# Lighting Level and Fixture Calculator

Lumen method fixture count, achieved lux and power density.

- Page: https://jianzhujisuan.com/en/lighting-lux
- Category: M&E (https://jianzhujisuan.com/en/c/me)
- Chinese version: https://jianzhujisuan.com/zh/lighting-lux.md
- Data checked: 2026-10-25

## Quick result

With the default inputs (Room area 50 m², Target illuminance 300 lx, Lumens per fixture 3000 lm, Utilisation factor 0.7, Maintenance factor 0.8, Watts per fixture 30 W): **Fixtures required 9 pcs**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Room area | number | 50 | m² |  |
| Target illuminance | number | 300 | lx | Office 300, classroom 300, corridor 100, fine work 500 |
| Lumens per fixture | number | 3000 | lm | 18 W LED about 1800, 36 W about 3600 |
| Utilisation factor | number | 0.7 | — | 0.7-0.8 for light small rooms, 0.4-0.5 for dark large ones |
| Maintenance factor | number | 0.8 | — | 0.8 clean, 0.6 dusty |
| Watts per fixture | number | 30 | W |  |

## 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

## Worked example

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.

## Questions

### 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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