Lighting
Lumen method: estimating luminaire numbers
- where
- number of luminaires
- target maintained illuminance (lx)
- area (m²)
- initial luminaire output (lm)
- utilisation factor
- maintenance factor
Explain this equation
Estimates how many luminaires are needed to reach a target maintained illuminance over an area. Use it for first-pass design of general lighting in offices, workshops and classrooms.
E_m is the target maintained illuminance in lux and A the area in m². Φ is each luminaire's initial output in lumens. UF, the utilisation factor, is the fraction of emitted light that reaches the working plane, depending on room shape and surface reflectances. MF, the maintenance factor, allows for falling output and dirt over time. N is the number of luminaires.
E_m × A is the total lumens needed on the working plane. Each luminaire only delivers Φ × UF × MF of useful light by the end of the maintenance period, so one is divided by the other. The calculator rounds up, because you cannot fit part of a luminaire and rounding down would miss the target.
500 lx over 20 m² with 4000 lm luminaires, UF 0.6 and MF 0.8 gives 5.2, so 6 luminaires.
Watch out: enter factors as decimals (0.8, not 80). Layout, spacing, uniformity and glare still need checking separately.
This estimates how many luminaires are needed to achieve a stated maintained average. The utilisation factor accounts for how emitted light reaches the working plane; maintenance allows for reductions with age and dirt.
Calculate the unrounded count, then round upward. A designer must still check layout and distribution using actual photometric data.
Enter your values to see the result.
Common mistake
Do not round the number downward or use 80 instead of 0.80 for an 80% factor.
Limits
Preliminary general-lighting estimate. Use the project illuminance target and manufacturer utilisation data on the same lumen basis. Do not double-count optical losses. Check layout, uniformity, glare and spacing separately.
Sources
The tools calculate the displayed equation. They do not automatically choose cable sizes, approve installations, supply current regulatory tables, certify equipment or replace inspection and testing.