Water supply fixture units (WSFU) and drainage fixture units (DFU), common fixture unit values, Hunter's curve and probability-based sizing, and how a real fixture-unit takeoff is organized.
A fixture unit is not a flow rate — it is a probability-weighted demand value that converts a raw fixture count into a number that already accounts for the statistical reality that not every fixture in a building operates at once. This module works through the WSFU/DFU distinction, the common fixture-unit values this program's worked examples use, and Hunter's curve — the nonlinear relationship that converts a WSFU total into an actual design GPM.
By the end of this module you should be able to explain why a large apartment tower's water service can be far smaller than a naive fixture-count extrapolation would suggest — the direct, practical consequence of Hunter's curve flattening at high totals, and the exact calculation Module 4's pipe sizing content picks up where this module leaves off.