In order to design pipe network, pump stations and sewage treatment plants, estimates need to be made of the volumetric flow rate which will be expected to be carried, pumped and treated.

Such flow rates are measured in cubic metres per second and need to be calculated for both existing land use and for expected future development.

There are many methods for calculating expected flow rates. One method is to calculate a design parameter called the "population equivalent" (PE) of a catchment and convert this to a flow rate.

The PE is an estimate of the usage made of sewage facilities. It is not a measure of population.

For residential areas the population equivalent is calculated as five per dwelling and is a direct measurement of the population in an area.

However for commercial areas it is calculated from the floor area, which is considered to be proportional to the number of people using a premises during the day. In this case it does not reflect the population living in an area.

The following table shows how the PE is calculated.