Lighting Lux Calculator
Estimate how many luminaires a room needs to reach a target illuminance, using the lumen method with utilisation and maintenance factors.
Last reviewed
Indicative only. Assumes a square room with an even grid and half-spacing to the walls. A real layout follows the fitting's spacing-to-mounting-height ratio and the actual room shape.
The lumen method
The lumen method gives the average illuminance a space will reach for a given number of fittings, or the number of fittings needed to hit a target:
where N is the number of luminaires, E is the target lux, A is the area in m², F is the lumen output per luminaire, UF is the utilisation factor and MF is the maintenance factor. The calculator rounds up, then shows the average lux you would actually achieve with that whole number of fittings.
About the factors
The utilisation factor depends on the room proportions and how reflective the ceiling, walls and floor are, and comes from the luminaire manufacturer's photometric data. The maintenance factor allows for dirt and lamp depreciation over the maintenance cycle. The defaults here are sensible starting points, not a substitute for the real figures for your fitting and space.