Lumen Method — How It Works
The lumen (or light-flux) method sizes general lighting for a uniform working plane. The number of luminaires is N = (E × A) ÷ (F × UF × MF): target illuminance times area, divided by the lumens each fitting actually delivers to the plane after the utilisation factor (how much light reaches the working plane, from the room index and reflectances) and the maintenance factor (lumen depreciation and dirt). Round up, then lay the fittings on a grid within their spacing-to-mounting-height ratio for acceptable uniformity.
Frequently Asked Questions
Use N = (E x A) / (F x UF x MF): target lux times area, divided by lumens per fitting, utilisation factor and maintenance factor. For 500 lux over 50 m2 with 4000 lm fittings, UF 0.5, MF 0.8 you need 16 luminaires.
The fraction of a luminaire output that reaches the working plane, typically 0.4-0.6. It depends on the room index (proportions), the fitting light distribution and the ceiling/wall/floor reflectances; read it from the manufacturer UF table.
It accounts for light loss over time from lamp lumen depreciation and dirt on the fitting and room surfaces, typically about 0.8. Cleaner environments and shorter cleaning intervals give a higher MF.
Arrange the fittings on a regular grid and keep the spacing within the luminaire spacing-to-mounting-height ratio (often around 1.5). Too-wide spacing gives bright spots and dark gaps.
Follow EN 12464-1 / local codes: about 300 lux for general office/living, 500 lux for kitchens and desks, 750-1000 lux for detailed work, lower for circulation areas.
Lumen Method Lighting Design
The lumen method is the standard way to work out how many fittings a room needs for uniform general lighting. It links the design illuminance to the installed light output through two efficiency factors.
The formula
N = (E × A) ÷ (F × UF × MF). The utilisation factor captures how much of each fitting's output actually reaches the working plane; the maintenance factor captures losses over the cleaning/replacement cycle.
Layout and uniformity
Once you have the count, place the fittings on a grid within their spacing-to-height ratio so light is even. Pair with the lumen-to-lux calculator to set the target and the cable sizing tool for the lighting circuit.
Related: Lumen to Lux, Cable Sizing.