Probable Peak Demand (WSFU to GPM)

Hunter's-curve probable peak flow in GPM from total water-supply fixture units, for flush-tank or flush-valve systems.

Run the calculator

Example

You enter

Total water-supply fixture units (WSFU) 120
System type flush_valve

You get

Probable peak demand 59.4

Details, formula, and sources

gpm = interpolate(curve, wsfu): piecewise-linear between published [WSFU, GPM] breakpoints for the selected system type (flush tank or flush valve). Flush-valve systems peak higher at low WSFU.

Hunter's curve (NBS BMS65, Methods of Estimating Loads in Plumbing Systems) and IPC 2021 Appendix E (Table E103.3(2)) by name; the curve ships as editable breakpoints, not a transcribed table.

IPC 2021 free read-only at codes.iccsafe.org; NBS BMS65 is public-domain (US government). The bundled curve is an editable approximation to tune to the published table.

Estimate. AHJ and licensed professional govern.

Field names used by the API: wsfu, system_type, gpm

Demand curve editable [WSFU, GPM] breakpoints per system type; tune to IPC Appendix EHunter's curve / IPC 2021 Appendix E