The formula
N = (E·S·k) / (F·u), where E — illuminance (lx), S — area (m²), k — maintenance factor (0.7-0.8), F — lamp flux (lm), u — utilization factor (0.5-0.7).
Standards (EN 12464 / SP 52)
Living room 150-200 lx, kitchen 200-300, bathroom 200, corridor 100, office 300-500, classroom 300-500, warehouse 150-200. For LED, take 100 lm/W and more.
Example: living room 20 m²
E=200 lx, S=20, k=0.8, u=0.6, F=3200 lm (LED 24 W). N = 200·20·0.8 / (3200·0.6) ≈ 1.67 → 2 fixtures of 24 W or 4 of 12 W. Add local lighting for reading.
Example: office 16 m²
E=400 lx: N = 400·16·0.8 / (3200·0.6) ≈ 2.7 → 3-4 fixtures with uniform placement. Check glare (UGR ≤ 19).
Placement rules
Uniform: fixtures in rows with a ratio of spacing to height ≤ 1.5. Above the table — the table lamp; under the upper cabinets — strip lighting.
GorkyCAD lighting
Draws the room, sets the required illuminance, places fixtures — and GorkyCAD calculates the number per the lumen method and checks it per the standard.