Lumen-Method Luminaire Count
How many fixtures to hit a target maintained footcandle level in a room, by the IES lumen method.
Example
You enter
- Target maintained (footcandles) 50
- Room area (sq ft) 1200
- Lumens per luminaire 5000
- Coefficient of utilization (CU) 0.7
- Light-loss factor (LLF) 0.8
You get
- Luminaires (rounded up) 22
- Exact count 21.43
- Achieved maintained 51.3 fc
- Total installed lumens 110,000 lm
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
ScopeTarget x area / (lumens x CU x LLF), rounded up, with the achieved footcandles at that count.
Formula or tablecount = ceil(target_fc x area / (lumens_per_lum x CU x LLF)); achieved_fc = count x lumens_per_lum x CU x LLF / area. 1 fc = 1 lumen per square foot.
Source and editionIES lumen method (zonal-cavity number-of-luminaires relation, by name).
Free accessMethod is public lighting practice; CU comes from the maker's photometric report.
What governsEstimate. AHJ and licensed professional govern.
API field namestarget_fc, area_sqft, lumens_per_lum, cu, llf, count, count_raw, achieved_fc, total_lumens
- Coefficient of utilization default 0.7; read from the photometric report at the room cavity ratio and reflectances; bounded (0, 1.5]IES lumen method
- Light-loss factor default 0.8 = lamp-lumen depreciation x luminaire-dirt depreciation; bounded (0, 1]IES lumen method
- Average maintained sizes the average level over the work plane, not a point value or uniformity; a photometric layout governs spacingIES lumen method