The peak water demand estimate is the most essential factor in determining the pipe sizes for the plumbing system in a building. The peak demand can be estimated for a known number of fixtures, fixture design flow rate, and the probability that the fixture is busy (i.e., water is flowing at the fixture) during the peak period of use. The fixture count and design flow rate are relatively easy parameters to estimate. However, the probability that a fixture is busy is somewhat elusive; it is a ratio of the flow duration to the duration of fixture observation for similar fixtures in a given location. The probability that a fixture is busy depends on the number of fixtures in a location, fixture accessibility (public or private), human use characteristics, fixture characteristics (automatic or manual operated), and other use conditions such as periods with high pedestrian traffic, i.e., congestion during the observation window. This paper presents several methods of arriving at the peak period probability that a fixture is busy. Results of data collected from observing fixture use, flow measurements at a fixture, and flow simulation at a fixture are consistent. The simulation exercise was further extended to create a relationship between fixture probability of use and fixture utilization. This relationship shows the full extent of fixture use from zero use to congestion when the fixture is always occupied. This relationship can be used to determine a fixture p-value depending on a building's end use and may provide guidance on the design process for determining the number of fixtures required in a building.
