Pipe Diameter for Flow

D = 2 × √(GPM / (velocity × π × 144 × 7.481 / 60)). Find pipe size to deliver target GPM.

Inputs

Domestic: 5-8 ft/s. Industrial: up to 10. Above 10: noise + erosion.

Result

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General calculation reads

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How to use this calculator

  • Enter target flow GPM.
  • Set max velocity (5-8 ft/s residential).
  • Read pipe size.

About this calculator

Pipe sizing balances flow rate against velocity (noise + erosion). Common limits: domestic 5-8 ft/s, commercial 8-10, industrial 10-12. Faster = smaller pipe but more wear, noise, and pressure drop. ½" pipe handles ~3 GPM at 5 ft/s; ¾" handles ~7 GPM; 1" handles ~12 GPM. Standard residential: ¾" mains, ½" branches. Verify with manufacturer flow charts for fixture types.

Frequently asked

Why velocity limit?+
Noise above 8 ft/s is audible. Above 10 ft/s erodes pipe walls (especially copper). Hammer-noise risk.
GPM by fixture?+
Sink/lav: 1-2 GPM. Shower: 2-2.5 GPM (2018 federal max). Toilet flush: 1.6 gallons. Washer: 4-7 GPM peak.
Hot vs cold supply?+
Same sizing rules. Plumbing trades often parallel hot+cold lines for easy labeling.
PVC vs copper?+
Different friction factors but similar size. Copper slightly smoother than PVC.
Pressure drop?+
Long runs lose pressure. Hazen-Williams equation: friction loss ∝ flow^1.85 / diameter^4.87. Sized pipes minimize.

Related calculators

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PSI = ft of head × 0.433. Or 1 PSI = 2.31 ft of water column.
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Pressure psi ↔ bar
1 bar = 14.5038 psi = 100 kPa = 0.987 atm.

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