Water Pressure & Flow Calculator
Estimate the flow rate (GPM) each fixture can get when several run at once, given pipe size, supply pressure, elevation change, and pipe length. Use this when sizing a plumbing system or diagnosing low-pressure complaints.
Last updated: September 2026
Formula below · 2 sources (epa.gov, Wikipedia) · Updated Sep 2026
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About this calculator
Water flow through a supply pipe is limited by the pressure left over after lifting the water and keeping a minimum pressure at the fixture. Elevation costs 0.433 psi per foot of rise, and plumbing codes require at least about 8 psi at a typical fixture, so the pressure available to push water through the pipe is: net = pressure − 0.433 × elevation − 8. The calculator then uses the Hazen-Williams friction formula for water, with C = 140 (smooth copper or plastic pipe) and the nominal pipe size as the inside diameter d in inches: pressure loss (psi) = 4.52 × Q^1.852 × length ÷ (140^1.852 × d^4.87), with Q in GPM. Solving for Q gives the largest total flow the pipe can carry while using up exactly the net pressure, and dividing by the number of fixtures gives the flow each fixture gets when they all run at once: Flow per fixture = (net × 140^1.852 × d^4.87 ÷ (4.52 × length))^(1/1.852) ÷ fixtures. Fittings, valves and meters add friction (often the equivalent of 25-50% more pipe length), velocity limits of about 5-8 ft/s are normally used in design, and real fixtures are limited by their own flow restrictors (about 1.5-2.5 GPM for faucets and showerheads), so treat the result as an upper bound on what the piping can deliver.
How to use
Suppose you have a 1-inch pipe, 60 PSI supply pressure, 100 ft of pipe, a 10 ft elevation rise, and 3 fixtures running together. Net pressure = 60 − (10 × 0.433) − 8 = 47.67 PSI. Total flow = (47.67 × 140^1.852 × 1^4.87 ÷ (4.52 × 100))^(1/1.852) ≈ 41.6 GPM, or about 13.85 GPM per fixture — far more than a 2.5 GPM showerhead needs, so this pipe is not the bottleneck (and design velocity limits would keep real flow lower). With a 1/2-inch pipe the same run delivers only about 2.24 GPM per fixture, which is why long runs of small pipe feel weak when several taps are open.
Frequently asked questions
How does elevation change affect water pressure in a plumbing system?
Every foot of vertical rise in a plumbing system reduces static water pressure by approximately 0.433 PSI, because water weighs about 0.433 pounds per square inch per foot of head. This means a fixture 20 feet above the meter loses roughly 8.66 PSI before any water even starts flowing. In multi-story buildings or homes on hilly lots, this elevation loss can be significant enough to require a booster pump. Always subtract the elevation head from your available supply pressure when sizing pipes.
What pipe diameter should I use for adequate flow rate in a residential system?
At a fixed pressure loss, flow through a pipe scales with roughly the 2.63 power of its diameter (Hazen-Williams), so small increases in diameter produce large gains: doubling the diameter carries about 6 times the flow. For most residential supply lines, ¾-inch to 1-inch main lines are standard, with ½-inch branches to individual fixtures. If you calculate inadequate flow with ¾ inch, upgrading to 1 inch is often far more cost-effective than boosting pressure.
Why does my water pressure drop when multiple fixtures run at the same time?
When several fixtures open simultaneously, the available flow must be shared among all of them, reducing pressure and flow at each point. The total friction loss in the supply piping also increases because more water is moving through the same pipe. This simultaneous-demand effect is why plumbers use fixture unit tables and peak-demand calculations rather than single-fixture flow rates. Installing larger-diameter mains or a pressure-boosting pump are the primary remedies for multi-fixture pressure drop.