The sizing of sanitary plumbing systems is based on a unit of measurement called the fixture unit. Each plumbing fixture has a unit rating, which is calculated by considering the rate of discharge, the frequency of use and the time between each use of a plumbing fixture. The fixture-unit rating represents the hydraulic load placed by that fixture on the sanitary plumbing system.
The fixture unit measurement is based on the discharge from a hand basin. One fixture unit is equal to the discharge from one hand basin. All plumbing fixtures have a fixture unit rating, which shows their hydraulic load when compared with a hand basin.
Table 6.1, 'Fixture unit ratings’, in AS/NZS 3500.2:2003 provides the fixture-unit ratings for plumbing fixtures.
A basin is assigned a fixture-unit rating of one (1), and a bath is assigned a fixture-unit rating of four (4). This would be important to know if we were sizing a discharge pipe to connect a basin and bath.
1 fixture unit + 4 fixture units = 5 fixture units
The discharge pipe would be sized on the basis of five (5) fixture units. The bath has a discharge equivalent to four basins.
The example above is a simple one, but similar principles apply when sizing discharge pipes, floor waste gullies, drainage vents and plumbing systems.