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Pipe thread chart: inch / mm conversion (BSP, 15/21, 20/27, S60x6)
A “1/2 inch” thread measures 20.96 mm, not 12.7: in plumbing, the inch size is not converted, it is read from a chart. Here you will find the inch / mm correspondence of every common thread (BSP or “gas thread”, French names 15/21 · 20/27 · 26/34, NPT, S60x6 IBC tanks, Guillemin, Camlock, PVC), a tool that identifies your thread and the fitting that goes with it, and the pitfalls to avoid.
Which thread do you have? Which fitting do you need?
No need to know the name of your thread. Tell us what you need to connect to, or type what you know (“20/27”, “3/4 inch”, “S60x6”, “26.4 mm”): we identify the thread and show you the fittings that go with it. Dimensions according to ISO 228, ISO 7 and ASME B1.20.1.
No need to know the name of your thread. Just answer the question: what do you need to connect to?
With a caliper, measure the outside diameter of a male (thread visible on the outside) or the inside diameter of a female (thread inside the hole), at the root of the thread.
You already know the name of your thread? Type it the way you say it: 20/27, 26/34, 3/4 inch, S60x6… everything works.
Tell us what you have and what you want to get: we look for the matching fitting in our catalogue.
Conversion charts and guides
One tab per standard: the exact dimensions (standard, male and female diameter, pitch), the pitfalls to avoid and, for each size, the fittings we have in stock.
BSP (gas) thread size chart
Measure the outside diameter of the thread if it is male, or the inside diameter if it is female, then read the matching row. The “threads per inch” column is a check: count the threads over 25.4 mm, it is the most reliable way to tell two neighbouring sizes apart. The “French name” column gives the name used by French plumbers: 15/21, 20/27, 26/34… these are the inside / outside diameters, in millimetres, of the original steel pipe (standard NF E 03-005).
| Size in inches | French name | Male: outside Ø | Female: inside Ø | Pitch | Threads per inch | Nominal Ø |
|---|---|---|---|---|---|---|
| 1/8" · G1/8 | 5/10 | 9,73 mm | 8,57 mm | 0,91 mm | 28 | DN 6 |
| 1/4" · G1/4 | 8/13 | 13,16 mm | 11,44 mm | 1,34 mm | 19 | DN 8 |
| 3/8" · G3/8 | 12/17 | 16,66 mm | 14,95 mm | 1,34 mm | 19 | DN 10 |
| 1/2" · G1/2 | 15/21 | 20,96 mm | 18,63 mm | 1,81 mm | 14 | DN 15 |
| 5/8" · G5/8 | — | 22,91 mm | 20,59 mm | 1,81 mm | 14 | DN 16 |
| 3/4" · G3/4 | 20/27 | 26,44 mm | 24,12 mm | 1,81 mm | 14 | DN 20 |
| 1" · G1 | 26/34 | 33,25 mm | 30,29 mm | 2,31 mm | 11 | DN 25 |
| 1"1/4 · G1"1/4 | 33/42 | 41,91 mm | 38,95 mm | 2,31 mm | 11 | DN 32 |
| 1"1/2 · G1"1/2 | 40/49 | 47,8 mm | 44,85 mm | 2,31 mm | 11 | DN 40 |
| 2" · G2 | 50/60 | 59,61 mm | 56,66 mm | 2,31 mm | 11 | DN 50 |
| 2"1/2 · G2"1/2 | 66/76 | 75,18 mm | 72,23 mm | 2,31 mm | 11 | DN 65 |
| 3" · G3 | 80/90 | 87,88 mm | 84,93 mm | 2,31 mm | 11 | DN 80 |
| 4" · G4 | — | 113,03 mm | 110,07 mm | 2,31 mm | 11 | DN 100 |
1/8" BSP thread (5/10) in mm 9,73 mm male · 8,57 mm female
A 1/8" BSP thread measures 9,73 mm in outside diameter (male) and 8,57 mm in inside diameter (female), with a 0,91 mm pitch, i.e. 28 threads per inch. French plumbers call it “5/10”. It is the parallel gas thread (G1/8), standard ISO 228-1; not to be confused with the American 1/8" NPT.
