Watts to Lux Calculator
Estimate illuminance in lux from a light source’s power output, its luminous efficacy, and the area it lights.
Why watts don’t directly equal lux
Watts measure raw power, while lux measures illuminance, how much visible light actually falls on a surface as perceived by the human eye. Converting between them requires knowing the light source’s luminous efficacy, since different technologies convert electrical power into visible light with very different efficiency.
How this calculator works
The calculator first estimates luminous flux in lumens using Φ (lm) = Power (W) × Luminous Efficacy (lm/W). It then spreads that flux over the illuminated area to get illuminance: Lux = Lumens ÷ Area (m²). The theoretical maximum efficacy of 683 lm/W applies only to pure monochromatic 555nm (green) light; real sources are far less efficient, so preset values are provided for common source types, or you can enter your own measured or specified efficacy.
This is a simplified estimate assuming light spreads evenly over the stated area. Real-world illuminance also depends on beam angle, distance, reflectance, and fixture design, so this tool is for general educational and planning purposes only.