When sizing pipes, the following criteria are applied in determining compliance with UK and USA recommendations.
Noncorrosive water is assumed.
For large pipes, UK recommendations are a maximum flow velocity of 3 m/s and in the USA 4.6 m/s is recommended.
UK Recommendations

Pipe diameter <= 50 mm

All pipes

flow velocity >= 0.75 m/s and <= 1.5 m/s

All pipes

pressure drop >= 50 Pa/m and <= 350 Pa/m

Pipe diameter > 50 mm

All pipes

flow velocity >= 1.25 m/s and <= 3.0 m/s

All pipes

pressure drop >= 50 Pa/m and <= 350 Pa/m

USA Recommendations

Pipe diameter <= 50 mm

All pipes

flow velocity >= 0.6 m/s and <= 1.2 m/s

All pipes

pressure drop >= 75 Pa/m and <= 400 Pa/m

Pipe diameter > 50 mm

Plastic pipes

flow velocity >= 1.2 m/s and <= 1.5 m/s

Steel, copper pipes

flow velocity >= 1.2 m/s and <= 3.0 m/s

All pipes

pressure drop >= 75 Pa/m and <= 400 Pa/m

