Lux is light per unit area — one lumen spread over one square meter. Divide total lumens by the area to find the illuminance.
What the terms mean
| Symbol | Meaning |
|---|---|
| E | Illuminance, in lux (lx) |
| Φ | Luminous flux, in lumens |
| A | Area, in square meters |
Worked example
6,000 lumens spread over 20 m² gives 6,000 ÷ 20 = 300 lux.
See the Lumens Formula and Foot Candle Formula.
The lux formula
Lux is light per unit area — one lumen spread over one square metre — so the basic relationship is lux = lumens ÷ area (m²). Spread 4,000 lumens over a 20 m² room and you average 200 lux. For a directional source measured on-axis, use the point method: lux = candela ÷ distance², the metric inverse-square law.
To size lighting with the lumen method, choose a target lux for the task (e.g., 300–500 lux for an office), multiply by floor area to find the lumens required on the work plane, then divide by delivered lumens per fixture for a count — applying a light-loss factor (typically 0.7–0.8) for dirt and lamp ageing. To convert to imperial, divide lux by 10.76 for foot-candles.
Worked example: to light a 5 m × 4 m office (20 m²) to 400 lux, you need about 400 × 20 = 8,000 lux-m² of light on the work plane. If each fixture delivers 2,000 usable lumens after optical and room losses, that’s roughly four fixtures — then nudge up slightly for light loss over time. The same method scales to any room: target lux × area ÷ delivered lumens per fixture.