Lighting Level and Fixture Calculator
Lumen method fixture count, achieved lux and power density.
Inputs
Result
Formula
| Fixture count | N = E × A ÷ (Φ × CU × MF) |
| Achieved level | N × Φ × CU × MF ÷ A |
| Power density | N × watts ÷ A |
| Area per fixture | A ÷ N |
| Square spacing | √(A ÷ N) |
| Symbols | E lux, A area, Φ lumens, CU utilisation, MF maintenance |
50 m² at 300 lx with 3000 lm fixtures, 0.7 utilisation, 0.8 maintenance and 30 W: 8.93 calculated, 9 installed, 302 lx achieved, 270 W total and 5.40 W/m².
Illuminance standards and power density limits
| Space | Illuminance lx | Power density W/m² | Note |
|---|---|---|---|
| Corridors and stairs | 100 | 5 | Emergency lighting at least 5 lx |
| Bedroom | 75 | 6 | |
| Kitchen | 100 | 6 | 300 lx at the worktop |
| Office | 300 | 9 | General office work |
| Meeting room | 300 | 9 | |
| Classroom | 300 | 9 | 500 lx on the blackboard |
| Retail floor | 300 | 12 | Good colour rendering needed |
| Fine machining | 500 | — | Set by the process |
| Operating theatre | 750 | — | Theatre lamp separate |
FAQ
Why does the measured level fall short of design?
Because design values are initial, while luminaires gather dirt and sources age. LEDs lose 20-30% of output in three years and dirty housings another 10-20%, which is what the maintenance factor is for. Plenty of installations pass at handover and measure 60% two years later. Dusty workshops should use 0.6 or even 0.5.
Is more light always better?
No. Beyond what the task needs you get glare, eye strain and wasted energy. Standards grade by task precision: 100 lx in corridors, 300 in offices and classrooms, 500 for fine machining, 750-1000 in operating theatres. Uniformity matters too — the minimum over average should not fall below 0.7, or patchiness proves more tiring than a slightly dim room.
How is power density controlled?
Through efficacy. The same 300 lx needs 20 W/m² with incandescent, 9 with fluorescent and only 5-6 with LED. Codes cap LPD by room type — 9 W/m² for offices, 12 for retail — and exceed it and the job fails sign-off. The right way to cut it is better efficacy and utilisation, through light finishes and good optics, not fewer fixtures and dimmer rooms.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.