Plumbing demand (Hunter's curve)

Estimates the probable simultaneous peak water demand on a plumbing main from a fixture count, using Hunter's fixture-unit method, then checks a candidate supply-main size against velocity and head-loss criteria.

Sums each fixture's Water Supply Fixture Units (WSFU, IPC Table E103.3(3)), then maps the total WSFU through Hunter's empirical demand curve (IPC Table E103.3(2), reproducing NBS BMS65 Fig.6 Curves 1 & 2 for flush-tank vs flush-valve systems) by linear interpolation — there is no closed-form equation, the curve is a published lookup table. The resulting demand is checked against the pipe using the same Darcy–Weisbach head-loss primitive as the pressure-drop calculator.