1/4" BSP thread (8/13) in mm 13,16 mm male · 11,44 mm female
A 1/4" BSP thread measures 13,16 mm in outside diameter (male) and 11,44 mm in inside diameter (female), with a 1,34 mm pitch, i.e. 19 threads per inch. French plumbers call it “8/13”. It is the parallel gas thread (G1/4), standard ISO 228-1; not to be confused with the American 1/4" NPT.
3/8" BSP thread (12/17) in mm 16,66 mm male · 14,95 mm female
A 3/8" BSP thread measures 16,66 mm in outside diameter (male) and 14,95 mm in inside diameter (female), with a 1,34 mm pitch, i.e. 19 threads per inch. French plumbers call it “12/17”. It is the parallel gas thread (G3/8), standard ISO 228-1; not to be confused with the American 3/8" NPT.
1/2" BSP thread (15/21) in mm 20,96 mm male · 18,63 mm female
A 1/2" BSP thread measures 20,96 mm in outside diameter (male) and 18,63 mm in inside diameter (female), with a 1,81 mm pitch, i.e. 14 threads per inch. French plumbers call it “15/21”. It is the parallel gas thread (G1/2), standard ISO 228-1; not to be confused with the American 1/2" NPT.
3/4" BSP thread (20/27) in mm 26,44 mm male · 24,12 mm female
A 3/4" BSP thread measures 26,44 mm in outside diameter (male) and 24,12 mm in inside diameter (female), with a 1,81 mm pitch, i.e. 14 threads per inch. French plumbers call it “20/27”. It is the parallel gas thread (G3/4), standard ISO 228-1; not to be confused with the American 3/4" NPT.
1" BSP thread (26/34) in mm 33,25 mm male · 30,29 mm female
A 1" BSP thread measures 33,25 mm in outside diameter (male) and 30,29 mm in inside diameter (female), with a 2,31 mm pitch, i.e. 11 threads per inch. French plumbers call it “26/34”. It is the parallel gas thread (G1), standard ISO 228-1; not to be confused with the American 1" NPT.
1"1/4 BSP thread (33/42) in mm 41,91 mm male · 38,95 mm female
A 1"1/4 BSP thread measures 41,91 mm in outside diameter (male) and 38,95 mm in inside diameter (female), with a 2,31 mm pitch, i.e. 11 threads per inch. French plumbers call it “33/42”. It is the parallel gas thread (G1"1/4), standard ISO 228-1; not to be confused with the American 1"1/4 NPT.
1"1/2 BSP thread (40/49) in mm 47,8 mm male · 44,85 mm female
A 1"1/2 BSP thread measures 47,8 mm in outside diameter (male) and 44,85 mm in inside diameter (female), with a 2,31 mm pitch, i.e. 11 threads per inch. French plumbers call it “40/49”. It is the parallel gas thread (G1"1/2), standard ISO 228-1; not to be confused with the American 1"1/2 NPT.
2" BSP thread (50/60) in mm 59,61 mm male · 56,66 mm female
A 2" BSP thread measures 59,61 mm in outside diameter (male) and 56,66 mm in inside diameter (female), with a 2,31 mm pitch, i.e. 11 threads per inch. French plumbers call it “50/60”. It is the parallel gas thread (G2), standard ISO 228-1; not to be confused with the American 2" NPT.
3" BSP thread (80/90) in mm 87,88 mm male · 84,93 mm female
A 3" BSP thread measures 87,88 mm in outside diameter (male) and 84,93 mm in inside diameter (female), with a 2,31 mm pitch, i.e. 11 threads per inch. French plumbers call it “80/90”. It is the parallel gas thread (G3), standard ISO 228-1; not to be confused with the American 3" NPT.
These values follow ISO 228-1 (formerly BS 2779). A male and a female of the same size always fit together: the 1/2" male at 20.96 mm goes into the 1/2" female at 18.63 mm, the difference is the thread depth. To find the matching fitting, see our BSP threaded fittings.
