Watts to Lux Calculator. 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. Also try the Focal Length Calculator.
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. See also our calculate Diffraction Angle (θₘ), Grating Spacing (d) & Path Difference (m·λ) — Diffraction Grating.
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. You might also find our find Brightness Ratio (Laser 1 / Laser 2) with Laser Brightness Calculator useful.