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Correspondence table of displays of units in inches and in mm (for pipes and plumbing fittings)
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How to keep track between the different denominations of piping units?
If you are wondering: “What does the reference DN25 on the fitting mean?” or “How many inches is a 26 x 34 valve?”…. this article is made for you!
The following table is useful in identifying the compatibility of 2 plumbing accessories (fittings, valves, etc.) for a new or existing installation. In everyday life, it is common that a material has one or other appellations according to whether it is expressed in British standard (imperial system) or in French standard (metric system). These 2 ratings coexist widely in our territory.
How to match 2 pieces of plumbing in inches and mm?
Conversion table of correspondence between inches and mm for standard dimensions of plumbing threads
A correspondence table inches ” / millimeter (mm) is of interest in the case of mechanical coupling of different piping elements (PE water irrigation pipes, pvc pipes, steel pipes, brass or copper ducts, meters, etc) via plumbing elements (flexible captive nut, nipple pipe fitting, fittings, etc..)
The table below allows you to obtain the correspondence between the name – or common name – in inches (conventionally used in the field of plumbing), name in millimeters (∅ inside x ∅ outside) or even the the DN designation – Nominal Diameter – (inside or outside diameter of a tube in mm depending on the case).
| Thread Dimensions in Inches ” | Appellation courante (ou dénomination métrique) – (∅ intérieur x ∅ extérieur en mm) |
DN (diamètre nominal) en mm* |
Applications point de puisage |
| 1/8″ | 5 x 10 | DN6 | – |
| 1/4″ | 8 x 13 | DN8 | |
| 3/8″ | 12 x 17 | DN10 | Washbasin mixer hose thread, WC tap fitting, Bath mixer hose thread, Sink mixer hose thread… |
| 1/2″ | 15 x 21 | DN15 | Distribution manifolds. Eccentric shower connection thread. PE pipe outlet (water) of Ø 20 mm. |
| 3/4″ | 20 x 27 | DN20 | Water meter outlet. PE pipe (water) of Ø 25 mm with outlet 3/4″. |
| 1″ | 26 x 34 | DN25 | PE pipe (water) of Ø 32 mm with outlet 1″. |
| 1″ 1/4 | 33 x 42 | DN32 | |
| 1″ 1/2 | 40 x 49 | DN40 | |
| 1″ 3/4 | 45 x 54 | – | |
| 2″ | 50 x 60 | DN50 | |
| 2″ 1/4 | 60 x 70 | DN60 | |
| 2″ 1/2 | 66 x 76 | DN65 | |
| 2″ 3/4 | 70 x 82 | – | |
| 3″ | 80 x 90 | DN80 | |
| 3″1/2 | 90 x 102 | DN90 | |
| 4″ | 102 x 114 | DN100 | |
| 4″1/2 | 115 x 127 | – | |
| 5″ | 125 x 139 | DN125 |
The most common are mentioned in bold. For specific dimensions see the detailed table.
The most common dimensions for individual housing are the 12 x 17, le 15 x 21, le 20 x 27.
(*) NB: the « DN » must be distinguished from the normative and commercial sense that sometimes differ. According to the EN ISO 6708 standard the DN corresponds to the inside diameter of a pipe expressed in mm (ex: DN10 = inside ∅ of 6 mm). However, in retail, wholesale and distribution, the DN is often used to indicate the outside diameter of the pipes. As it is confusing it’s better to know if we are talking about the inside or outside diameter.
>> See the applicable sealing types according on the types of fittings
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