WSFU to GPM Conversion Chart

Count the water supply fixture units for everything on the segment — per-fixture values are on the WSFU chart and the fixture unit calculator totals them. Decide which column governs: if the water closets are tank type, read the flush-tank column; if they're flushometer valves, read the flushometer column. The resulting GPM, together with available pressure and developed length, is what actually sizes the pipe — via the pipe sizing calculator or the code's procedure.

Fixture units do not apply to constant-use loads such as hose bibbs, irrigation, or cooling equipment — assign those their actual GPM directly and add them on top of the fixture-unit demand.

The table is Roy Hunter's probability curve from the 1940 National Bureau of Standards work, as tabulated in IPC Appendix E. It answers a statistical question — the peak flow a group of intermittently used fixtures will probably demand — which is why continuous loads like irrigation and cooling makeup are excluded and added in GPM on top. IPC 101.2: appendix provisions apply only where the jurisdiction has specifically adopted that appendix. Confirm Appendix E is adopted before citing it as enforceable. Independently computed Hunter tables in engineering handbooks do not match the code's printed rungs cell-for-cell, so cite the code table, not a recomputation. Local amendments override everything; confirm with your jurisdiction.