Office, workshop or kitchen – the question is always the same: how much light does the room need, and how many luminaires provide it? The Lighting Calculator (Lumen, Lux, Number of Luminaires) uses the utilisation factor method of lighting design.
The formula
Φ = E × A ÷ (η × MF) – the required total luminous flux Φ (lumens) results from illuminance E (lux), room area A, utilisation factor η and maintenance factor MF. The utilisation factor captures how much light actually reaches the working plane – bright rooms achieve 0.5–0.6, dark furniture and high ceilings push it down to 0.3–0.4.
Step by step
- Select target illuminance: per EN 12464-1 e.g. 500 lx for office work, 300 lx for rough bench work, 150 lx for circulation areas in warehouses
- Enter room data: length, width, room brightness (bright/medium/dark)
- Choose the luminaire: luminous flux per luminaire in lumens (datasheet)
- Read the result: total luminous flux, number of luminaires and a grid suggestion for uniform lighting
Worked example
Office 5 × 6 m, 500 lx, bright room (η = 0.6), maintenance factor 0.9: Φ = 500 × 30 ÷ (0.6 × 0.9) = 27,778 lm. With LED panels of 3600 lm each, that is 8 luminaires – sensibly arranged as a 2×4 grid.
Practical tips
The maintenance factor (usually 0.8–0.9) compensates for ageing and soiling – omit it and you design a system that is only bright enough on day one. Calculate with the luminaire's luminous flux, not the bare LED's (optics and cover losses). And: the lux values of EN 12464-1 apply on the working plane (0.75 m desk height), not on the floor. For demanding projects, a DIALux calculation remains the gold standard – but as pre-planning and sanity check, the utilisation factor method is unbeatably fast.
