The utilisation factor method is used for an estimated calculation of lighting installations. It is used to calculate the number of luminaires required for the target illuminance on the working plane or the illuminance achieved by a specified number of luminaires. The utilisation factor method is based on the fact that the average horizontal illuminance for a room of a specific size can be calculated using the total luminous flux of the installed luminaires and the light output ratio along with the utilisation factor.
The utilisation factor method is rarely relevant to routine planning any more since it is based on standardised rooms. Today, it is much easier and quicker to calculate individual rooms using computer programs. The utilisation factor method is still used as the basis for the relevant European standard and for planning programs, to calculate the average illuminance for rooms on regular luminaire grids.