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Water Supply Pipe Sizing Calculator

Work out water supply pipe sizing instantly with clear inputs, formula shown and shareable results.

About Water Supply Pipe Sizing Calculator

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.

Formulas

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)

C
150 copper or plastic, 130 new steel, 100 old steel

Worked example

With the default values below, Water Supply Pipe Sizing Calculator returns Bore required of 1.011. Change any input above and every figure here is recalculated in your browser.

Inputs used

Design flowminimum 0.1 GPM
20 GPM
Maximum velocityminimum 1 ft/s
8 ft/s
Developed pipe lengthminimum 1 ft
120 ft
Hazen-Williams C factorminimum 40
150

Results

Bore required
1.011 in
Nominal size to use
1-1/4 in
Velocity in chosen size
5.23 ft/s
Friction loss over the run
4.39 psi

Step by step

  1. 1Values used
  2. 2Velocity limit
  3. 3Hazen-Williams
  4. 4Bore required= 1.011 in
  5. 5Nominal size to use= 1-1/4 in
  6. 6Velocity in chosen size= 5.23 ft/s
  7. 7Friction loss over the run= 4.39 psi

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Preliminary estimate only. Final sizing must follow the plumbing code in force, including allowances for fittings, meters and available street pressure.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Final sizing must follow the plumbing code in force, including allowances for fittings, meters and available street pressure.

Frequently Asked Questions

How is the Water Supply Pipe Sizing figure worked out?

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.

Which limit usually governs?

On short branch runs velocity governs. On long mains, pressure loss governs and often forces a size larger than velocity alone would suggest.

What values do I need to enter?

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.

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