Work out water supply pipe sizing instantly with clear inputs, formula shown and shareable results.
Water supply pipe is sized first on velocity, because noise and erosion set the practical limit near 8 ft/s for cold water. The chosen size is then checked for pressure loss using Hazen-Williams, hf = 10.44 L Q^1.852 / (C^1.852 d^4.8704), which returns head in feet and is converted to psi at 0.4335 psi per foot.
Velocity limit
Bore in = sqrt(4 x (Q cfs / Vmax) / pi) x 12, Q cfs = GPM / 448.83
Hazen-Williams
hf ft = 10.44 x L x Q^1.852 / (C^1.852 x d^4.8704)
Preliminary estimate only. Final sizing must follow the plumbing code in force, including allowances for fittings, meters and available street pressure.
Bore in = sqrt(4 x (Q cfs / Vmax) / pi) x 12, Q cfs = GPM / 448.83. Water supply pipe is sized first on velocity, because noise and erosion set the practical limit near 8 ft/s for cold water.
On short branch runs velocity governs. On long mains, pressure loss governs and often forces a size larger than velocity alone would suggest.
This calculator takes 4 inputs: Design flow, Maximum velocity, Developed pipe length, Hazen-Williams C factor. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.