millimetres, you read the chart. The two most common mistakes are confusing 3/8" (16.66 mm) with 1/2" (20.96 mm), and 1" (33.25 mm) with 1"1/4 (41.60 mm).
Parallel or tapered: BSPP and BSPT
Two thread forms share the BSP name, with the same diameter and the same pitch:
| Type | Also called | Form | Sealing |
|---|---|---|---|
| BSPP | G, parallel gas thread | Constant diameter over its whole length | With a flat washer or an O-ring at the base |
| BSPT | R, tapered gas thread | Diameter grows towards the base, 1 in 16 taper | By thread interference, with PTFE tape or hemp |
A tapered male screws into a parallel female: that is the most common assembly in plumbing. The other way round, a parallel male into a tapered female, never tightens properly. If your fitting leaks although the size is right, this is usually where to look.
BSP or NPT: the mix-up to avoid
NPT is the American standard. The sizes carry the same names and the diameters are very close, but the threads are not compatible: the thread angle is 55° for BSP and 60° for NPT. An NPT fitting starts to screw into a BSP one, then jams after two turns and damages both parts.
| Size | BSP: male Ø / threads per inch | NPT: male Ø / threads per inch |
|---|---|---|
| 1/8" | 9.73 mm / 28 | 10.29 mm / 27 |
| 1/4" | 13.16 mm / 19 | 13.72 mm / 18 |
| 3/8" | 16.66 mm / 19 | 17.15 mm / 18 |
| 1/2" | 20.96 mm / 14 | 21.34 mm / 14 |
| 3/4" | 26.41 mm / 14 | 26.67 mm / 14 |
| 1" | 33.25 mm / 11 | 33.40 mm / 11.5 |
| 2" | 59.61 mm / 11 | 60.33 mm / 11.5 |
In 1/2" and 3/4", both standards have the same number of threads per inch: only the thread angle and a few tenths of a millimetre separate them. That is the most treacherous case, and a common one on imported equipment.
Identifying a thread in three measurements
- Diameter. Caliper on the outside of a male thread, on the inside of a female one. Read the value in millimetres and find the closest line in the chart, within 0.3 mm.
- Pitch. Count the threads over 25.4 mm, or use a thread gauge. This is what separates a 1/4" (19 threads) from a 1/2" (14 threads).
- Form. Lay a rule along the thread: constant diameter means parallel (BSPP, G); flaring towards the base means tapered (BSPT, R, or NPT).
These three measurements are enough in nearly every case. For a conversion chart covering plumbing threads more widely, see our complete guide to plumbing threads and the different fitting threads.
IBC tanks: S60x6 and the buttress thread
On the outlet valve of a 1000 litre tank, by far the most common thread is S60x6: 60 mm diameter, 6 mm pitch, only three to four threads. It is the standard on the vast majority of IBCs sold in Europe. Other threads do exist, though, depending on the tank or valve manufacturer — and those are usually where people order the wrong adapter.
| Thread | Diameter | Pitch | Where you find it |
|---|---|---|---|
| S60x6 | 60 mm | 6 mm | Most common — outlet valve on most IBC tanks |
| S100x8 | 97 to 100 mm | 8 mm | Less frequent — Schütz, Sotralentz and Greif tanks |
| S92x4 | 88 to 92 mm | 4 mm | Rare — older Werit valves |
| M80x3 | 80 mm | 3 mm | Rare — valves of Chinese origin |
| 2" BSP (gas thread) | 59.4 mm | 2.3 mm | Rare — outlet of some valves |
| Camlock outlet | depends on type | no thread | Rare — quick-coupling valve, factory-fitted or retrofitted |
Watch out for one trap: S60x6 (60 mm) and 2" BSP (59.4 mm) have almost the same diameter, but the first has a pitch six times coarser. They do not screw into each other. Our page on IBC valve threads details each standard with its dimensions, and the article identifying your IBC tank thread explains what to do when facing an unknown tank.
To move from one standard to another, we manufacture and stock the matching adapters: S60x6 fittings, male threaded outlets, tank taps, caps and BSP threaded fittings.
What about Camlock couplings?
Camlock quick couplings, used for liquid transfer, are identified by their nominal size rather than by a thread: a 2" Camlock mates with 2" BSP through a male threaded adapter. The dimensions of each type, A to F, are gathered in our Camlock size chart.
Frequently asked questions
What is the diameter of a 1/2 BSP thread in mm?
20.96 mm for the male, measured across the crests, and 18.63 mm for the female. The pitch is 1.81 mm, that is 14 threads per inch.
Are gas thread and BSP the same thing?
Yes. "Gas thread", "BSP" and "G" all refer to the same 55° thread, standardised as ISO 228. The name comes from its original use on gas pipework.
Can a BSP fitting screw onto an NPT one?
No. The two standards differ by only a few tenths of a millimetre in diameter, but the thread angle is not the same: the joint jams and the threads get damaged. You need a BSP to NPT adapter.
How do you measure a female thread?
With a caliper, inside, across the roots of the thread. You get the "female" column of the chart: 18.63 mm for 1/2", 24.12 mm for 3/4", 30.29 mm for 1".
Is the thread on 1000 litre tanks a BSP thread?
In the vast majority of cases, no: it is a coarse S60x6. You also meet, more rarely, S100x8, S92x4, M80x3, a Camlock outlet or 2" BSP depending on the manufacturer. Beware of S60x6 (60 mm) versus 2" BSP (59.4 mm): close diameters, pitches six times apart.