Fitting pipe inch sizes
Guide: Pipe and Fitting Sizes Explained Simply
Many DIY enthusiasts and professionals face the same problem: One inch is not always the same. A common mistake is to use a 1:1 conversion (1 inch = 25.4 mm). In the heating and plumbing industry, this leads to incorrect orders because the inch measurement historically refers to the internal diameter of steel pipes from the early 20th century.
Why doesn't the conversion work?
Back then, 1" pipes had a wall thickness that resulted in an outside diameter of approx. 33 mm. Despite modern, thinner materials, this outside diameter has remained constant so that old tools and fittings continue to fit. Today, these dimensions are defined in the standards DIN ISO 228-1 and EN 10226-1 (designations such as G, R, or Rp).
Conversion Table: Threads, Pipe Sizes, and Nominal Diameter (DN)
The following table helps you find the right size for your project based on measurements.
| Male thread | Female thread | Inch size | Nominal diameter | Composite pipe | Copper pipe |
| ~ 9.0 mm | ~ 8.5 mm | 1/8" | DN 6 | - | - |
| ~ 13.0 mm | ~ 12.0 mm | 1/4" | DN 8 | 12 x 1 mm | - |
| ~ 16.0 mm | ~ 15.0 mm | 3/8" | DN 10 | 16 x 2 mm | 15 x 1 mm |
| ~ 20.0 mm | ~ 18.5 mm | 1/2" | DN 15 | 20 x 2 mm | 18 x 1 mm |
| ~ 26.0 mm | ~ 25.0 mm | 3/4" | DN 20 | 26 x 3 mm | 22 x 1 mm |
| ~ 33.0 mm | ~ 31.0 mm | 1" | DN 25 | 32 x 3 mm | 28 x 1.5 mm |
| ~ 42.0 mm | ~ 39.0 mm | 1 1/4" | DN 32 | 40 x 3.5 mm | 35 x 1.5 mm |
| ~ 47.0 mm | ~ 45.0 mm | 1 1/2" | DN 40 | 50 x 4 mm | 42 x 1.5 mm |
| ~ 59.0 mm | ~ 57.0 mm | 2" | DN 50 | 63 x 4.5 mm | 54 x 2 mm |
| ~ 76.0 mm | ~ 67.0 mm | 2 1/2" | DN 65 | - | 64 x 2 mm |
| ~ 89.0 mm | ~ 79.0 mm | 3" | DN 80 | - | - |
| ~ 114.0 mm | ~ 102.5 mm | 4" | DN 100 | - | - |
Note: The mm measurements for threads are averages and are intended as a guide. Slight variations may occur depending on the manufacturer and standard.
What does nominal diameter (DN) mean?
The nominal diameter (Diameter Nominal) is the central reference size in installation technology.
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Sanitary/Heating: Usually refers to the approximate internal diameter.
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Drainage: Often corresponds more closely to the outside diameter.
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Important: Since the outside diameter must remain constant for thread cutting, the actual internal diameter varies depending on the material and pressure rating (wall thickness).
Practical example:
You are replacing a 22x1 mm copper pipe (DN 20) with an aluminum composite pipe. To maintain the same flow rate, according to the table, you should choose a composite pipe with dimensions of 26x3 mm (DN 20).