Four different light quantities
Lumens (lm) measure the total visible light a source emits. Candela (cd) measure intensity in a particular direction. Lux (lx) measure light arriving on a surface: one lumen per square meter. Watts measure the electrical power a lamp draws.
None of these converts to another by a fixed factor. Lumens to lux needs the area lit; candela to lux needs the distance; lumens to candela needs the beam angle; lumens to watts needs the lamp's efficacy. The calculator asks for each one explicitly.
| Conversion | Formula | Extra input |
|---|---|---|
| Lumens → lux | E = Φ ÷ A | Area (m²) |
| Candela → lux | E = I ÷ d² | Distance (m) |
| Lumens → candela | I = Φ ÷ [2π(1 − cos(θ/2))] | Full beam angle θ |
| Watts → lumens | Φ = P × η | Efficacy η (lm/W) |
| Lux → watts | P = E × A ÷ η | Area (m²) and efficacy η |
| Watts → lux | E = P × η ÷ A | Efficacy η and area (m²) |
Worked examples
An 800 lm lamp lighting a 10 m² floor evenly would give 80 lx. A spotlight of 1,000 cd pointed at a surface 2 m away gives 1,000 ÷ 2² = 250 lx.
A 500 lm LED with a 40° beam spreads its light over 0.379 steradians, giving an average intensity of about 1,320 cd across the beam.
To light a 20 m² room to 300 lx with 100 lm/W LEDs you need 300 × 20 ÷ 100 = 60 W of lamps before losses. Going the other way, 60 W × 100 lm/W over 20 m² gives 6,000 ÷ 20 = 300 lx.
Assumptions
Lumens to lux assumes all the light lands evenly on the area. The inverse-square law assumes a point source aimed straight at the surface. Lumens to candela assumes uniform intensity across a cone. Real fixtures, rooms and beams differ, so lighting designers use photometric files and utilization factors for final layouts.
One foot-candle is one lumen per square foot, which equals 10.764 lux.