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Watts to Lux Calculator

Watts to lux calculator will help you measure how much luminous flux is spread over a given area.

Clear
Illuminance at 2 m79.58 lux7.393 foot-candles
Luminous flux1,000 lumens10 W × 100 lm/W
Luminous intensity318.31 candelalumens ÷ 3.1416 steradians of beam
Beam solid angle3.14159 sra full sphere is 4π ≈ 12.566 sr, so this beam covers 25% of all directions
Illuminated spot radius3.464 m6.928 m across
At twice the distance19.89 luxa quarter — illuminance follows the inverse square law, because the same flux spreads over four times the area
Radiant efficiency14.64%against the 683 lm/W theoretical maximum for green light at 555 nm, where the eye is most sensitive — no white source can approach it, because white light must include wavelengths the eye sees poorly
Against direct sunlight0.0008×about 100,000 lux
Against overcast daylight0.0796×about 1,000 lux
Against office lighting0.1592×about 500 lux
Against living room0.5305×about 150 lux
Against full moon318.3099×about 0.25 lux

The formula

lux = lumens ÷ area; lumens = watts × luminous efficacy

Photometry is weighted by the eye

Radiometric units measure energy; photometric units measure what a human eye makes of it. A watt of green light at 555 nm, where the eye is most sensitive, corresponds to 683 lumens; a watt of deep red or violet contributes far fewer, and a watt of infrared none at all. Every lumen figure therefore carries the eye's response curve inside it.

That is why 683 lm/W is an absolute ceiling no white light source can approach — white light must contain wavelengths the eye sees poorly. A good LED reaches 150 lm/W, about 22% of the monochromatic limit, and that is genuinely excellent.

Lumens measure total output, candela measure output per unit solid angle, and lux measure what lands on a surface. A torch and a bare bulb can emit the same lumens while the torch delivers hundreds of times the lux on target, purely by concentrating the beam. Illuminance then falls as the inverse square of distance, because the same flux spreads over an area growing with the square.