Number of luminaires N = E × A ÷ (lumens × UF × MF). A 10 × 6 m office at 500 lux with 4,000 lm luminaires, UF 0.6 and MF 0.8 needs 15.6, so 18 in a 3 × 6 grid at about 1.67 × 2.0 m spacing, giving about 576 lux and 9.6 W/m² at 32 W each.
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Lumen method: N = E × A ÷ (Φ × UF × MF) = 500 × 60.0 ÷ (4,000 × 0.60 × 0.80) = 15.6
Grid of 3 × 6 = 18, spaced 1.67 × 2.00 m
Achieved = 18 × 4,000 × UF × MF ÷ area = 576 lux
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Generated with myconstructioncalculator.com. Estimates for planning and ordering only; verify against drawings and specifications.
Formulas
N = E × A ÷ (Φ × UF × MF)Grid = rows ≈ √(N × W ÷ L), columns = ⌈N ÷ rows⌉Achieved = rows × columns × Φ × UF × MF ÷ A
Worked example
10 × 6 m office, 500 lux, 4,000 lm / 32 W luminaires, UF 0.6, MF 0.8, 2.2 m above the desk:
N = 500 × 60 ÷ (4,000 × 0.6 × 0.8) = 15.6
Grid 3 × 6 = 18 at 1.67 × 2.0 m
Achieved ≈ 576 lux; spacing-to-height 0.91; 576 W (9.6 W/m²)
Typical maintained illuminance
Space
Lux
Corridors, stairs
100
Living areas, lobbies
150
Stores, warehouses
200
Classrooms, retail
300
Offices
500
Technical drawing
750
Frequently Asked Questions
How many lights do I need for a room?
Use the lumen method, required lux × floor area ÷ (lumens per luminaire × utilisation factor × maintenance factor), then round up to a tidy grid.
What lux level do different rooms need?
Typical maintained levels are about 500 lux for offices, 300 for classrooms and shops, 100–200 for corridors and stores, and 750 for fine work. EN 12464-1 and local codes give the exact values.
How far apart should ceiling lights be?
Keep the spacing no more than the luminaire's spacing-to-height ratio (often about 1.0–1.5 × mounting height) and about half a spacing from the walls.
