Watts to Lux Calculator

Enter a light source's wattage, luminous efficacy, distance, and beam angle to estimate the illuminance (lux) it produces on a surface.

Quick Facts

Core relationship
Lux = Lumens ÷ Illuminated Area (m²)
Illuminance is light flux spread over a surface, not electrical power.
Watts to lumens
Lumens = Watts × Luminous Efficacy (lm/W)
Efficacy depends on the bulb technology, not the wattage rating.
Typical efficacies
LED ≈ 80–120 lm/W · Incandescent ≈ 10–17 lm/W
Two bulbs of the same wattage can differ 5–8x in light output.
Unit conversion
1 lux = 1 lm/m² ≈ 0.0929 footcandles
Footcandles are the imperial illuminance unit used in US lighting codes.

Your Results

Calculated
Illuminance
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E = Φ ÷ A, in lux (lm/m²)
Luminous Flux
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Φ = Watts × Luminous Efficacy, in lumens
Illuminated Area
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Beam footprint at your distance, in m²
Illuminance (Footcandles)
-
1 fc ≈ 10.764 lux

Ready

Enter your light source details, then press Calculate.

Formula and Method for Converting Watts to Lux

Watts measure how much electrical power a light source consumes, but they say nothing about how bright the light looks on a surface. Lux measures illuminance — light striking an area — so converting from watts to lux requires two extra facts: how efficiently the source turns electricity into visible light (luminous efficacy, in lumens per watt) and how that light spreads out over distance and beam angle before it lands.

How the calculation works

The conversion happens in two stages. First, multiply the power by the light source's luminous efficacy to get luminous flux: Φ = P × η, where P is power in watts and η is efficacy in lumens per watt (lm/W). Second, spread that flux over the illuminated area. For a beam with full angle θ striking a flat surface at distance d, the footprint is a circle of radius d·tan(θ/2), so its area is A = π × (d × tan(θ/2))². Illuminance is then flux divided by area: E = Φ / A, measured in lux (lumens per square meter).

Common mistakes

  • Treating watts as a brightness rating: a 10 W incandescent bulb and a 10 W LED can differ in light output by 5–8x because their luminous efficacies are so different.
  • Ignoring the inverse-square relationship: because illuminated area grows with distance squared, doubling the distance to a light source cuts illuminance to roughly one-quarter, not one-half.
  • Forgetting beam angle: the same lumens concentrated into a narrow spotlight beam produce far higher lux than when spread across a wide floodlight beam.

Real-world applications

  • Choosing grow lights that deliver a target illuminance across a plant canopy of a known size.
  • Checking whether a replacement LED bulb will meet workspace lighting standards (roughly 300–500 lux for office desks, 750+ lux for detailed tasks).
  • Comparing security or outdoor floodlights by the illuminance they actually deliver at the distance you plan to mount them, not just their wattage.
  • Estimating camera or video lighting setups where a target lux level at the subject matters more than the fixture's power draw.

Frequently Asked Questions

Can I convert watts directly to lux?
Not directly. Watts measure electrical power, while lux measures illuminance (light landing on a surface). You first need the light source's luminous efficacy (lumens per watt) to find luminous flux, then divide by the illuminated area to get lux — that's why this calculator asks for efficacy, distance, and beam angle in addition to wattage.
Why do two bulbs with the same wattage produce different lux?
Because luminous efficacy varies by technology. An incandescent bulb converts only about 10–17 lumens per watt, while an LED can produce 80–120+ lumens per watt for the same electrical input. A 10 W LED can output 5–8 times more light than a 10 W incandescent bulb, so it produces far more lux at the same distance.
How does distance affect the lux reading?
Illuminance drops as light spreads over a larger area with distance. In this calculator the beam's footprint area grows with distance squared, A = π × (d × tan(θ/2))², so lux falls roughly with the square of distance — doubling the distance to a light source cuts illuminance to about one-quarter, all else equal.
What beam angle should I use if I don't know it?
Check the bulb or fixture's spec sheet — beam angle is usually listed in degrees (for example 25° for a narrow spotlight, 120° for a standard household bulb, up to 180° for a wide floodlight). If it's unspecified, 120° is a reasonable default for an omnidirectional household LED bulb.

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