Parallel or tapered: BSPP and BSPT
The name BSP covers two thread forms, with the same diameter and the same pitch:
| Type | Other name | Form | Sealing |
|---|---|---|---|
| BSPP | G, parallel gas thread | Constant diameter over the whole length | By a flat washer or an O-ring at the thread root |
| BSPT | R, tapered gas thread | Diameter increases towards the base, 1/16 taper | By tightening the threads, with PTFE tape or hemp |
A tapered male screws into a parallel female: it is the most common assembly in plumbing. The reverse, a parallel male in a tapered female, never tightens properly. If your fitting leaks although the size is right, this is often where to look.
BSP or NPT: the confusion to avoid
NPT is the American standard. The sizes have the same names, the diameters are very close, but the threads are incompatible: the thread angle is 55° for BSP and 60° for NPT. An NPT fitting starts to screw into a BSP, then jams after two turns, damaging both parts.
| Size | BSP: male Ø / threads per inch | NPT: male Ø / threads per inch | What sets them apart |
|---|---|---|---|
| 1/8" | 9,73 mm / 28 | 10,29 mm / 27 | different pitch |
| 1/4" | 13,16 mm / 19 | 13,72 mm / 18 | different pitch |
| 3/8" | 16,66 mm / 19 | 17,15 mm / 18 | different pitch |
| 1/2" | 20,96 mm / 14 | 21,34 mm / 14 | same number of threads, only the angle and a few tenths of a millimetre differ |
| 3/4" | 26,44 mm / 14 | 26,67 mm / 14 | same number of threads, only the angle and a few tenths of a millimetre differ |
| 1" | 33,25 mm / 11 | 33,4 mm / 11,5 | different pitch |
| 1 1/4" | 41,91 mm / 11 | 42,16 mm / 11,5 | different pitch |
| 1 1/2" | 47,8 mm / 11 | 48,26 mm / 11,5 | different pitch |
| 2" | 59,61 mm / 11 | 60,33 mm / 11,5 | different pitch |
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. It is the trickiest case, and a frequent one on imported equipment. Our guide no more confusion between BSP, NPT and gas details the identification methods.
How to identify a thread in three measurements
- The diameter. Caliper on the outside of a male, on the inside of a female. Note the value in millimetres and look for the closest row in the chart, within 0.3 mm — or type it into the tool at the top of the page.
- The pitch. Count the threads over a length of 25.4 mm, or use a thread gauge. This is what separates a 1/4" (19 threads) from a 1/2" (14 threads).
- The form. Lay a ruler along the thread: if the diameter is constant, it is parallel (BSPP, G); if it widens towards the base, it is tapered (BSPT, R, or NPT).
These three measurements are enough in almost every case. When in doubt, the tool at the top of the page accepts a measurement in millimetres directly (tab “I don’t know what I have” → “I’d rather measure”).
The IBC tank case: S60x6 and the coarse “artillery” thread
On the valve of a 1000 litre tank, by far the most common thread is the S60x6: 60 mm diameter, 6 mm pitch, only three or four threads. In France it is commonly called the “pas d'artillerie” (artillery thread), and it is the standard on the vast majority of IBCs sold in France and Europe. Other threads do exist, more marginal, depending on the maker of the tank or valve — and those are usually the ones people get the wrong fitting for.
| Thread | Outside Ø | Pitch | Where it is found |
|---|---|---|---|
| S60x6 | 60 mm | 6 mm | The most common on 1000 L IBC tanks. Also written DIN 61, or 2" S60x6. Nicknamed “artillery thread”. |
| S56x4 | 56 mm | 4 mm | 2" cap of plastic (HDPE) drums and jerrycans. |
| S64x5 | 64 mm | 5 mm | Found on drums, connected to BSP through adapters. |
| S70x6 | 70 mm | 6 mm | Cap of 200 litre plastic drums. |
| S75x6 | 75 mm | 6 mm | Wide-mouth tanks, valve with 70 mm floating nut. |
| S80x6 | 80 mm | 6 mm | Valve with 75 mm inside floating nut. |
| S92x4 | 92 mm | 4 mm | Measured diameter 88 to 92 mm, 4 mm pitch. Old WERIT valves. |
| S100x8 | 100 mm | 8 mm | Second IBC standard after S60x6. Diameter 97 to 100 mm, 2 to 3 threads. Makers Schütz, Sotralentz, Greif. |
| M80x3 | 80 mm | 3 mm | Metric thread ISO 68, 7 to 8 threads. Mainly on valves of Chinese origin. |
| RD78 | 78 mm | 4,23 mm | Round thread RD78, DIN 50, 1/6" pitch. On drums and containers. |
Beware of the trap: the S60x6 (60 mm) and the 2" BSP (59.6 mm) have almost the same diameter, but a pitch nearly three times coarser for the first (6 mm against 2.31). They do not screw into each other. The article identify the thread of your IBC tank explains what to do in front of an unknown tank.
