Pipework velocity, without a pipe design
Velocity tells you whether a straight pipe is in a familiar band. Many building-services water lines are talked about around 1 to 2 m/s. That is a habit, not a specification. Head loss tells you the straight-pipe cost of that velocity. Fittings can exceed it.
Worked pipe
0.02 m³/s in a 150 mm internal bore is about 1.13 m/s. With a friction factor of 0.02 over 50 m, Darcy–Weisbach head loss is about 0.43 m. The same bore, 40 m long at 100 mm, holds 314 litres. Those two results do not size a pump or a support.
What is missing
Roughness was not looked up. Bends, valves and tees are absent. Temperature, lining and a minimum velocity for self-cleansing are absent. A pipework design still needs the project specification and a competent check.
Tools: velocity and head loss, pipe volume.
Last checked 2 October 2026. Original guidance. Standards text stays with the publisher.