Starting pressure and flow
Source pressure is what comes out of your municipal water main or well pump – typically 40-80 PSI. Flow rate depends on the fixture: shower 2-2.5 GPM, garden hose 5-8 GPM, irrigation zone 8-15 GPM.
Pipe details
Copper and CPVC have the lowest friction loss. PEX has higher friction loss per foot but is acceptable for most homes. Old galvanized has the most loss because of mineral buildup.
Measure the actual pipe run from source to the fixture. Use the longest path for worst-case calculation.
Fittings and elevation
Every 2.31 feet of upward rise costs 1 PSI. Going downhill adds pressure. For a typical two-story house, the second floor is about 10-12 ft above the basement.
Pressure at the fixture
Pipe friction loss
0
PSI lost in pipe run
Fittings loss
0
PSI lost in fittings
Elevation loss
0
PSI gain/loss from height
Pressure at fixture
0 PSI
working pressure available
Most fixtures need at least 20 PSI to work well; 30-40 PSI is preferred. Showers feel weak below 30 PSI. Irrigation systems typically need 30-50 PSI at the head. Estimates use the Hazen-Williams equation for common residential conditions and approximate values for typical fittings.