Fixture units add up — flow rates (GPM) don't
WSFU values already discount how often each fixture runs, so they add up in a way raw GPM can't. 5.7 WSFU at 100 ft and 45 psi reads as a 1″ building supply off a 3/4″ meter (IRC Table AP201.1).
What this diagram shows
A water meter feeding a trunk line with four fixtures tapping off it — a toilet, lavatory, shower, and kitchen sink — each labeled with its Water Supply Fixture Unit value and a running total that climbs from 2.2 to 2.9 to 4.3 to 5.7 as each fixture is added. Below, three input boxes for total demand (5.7 WSFU), developed length (100 feet), and available pressure (45 psi) converge with arrows into a result box reading a 1-inch building supply off a 3/4-inch meter, read from the IRC sizing table rather than calculated from flow rate.
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Water Supply Pipe Size Calculator
Free water supply pipe sizing calculator — total water supply fixture units and minimum main size, with the real IPC-vs-UPC fixture-unit difference.