To go from one standard to another, we make and stock the matching adapters: S60x6 fittings, threaded male connectors, tank taps, caps and BSP threaded fittings. The “Find the fitting” tab at the top of the page lists, for each connection, the references in stock.
Guillemin symmetrical couplings: DN sizes, not threads
The Guillemin coupling (called “symmetrical” or “fire hose” coupling) locks with a half turn, without a thread: it is designated by its nominal diameter DN, in millimetres. A DN50 matches the bore of a 2". To connect a tank, use an adapter threaded on one side and Guillemin on the other.
| DN | Inch equivalent |
|---|---|
| DN20 | 3/4" |
| DN25 | 1" |
| DN32 | 1"1/4 |
| DN40 | 1"1/2 |
| DN50 | 2" |
| DN65 | 2"1/2 |
| DN80 | 3" |
| DN100 | 4" |
| DN125 | 5" |
| DN150 | 6" |
What about Camlock couplings?
Camlock quick couplings, used for liquid transfer, are identified by their nominal size and not by a thread: a 2" Camlock mates with a 2" BSP through a threaded male adapter. The types pair up two by two, adapter (A, E, F) against coupler (B, C, D):
| Type | Configuration | Connects to |
|---|---|---|
| Type A | Male adapter x female thread | Connects to a type B, C or D coupler |
| Type B | Female coupler x male thread | Connects to a type A, E or F adapter |
| Type C | Female coupler x hose barb | Connects to a type A, E or F adapter |
| Type D | Female coupler x female thread | Connects to a type A, E or F adapter |
| Type E | Male adapter x hose barb | Connects to a type B, C or D coupler |
| Type F | Male adapter x male thread | Connects to a type B, C or D coupler |
| Type DC | Female dust cap | Closes a male adapter |
| Type DP | Male dust plug | Closes a female coupler |
The size of a Camlock is checked with a caliper on the mating diameter (not on the thread or the hose tail, which may differ):
| Camlock size | Inside Ø of the coupler (female) | Outside Ø of the adapter (male) |
|---|---|---|
| 3/4" | 32,4 mm | 32,1 mm |
| 1" | 37,3 mm | 36,7 mm |
| 1"1/4 | 46 mm | 45,5 mm |
| 1"1/2 | 54 mm | 53,4 mm |
| 2" | 63,8 mm | 63 mm |
| 3" | 92,2 mm | 91,5 mm |
| 4" | 120,3 mm | 119,5 mm |
A coupler and an adapter of the same size fit together whatever the brand: standard A-A-59326 sets these dimensions. Our Camlock couplings come in polypropylene, aluminium, brass and stainless steel.
Pressure PVC pipe: pipe diameter and fitting thread
A pressure PVC pipe is designated by its outside diameter in millimetres (Ø 32, Ø 50, Ø 63…). The mixed fittings, solvent-weld on one side and threaded on the other, carry the matching BSP thread:
| Pipe outside Ø | Thread of the screwed fittings |
|---|---|
| Ø 16 mm | 3/8" |
| Ø 20 mm | 1/2" |
| Ø 25 mm | 3/4" |
| Ø 32 mm | 1" |
| Ø 40 mm | 1"1/4 |
| Ø 50 mm | 1"1/2 |
| Ø 63 mm | 2" |
| Ø 75 mm | 2"1/2 |
| Ø 90 mm | 3" |
| Ø 110 mm | 4" |
| Ø 125 mm | 5" |
| Ø 160 mm | 6" |
Our fitting catalogue, by material and by standard
Each family groups the references in stock, with the number of products.
