Sizes the number of luminaires for a target maintained illuminance using the CIBSE/SLL lumen method: E = (N·n·F·UF·MF) ÷ A.
Design-stage indicative. Verify final layout to BS EN 12464-1 and the luminaire UF tables.
For design guidance only. Always verify with a qualified engineer.
About this lighting lumen method calculator
This free lighting lumen method calculator works out how many luminaires a room needs to reach a target maintained illuminance (in lux) over the working plane. It is aimed at electrical and building-services designers, contractors and estimators carrying out an early-stage interior lighting layout, offices, classrooms, workshops, retail and similar regular spaces. Enter the target lux, the room area, the lamp lumens and lamps per luminaire, plus the utilisation and maintenance factors, and it returns the exact and rounded-up luminaire count, the total lamps and the illuminance the installed count actually delivers. Everything runs in your browser. Nothing is uploaded.
How the lumen method is calculated
The average maintained illuminance is E = (N × n × F × UF × MF) ÷ A, where N is the number of luminaires, n the lamps per luminaire, F the initial lamp flux in lumens, UF the utilisation factor, MF the maintenance factor and A the working-plane area. Rearranged for design, the number of luminaires is N = (E × A) ÷ (n × F × UF × MF), rounded up to a whole count because you cannot install a fraction of a fitting. The calculator then reports the illuminance that whole-number count delivers, which is always at or above target. The method follows The SLL Code for Lighting (CIBSE/SLL); target lux levels such as 500 lux for general office work come from BS EN 12464-1:2021 (also cited in CIBSE/SLL LG7). The utilisation factor is read from the luminaire's UF table against the room index and surface reflectances, and the maintenance factor is the product LLMF × LSF × LMF × RSMF.
Frequently asked questions
What utilisation factor should I use?
Read it from the specific luminaire's photometric UF table using the room index (room geometry) and the ceiling, wall and floor reflectances. Typical values fall between about 0.4 and 0.7. The lumen method is only as accurate as this figure, so use the manufacturer's table for the actual fitting rather than a generic value where you can.
Why is the achieved illuminance higher than my target?
Because the luminaire count is rounded up to the next whole number. You cannot install a fraction of a fitting. The extra fittings push the delivered illuminance slightly above target, which is shown as a percentage over. If the over-lighting is significant you can reduce lamp output, use a lower-lumen lamp, or accept it against the project tolerance.
What is the maintenance factor for?
The maintenance factor (MF) allows for light loss over time: lamp lumen depreciation (LLMF), lamp survival (LSF), luminaire dirt (LMF) and room-surface dirt (RSMF). It is the product of those four terms, typically 0.7–0.8 for a clean LED installation on a sensible cleaning cycle. Designing to the maintained illuminance means the room still meets target near the end of the cleaning/replacement interval.