In creating Hunter’s Curve, the beauty of what Dr. Roy Hunter did was to take an incredibly complex problem in probability theory and make it simple — so simple, in fact, that all it required was basic arithmetic. The complexity involved a “mixed system” combining the calculation of instantaneous demand in a water distribution system with fixture types that differ from each other in frequency of use and flow rate. The solution resulted in a fixture list with unique unit values (fixture units) and a design curve predicting the 99th percentile flow rate for each fixture unit value. In order to properly size the pipes in a domestic water (i.e. potable water) piping system, each plumbing fixture (e.g. toilet, sink, shower, etc.) had an assigned value, known as the fixture unit (specifically, the water supply fixture unit). All the plumbing system engineer or designer needed to do was count the number of each type of plumbing fixture inside a building or area inside the building, multiply by each plumbing fixture’s “fixture unit” value and total the fixture units. From there, referencing the “Hunter’s Curve” chart would lead to a given instantaneous peak flow rate. Referring to Table 1.1 and Figure 1.1 you can see an example of this process.