Pipe size example
How much water through a 15 mm pipe?
A 15 mm pipe can carry a useful domestic flow if the velocity stays in a reasonable range.
Quick estimate
Using a typical 1.5 m/s velocity
Example build-up
Internal diameter: 15 mm. Velocity: 1.5 m/s.
Estimated flow is about 15.9 L/min, or 0.95 m3/h.