Convert electrical power to illuminance with the Watts to Lux Calculator. Enter power in watts, luminous efficacy (lm/W), and surface area (in square meters or square feet) to calculate illuminance in lux (lx). Choose your light source type to auto-fill typical efficacy values, or enter a custom value. Results include the calculated lux value and total luminous flux in lumens.
Illuminance
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Luminous Flux
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Efficacy Used
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Area (m²)
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Lux (lx) measures illuminance — the amount of light falling on a surface per square meter. Watts measure electrical power consumed by the light source. A bulb may consume the same wattage but produce very different lux depending on its luminous efficacy and how large an area it illuminates.
For area in square meters: Ev(lx) = P(W) × η(lm/W) / A(m²). For area in square feet: Ev(lx) = 10.76391 × P(W) × η(lm/W) / A(ft²). Here, η is the luminous efficacy of the light source in lumens per watt.
It depends on the luminous efficacy of the light source and the surface area. For example, a 1 W LED with 90 lm/W illuminating 1 m² produces 90 lux. The same watt with an incandescent bulb at 15 lm/W over 1 m² produces only 15 lux.
Luminous efficacy (η) is the ratio of luminous flux (lumens) produced to electrical power (watts) consumed, measured in lm/W. LEDs typically achieve 80–120 lm/W, making them far more efficient than incandescent bulbs which average only 10–17 lm/W.
Rearrange the formula: P(W) = Ev(lx) × A(m²) / η(lm/W). For example, to achieve 300 lux over 10 m² using a fluorescent lamp (75 lm/W): P = 300 × 10 / 75 = 40 W. You can use our companion Lux to Watts calculator for this conversion.
Common values are: LED lamp ~90 lm/W, Tungsten incandescent ~15 lm/W, Halogen lamp ~25 lm/W, Fluorescent lamp ~75 lm/W, and Metal halide lamp ~87 lm/W. These are averages; actual values vary by manufacturer and model.
This calculator uses flat surface area (m² or ft²). For spherical sources, you would use the formula with the sphere's surface area A = 4πr², where r is the radius. You can manually compute 4πr² and enter it as the surface area.
Lux is illuminance — light per unit area. A fixed amount of luminous flux (lumens) spread over a larger area produces fewer lux. Doubling the room area halves the lux, even with the same bulb. This is why larger rooms require higher-wattage or more efficient light sources.