Lumens to Watts / Lux to Footcandles Converter

What each unit actually measures

The distinction that trips people up is between light produced and light arriving. A bulb’s lumens never change; the lux on your worktop depends entirely on how far away it is and how big the room is.

Luminous Flux (Lumens) Lumens (lm) measure the total visible light a source emits in every direction. A candle puts out about 12 lumens. A 60 W incandescent bulb manages roughly 800.

Power (Watts) Watts measure electrical energy consumed per second. Dividing lumens by watts gives luminous efficacy, which is the honest measure of a bulb. Wattage stopped being a useful proxy for brightness the moment LEDs arrived, which is why bulb packaging now leads with lumens.

Luminous Efficacy Efficacy (lm/W) = Lumens ÷ Watts. Higher is better:

• Incandescent: 10–15 lm/W (most energy becomes heat)
• Halogen: 15–20 lm/W
• Compact Fluorescent (CFL): 45–75 lm/W
• LED: 80–120 lm/W (typical modern LED)
• High-Efficiency LED: up to 200 lm/W (laboratory conditions)
• HID (High Intensity Discharge): 80–150 lm/W

Illuminance: lux and footcandles Illuminance measures how much light lands on a surface.

• Lux (lx): lumens per square metre. The SI unit.
• Footcandles (fc): lumens per square foot. Still standard in the United States.
• Conversion: 1 footcandle = 10.7639 lux. That is not a rounded figure of convenience; it falls out of the foot being defined as exactly 0.3048 m, so a square foot is exactly 0.09290304 m² and the ratio is exactly 1/0.09290304.

Lumens from Area Illuminance (lux) = Luminous flux (lumens) ÷ Area (m²) Rearranging: Lumens = Lux × Area (m²)

Inverse Square Law For a point source of light: Illuminance = Luminous Intensity / distance². Doubling the distance reduces illuminance to one quarter.