How the water content of a pipe run is calculated geometrically, and what it is used for, e.g. with the expansion vessel and filling.
Introduction
The total water content of a system is useful to know when sizing an expansion vessel, calculating fill quantity, or judging how much liquid is shut off in a section of pipework.
It's pure geometry: the area of the circle multiplied by the length. A pipe with a 20 mm inner diameter holds about 0.314 l per metre, so 10 m gives a good 3.1 litres.
Assumptions
- 01A fully filled, straight pipe with a constant internal diameter.
- 02Bends and fittings contribute only marginally and are not included.
- 03Use the internal diameter — not the nominal DN or outer dimension.
Reference table: water content per metre
In practice, the circle area is rarely calculated every time — the water content per metre is looked up. Since it is pure geometry, the table can be derived directly from the internal diameter: V [l/m] = π · di² / 4000, with di in mm. E.g. di = 20 mm gives π · 400 / 4000 = 0.314 l/m. The crucial thing is that you use the CORRECT internal diameter for the exact material and wall thickness you are installing — the nominal designation (DN or outside diameter) does not in itself say how large the cross-section is.
| Copper (DS/EN 1057), outer × thickness | Int. diameter | Water content |
|---|---|---|
| 15 × 1,0 | 13 mm | 0.13 l/m |
| 18 × 1,0 | 16 mm | 0.20 l/m |
| 22 × 1,0 | 20 mm | 0.31 l/m |
| 28 × 1,5 | 25 mm | 0.49 l/m |
| 35 × 1,5 | 32 mm | 0.80 l/m |
| 42 × 1,5 | 39 mm | 1.19 l/m |
| 54 × 2,0 | 50 mm | 1.96 l/m |
The copper table uses the internal diameters that follow from EN 1057's standard thicknesses (outside diameter minus 2 × thickness); water content is the geometric consequence. Other thicknesses (e.g. heavy-wall for gas) give a different internal diameter and thus a different content.
| Threaded pipe, steel (DS/EN 10255, medium) | Approx. internal diameter | Water content |
|---|---|---|
| DN 15 (½") | approx. 16.0 mm | approx. 0.20 l/m |
| DN 20 (¾") | approx. 21.6 mm | approx. 0.37 l/m |
| DN 25 (1") | approx. 27.2 mm | approx. 0.58 l/m |
| DN 32 (1¼") | approx. 35.9 mm | approx. 1.01 l/m |
| DN 40 (1½") | approx. 41.8 mm | approx. 1.37 l/m |
| DN 50 (2") | approx. 53.0 mm | approx. 2.21 l/m |
The steel table is based on the internal diameters of the EN 10255 medium series; the galvanised version has the same dimensions. For PEX and PP the content cannot be looked up from the outside diameter alone, because the wall thickness (SDR class) varies a lot — e.g. 16 mm PEX often has a 2.0 mm wall (di ≈ 12 mm → approx. 0.11 l/m) and 20 mm PEX approx. 16 mm internal (≈ 0.20 l/m). Always use the manufacturer's data sheet (Wavin, Uponor) for the internal diameter, and insert it in V = π · di² / 4000. The same applies to stainless press pipes (Geberit Mapress, Viega). (Source: DS/EN 1057; DS/EN 10255; manufacturer data sheets — internal diameters.)
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