Plumbing
Pipe Flow Calculator
Calculate the flow rate a pipe carries at a given velocity, or the velocity at a given flow.
Result
Pipe and drain sizing is governed by plumbing code (IPC/UPC or local equivalent) and must be designed and installed by a licensed plumber. This is an educational estimate, not a design. Friction loss uses a Hazen-Williams coefficient of 150, typical for smooth plastic pipe. Older galvanised or scaled pipe loses far more, and fittings add equivalent length this does not count.
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About this calculator
How much water can a 3/4 inch pipe carry?
Roughly 14 GPM at 5 ft/s, though usable flow depends on available pressure and the length of the run. Long runs lose pressure to friction.
Why does my flow drop at the far end of the house?
Friction loss accumulates with distance and rises sharply with flow rate. Doubling the flow roughly triples the loss, which is why simultaneous fixtures cause the problem.
The formula
GPM = 2.448 × d² × velocity; friction loss estimated by Hazen-Williams (C = 150)
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