How many luminaires does the room need? Calculate luminous flux demand and number of fixtures using the utilisation factor method with EN 12464-1 target values.
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Lighting design: from target lux to luminaire count
How many LED panels for the office, how many battens for the workshop? The utilisation factor method answers this with one formula: Φ = E × A / (η × MF) – required luminous flux equals target illuminance times area, divided by utilisation factor and maintenance factor.
Worked example: 5 × 4 m office
EN 12464-1 requires 500 lx for office work. With 20 m², a normal room (η = 0.45) and LED maintenance factor 0.8: Φ = 500 × 20 / (0.45 × 0.8) = 27,778 lm. With 3600 lm LED panels that is 8 luminaires, sensibly arranged in a 4 × 2 grid.
Why lumen ÷ area is not enough
Only part of the luminous flux reaches the working plane: walls and ceiling absorb light (utilisation factor η ≈ 0.35–0.55), and output decreases over time through ageing and dirt (maintenance factor MF ≈ 0.57–0.8). Ignoring both under-sizes the installation by 40–60 %.
Illuminance targets per EN 12464-1 (selection)
Circulation areas: 100 lx · storage: 100–200 lx · bathrooms: 200 lx · kitchens: 300 lx · office/reading: 500 lx · technical drawing: 750 lx · precision work: 1000 lx and above.
FAQ
How many lumens per m²? Rule of thumb for a normal room: target lux × 2.8 of installed flux. For 300 lx about 840 lm/m².
