Code Applied by the app plumb.wsfu.demand

Demand curve: fixture units to gallons per minute

The design flow a load of water supply fixture units stands for, in gallons per minute, read off the code's demand table in its flush-tank and flush-valve columns.

Fixture units are not a flow. The code converts a load to a design flow with a demand table (the Hunter curve): the more fixtures on a pipe, the smaller the share that runs at once, so the flow climbs more slowly than the load. It prints two columns: systems predominantly for flush tanks and systems predominantly for flush valves, whose short heavy draws need more water at the same load. The table prints points; between them the app interpolates on a straight line, which is how the table is ordinarily read (the code text itself gives no interpolation instruction that the sign-off could find).

What the app does with it

Every size suggestion in the water-sizing ladder starts here: the fixture units still to serve beyond a point on a run are converted to gallons per minute in the column the run's fixtures call for, then to a pipe size at the velocity cap. The app interpolates between the printed rows; a load under 1 WSFU reads down to zero with the load; a flush-valve load under 5 WSFU (below the first printed row) reads the flush-tank column; a load past 5,000 WSFU holds the last value, and the schedule says so, because a main that size is an engineer's calculation.

What it does not do

It does not size by pressure and developed length (Appendix E's full method, Tables E103.3(4) to E103.3(7)). That is the master's calculation, and the app leaves it to Bid Check as a manual row until the critical-path math exists.

Verify against your edition

All 100 points, both columns, matched the 2021 text of Table E103.3(3) as a state adoption prints it; the 2018 table was not opened. The three readings under What the app does with it (the flush-tank column under 5 WSFU for flush valves, down to zero under 1 WSFU, the last value held past 5,000) are the app's, not the code's: sensible for a walk-through size, and replaced by the engineer's calculation on a real design. Settled 2026-09-27 by Claude on the owner's delegation, from the dossier's research and the model code text; not checked against a printed book or a local amendment.

Cited by section, not reprinted: the code text belongs to its publisher. Company practice is not a rule here — it lives with your pricing. Updated Sep 27, 2026.