IBC / 1000 L tank
IBC tank fittings and accessories (43)S100x8 fittings (31)Tap for 1000 litre water tank (22)Accessories for IBC tees (20)Male threaded S60x6 adapter (17)Cap for 1000 litre IBC tank (14)S60x6- straight hose tail male (14)S60x6 - female brass valve (13)Compression
Compression fittings (37)Compression fittings (30)Plastic compression fitting (15)Compression fitting for swimming pools (7)Compression outlet (3)Garden hose / quick-connect
Watering fittings (43)Quick couplings (12)Watering fittings (10)DRY SHUT quick couplings (3)Valves and check valves
Non-return valves (11)Mini valves (11)High-strength valves (5)Frost-proof polypropylene valves (3)Fitting and valve packs (2)Valves (2)Hose barb
Hose tail fittings (21)Polypropylene hose tail fittings (10)Male BSP threaded fitting - straight hose tail (7)Male barbed fitting - hose tail male output (5)Barbed female fitting - hose tail straight exit (5)G2 2'' BSP fitting with hose tail output (4)Camlock
Camlock fittings (27)Polypropylene camlock fittings (19)Aluminium camlock fittings (8)G2 2'' BSP fitting with male camlock output (4)Camlock male - female thread BSP (2)Camlock caps (2)Female Camlock - threaded male (2)Female Camlock - straight hose tail male (2)Brass
Screw-on brass fittings (28)Brass fittings (23)Brass compression fitting (14)Brass camlock fittings (8)Brass hose tail fittings (5)Brass compression sleeve (4)Brass compression elbow (4)Chrome brass valve (4)Serrated
Barbed hose fittings (10)Pressure PVC
PVC PN16 fittings (29)Solvent-weld PVC PN16 fittings (16)Threaded PVC PN16 fittings (15)Mixed PVC PN16 fittings (threaded and solvent-weld) (10)Threaded PVC nuts (7)Threaded PVC sleeves (6)PVC sleeve (6)PVC elbow (5)Express
Push-fit fittings (9)Stainless steel
316 stainless steel fittings (23)Stainless steel (11)Stainless camlock fittings (8)Stainless steel Macon fittings (3)Stainless SMS fittings (3)DIN stainless steel fittings (3)Frequently asked questions
Why doesn't a 1/2 inch thread measure 12.7 mm?
The inch size refers to the inside diameter of the original steel pipe, not to the thread: a pipe with a half-inch bore had an outside of about 21 mm, and it is this outside that was threaded. So inches are not converted into millimetres, the correspondence is read from the chart. Frequent mistakes: confusing the 3/8" (16.66 mm) with the 1/2" (20.96 mm), and the 1" (33.25 mm) with the 1"1/4 (41.91 mm).
What does “20/27” mean?
It is the French name of the 3/4" BSP thread: 20 mm inside diameter and 27 mm outside diameter for the original steel pipe. The male thread measures 26.44 mm. Likewise, 15/21 = 1/2", 26/34 = 1", 33/42 = 1"1/4, 40/49 = 1"1/2, 50/60 = 2".
What is the diameter of a 1/2 BSP thread in mm?
20.96 mm for the male, measured over the thread crests, and 18.63 mm for the female. The pitch is 1.81 mm, i.e. 14 threads per inch.
Are gas thread and BSP the same thing?
Yes. “Gas thread”, “BSP” and “G” designate the same 55° thread, standardised as ISO 228. The name comes from its original use on gas pipes.
Can a BSP fitting be screwed onto an NPT?
No. The two standards differ by only a few tenths of a millimetre in diameter, but the thread angle is not the same: the assembly jams and damages the threads. A BSP/NPT adapter is needed.
How do you measure a female thread?
With a caliper, inside, at the root of the threads. You get the diameter of the “female” column of the chart: 18.63 mm for a 1/2", 24.12 mm for a 3/4", 30.29 mm for a 1".
Is the thread of 1000 litre tanks BSP?
In the vast majority of cases, no: it is a coarse-pitch S60x6. More rarely you also find S100x8, S92x4, M80x3, a Camlock outlet or 2" BSP depending on the maker. Beware of the S60x6 (60 mm) and the 2" BSP (59.6 mm): close diameters, very different pitches.