How the lumen method sizes a lighting layout
The lumen method is the standard first pass for interior lighting: given a room area, a target maintained illuminance and the output of one luminaire, it returns how many fixtures the space needs. It is fast enough for budget pricing and retrofit studies, which is where most designers still start.
The formula is luminaires = (target footcandles × area in ft²) ÷ (lumens per luminaire × CU × LLF). Because a footcandle is one lumen per square foot, the units cancel directly with no conversion factor. CU is the share of emitted light that reaches the work plane, and LLF is the allowance for dirt accumulation and lamp lumen depreciation.
For a 1,000 ft² office at 50 fc with 6,000 lm luminaires, CU 0.6 and LLF 0.8, the math gives 17.4 fixtures, so 18 go in and the installed average lands near 51.8 fc, which is about 558 lux if you need the metric equivalent (1 fc ≈ 10.76 lux).
| Space type | Typical IES target (fc) |
|---|---|
| Corridors and stairs | 5-10 |
| Bulk warehouse storage | 5-15 |
| General office and classroom | 30-50 |
| Retail sales floor | 50-75 |
| Drafting, inspection, fine assembly | 75-100 |
Those targets follow IES Lighting Handbook practice and are illustrative only; check the current IES recommended practice for your specific task, and note that energy codes such as ASHRAE 90.1 cap lighting power density regardless of the count you arrive at. Uniformity, glare and emergency lighting still need a point by point model and a licensed electrical engineer before construction.
Frequently asked questions
The coefficient of utilization comes from the luminaire photometric report combined with room cavity ratio and surface reflectances; recessed troffers in a normal office usually land between 0.5 and 0.7. A light loss factor of 0.8 suits clean interiors with LED sources, while dusty industrial spaces are closer to 0.6 or 0.7.
No. The lumen method gives an average maintained illuminance across the room and says nothing about uniformity, task plane values or glare. Use it for budgeting and early layout, then confirm with point by point software and a licensed engineer.