Water Pressure & Flow Calculator
Estimate the flow rate (GPH) available at your fixtures given pipe size, supply pressure, elevation change, and pipe length. Use this when sizing a plumbing system or diagnosing low-pressure complaints.
About this calculator
Water flow through a pipe depends on the net driving pressure after accounting for static head loss from elevation and friction loss along the pipe. Elevation reduces pressure by roughly 0.433 PSI per foot of rise, and friction loss is approximated here at 0.1 PSI per foot of pipe. The net pressure drives flow through a pipe whose capacity scales with diameter to the fourth power — a relationship rooted in the Hagen-Poiseuille principle. The formula used here is: Flow = √(pipeSize⁴ × max(pressure − elevation × 0.433 − pipeLength × 0.1, 0.1)) / fixtures × 7.48 × 60, converting cubic feet per minute to gallons per hour. Dividing by the number of fixtures distributes the available flow across all simultaneous draw points. This gives a practical estimate for residential and light-commercial plumbing design.
How to use
Suppose you have a 1-inch pipe (pipeSize = 1), 60 PSI supply pressure, 100 ft of pipe, 10 ft elevation rise, and 3 fixtures. Net pressure = 60 − (10 × 0.433) − (100 × 0.1) = 60 − 4.33 − 10 = 45.67 PSI. Flow = √(1⁴ × 45.67) / 3 × 7.48 × 60 = √45.67 / 3 × 448.8 = 6.758 / 3 × 448.8 ≈ 1,011 GPH across all fixtures, or about 337 GPH per fixture. Enter your own values to check whether your system meets demand.
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?
Because pipe capacity scales with the fourth power of diameter (from Hagen-Poiseuille), even small increases in pipe diameter produce large gains in flow. Doubling the diameter increases theoretical flow capacity by a factor of 16. 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.