Water Pipe Sizing Calculator

This calculator sizes a water supply pipe from Total Fixture Units and Longest Pipe Run alone. Total Fixture Units feeds Hunter's curve approximation to produce Estimated Flow Rate, and it is both the dominant and the sole driver of that result, since it is the only input the formula uses. Estimated Flow Rate is, in turn, the sole input to Required Pipe Size.

The calculator then picks the smallest standard trade pipe size that keeps velocity at or below an 8 ft/s design maximum, and Longest Pipe Run feeds a Hazen-Williams friction-loss formula against that selected pipe size to report Friction Pressure Loss, so a longer run always raises the reported loss. Number of Floors and Available Pressure are both collected on this form, but neither one factors into any of the four results shown -- they exist for you to record project context and to compare Friction Pressure Loss against your own supply pressure by hand, not because the underlying sizing math uses them. That gap matters most on tall buildings, where elevation head loss (roughly 0.43 psi per foot of rise) can matter as much as friction loss; this calculator does not add that separately, so treat Friction Pressure Loss as pipe friction only, not total system pressure loss.