Water Pressure Loss Calculator

It splits the loss into two parts. Friction loss comes from the Hazen-Williams equation, which uses your flow rate in GPM, the pipe inner diameter in inches, the pipe length, and a roughness coefficient set by the material. Elevation loss adds 0.433 PSI for every foot the fixture sits above the supply. The total is friction plus elevation.

The C factor is the Hazen-Williams roughness coefficient, a measure of how smooth the pipe walls are. Smooth materials like copper, PVC, and PEX use about 150, new galvanized steel about 120, and old corroded steel about 100. A lower C means rougher pipe and more pressure loss, so picking the wrong material can throw the result off noticeably. Choose your material from the list and the calculator sets C for you.

A one-foot column of water exerts about 0.433 PSI of static pressure at the bottom, so lifting water one foot higher costs that much pressure regardless of flow. Multiply the vertical rise from the supply to the fixture by 0.433 to get the elevation loss in PSI. If the fixture sits below the supply, enter a negative rise and the calculator adds that pressure back instead.

Use the actual inner diameter, because friction loss is extremely sensitive to it (the diameter is raised to the power of about 4.87). The size dropdown maps common nominal sizes to a typical inner diameter, but real inner diameters vary by material, schedule, and wall thickness. If you know the exact inside diameter of your pipe, choose Custom diameter and enter it for the most accurate result.

Most plumbing practice keeps supply lines under about 8 feet per second to limit noise, water hammer, and long-term erosion of the pipe walls. The calculator reports the flow velocity for your inputs and flags it when it climbs above 8 ft/s. If you see that warning, step up to a larger pipe size, which lowers both the velocity and the friction loss.

The Hazen-Williams equation is a widely used engineering estimate for water near room temperature in full, turbulent, pressurized pipe, which covers most home and light-commercial supply lines. It does not include extra losses from fittings, valves, meters, or a water heater, so add those separately or use an equivalent-length allowance. Treat the number as a sizing estimate, not a guaranteed measured value.