Pipe Size & Flow Rate Calculator

The IPC and most plumbing codes recommend a maximum velocity of 8 ft/s for cold water and 5 ft/s for hot water supply piping. At higher velocities, turbulent flow creates pipe noise, water hammer, and accelerated erosion at fittings -- particularly copper fittings at elbows and tees. Velocity above 8 ft/s is associated with accelerated pipe wear and noise complaints in residential installations.

Galvanized steel pipe corrodes internally over 20 to 40 years, building up scale (zinc oxide and mineral deposits) that reduces the effective inside diameter. A 3/4-inch galvanized pipe with moderate scale has an effective flow area 30-50% smaller than new pipe. This is why the Hazen-Williams C coefficient for galvanized drops from 120 when new to 80-100 in aged systems. Low pressure complaints in older homes almost always trace back to degraded galvanized pipe.

Pressure drop (or friction loss) is the reduction in pressure caused by fluid flowing through a pipe -- it is a result of wall friction and turbulence. Elevation pressure loss is a separate component: for every foot of elevation gained, the system loses 0.433 PSI. A second-floor fixture 10 feet above the meter has 4.33 PSI less pressure than the ground floor supply before any pipe friction is accounted for. Both must be subtracted from supply pressure to determine fixture delivery pressure.

Manifold systems use individual runs from a central manifold to each fixture -- eliminating trunk-and-branch sizing complexity. Each run is sized for the single fixture it serves. PEX 1/2-inch handles most individual fixtures up to 50 feet at acceptable pressure drop. Runs over 50 feet or for high-demand fixtures (bathtub fill) should use 3/4-inch PEX. The manifold supply line is sized for total simultaneous demand, typically 1-inch from the main valve to the manifold for homes with more than 4 bathrooms.