Galvanised water tubes compared to alpex. As I have had to look this up repeatedly, just noting it down for my own and anyone’s reference. Below a list of the most common galvanised tubes and alpex tubes so you can make an educated decision on what to use.

The internal cross-sectional area of ​​galvanized steel pipes is based on the European standard EN 10255 (formerly DIN 2440 / medium thickness), which is the standard used throughout Europe:

• 1/2-inch galvanized pipe (outer diameter 21.3mm; wall thickness 2.65mm; inner diameter 16.0 mm): Area: 201.06 mm² (or 2.01 cm²)
• 3/4-inch galvanized pipe (outer diameter 26.9mm; wall thickness 2.65mm; inner diameter 21.6 mm): Area: 366.44 mm² (or 3.66 cm²)

The thickness of an Alpex multilayer pipe is usually not expressed in inches, but rather millimeters: the outer diameter and the wall thickness. eg. alpex 20×2 has an outer diameter of 20mm, a wall thickness o 2mm so an inner diameter of 16mm.

The most common types are:

• Alpex 16×2 (internal diameter 12 mm / radius 6 mm): Area: 113.10 mm² (or 1.13 cm²)
• Alpex 20×2 (internal diameter 16 mm / radius 8 mm): Area: 201.06 mm² (or 2.01 cm²)
• Alpex 26×2 (internal diameter 20 mm / radius 10 mm): Area: 314.16 mm² (or 3.14 cm²)
  

Important installation considerations:

• A multilayer piper has a much smoother inner surface which allows for better flow even through narrower tubes.
• For a single tap, the standard pipe in galva is 1/2′, the standard multilayer pipe is 16×2.
• The internal diameter of alpex 20×2 tube has a comparable diameter to a 1/2′ brass coupling
• The internal diameter of alpex 26×2 tube is most closely matched to a 3/4′ brass coupling