Intermediate
Lumen Method Formula
Calculate the number of luminaires needed for a space using the lumen method — foot-candle targets, CU, and maintenance factors.
Formulas
N = (E × A) / (Φ × CU × MF)
Number of luminaires required
E = (N × Φ × CU × MF) / A
Resulting illuminance from N luminaires
Variable Definitions
N=Number of luminaires(count)
E=Target illuminance(foot-candles (fc) or lux)
A=Area of the space(square feet (ft²) or m²)
Φ=Lumens per luminaire(lumens (lm))
CU=Coefficient of utilization(dimensionless (0-1))
MF=Maintenance factor (light loss factor)(dimensionless (0-1))
Worked Example
Office space: 40×60 ft, target 50 fc, using 4,000 lm luminaires, CU=0.65, MF=0.80.
- A = 40 × 60 = 2,400 ft²
- N = (50 × 2,400) / (4,000 × 0.65 × 0.80)
- N = 120,000 / 2,080 = 57.7
Answer: 58 luminaires required
When to Use
- •Designing general lighting layouts for offices, warehouses, retail
- •Verifying existing lighting meets IES illuminance recommendations
- •Comparing luminaire options for a given space
- •Estimating lighting electrical load for panel sizing
Standards & References
- NEC 220.12 — General Lighting Loads by Occupancy
- IES Lighting Handbook — Recommended Illuminance Levels