What velocity range should water supply pipe stay in?

Most plumbing guides keep cold and hot water supply velocity between 4 and 8 ft/s. Below 4 ft/s pipes can be oversized and sluggish, and above 8 ft/s you risk erosion-corrosion, water hammer, and noise. Hot recirculation lines are often kept under 4 ft/s to limit copper erosion.

What is the Hazen-Williams C-value?

C is an empirical roughness coefficient: higher means smoother. Smooth plastics like PVC and CPVC are around 150, new copper near 140, and older steel drops toward 100 to 120 as it scales. A lower C produces more friction loss for the same flow.

How accurate is the Hazen-Williams equation?

It is a reliable empirical fit for water near room temperature in the turbulent flow range typical of plumbing, which is why most plumbing codes use it. It loses accuracy for very hot or very cold water, very small pipes, or non-water fluids, where the Darcy-Weisbach method is preferred.

Why does pressure drop matter for sizing?

Total friction loss along a run, plus elevation and fitting losses, must stay within the pressure available from the supply minus the fixture demand. Sizing each segment so velocity and psi-per-100-ft stay reasonable keeps the worst fixture above its minimum flow pressure.

Does this include fitting losses?

No. The result is straight-pipe friction loss only. Add equivalent length for elbows, tees, and valves, plus 0.433 psi per foot of elevation rise, to get the true total pressure drop on a real run.

Pipe Flow Velocity & Pressure Drop Calculator

Sizing water supply pipe is a balance: too small and velocity climbs until the pipe erodes and bangs, too large and you waste material and let the water go stale. This calculator computes the flow velocity and the Hazen-Williams friction loss for a given flow rate, pipe size, and material so you can keep a run inside the sensible 4 to 8 ft/s window.

How it works

Velocity comes straight from continuity — flow divided by cross-sectional area. Friction loss uses the Hazen-Williams equation, the workhorse of plumbing hydraulics for water at normal temperatures:

V (ft/s)   = 0.4085 × GPM / d^2          (d = inside diameter, inches)
S (ft/ft)  = 4.52 × Q^1.852 / (C^1.852 × d^4.8704)   (Q in GPM, d in inches)
psi/100ft  = S × 100 × 0.4331

C is the material roughness coefficient: about 150 for PVC and CPVC, 140 for new copper, and 100 to 130 for steel depending on age. A higher inside diameter or a higher C both cut the friction loss sharply because of the large exponents.

Tips and example

Take 10 GPM through 3/4 inch type-L copper (inside diameter about 0.785 in, C = 140). Velocity works out near 6.8 ft/s — inside range but on the high side — with roughly 9 psi per 100 ft of friction loss. Step up to 1 inch copper and velocity falls to about 3.7 ft/s with under 3 psi per 100 ft. Remember this is straight-pipe loss only: add equivalent length for every fitting and 0.433 psi per vertical foot before judging whether the farthest fixture still has enough pressure.