Watts to lux calculator
Convert power to an average illuminance estimate with explicit assumptions.
Formula
Lumens = watts × lm/W; average lux = lumens × received fraction / area (m²).
Electrical power: 20 W · Receiving area: 100 ft² · Emitted luminous flux ≈ (20 W)×(100) · Average illuminance ≈ (2,000)×(100)÷100÷(100 ft² × 0.09290304 m²/ft²)
How to use
- Enter electrical power and rated luminous efficacy.
- Enter the receiving area and light reaching it (%).
- Read emitted lumens and average lux.
Examples
- 20 W at 100 lm/W gives 2,000 lm; 100% reaching 10 m² gives 200 lx.
Frequently asked questions
Can watts be converted directly to lux?
No. You also need luminous efficacy, area and how much light reaches it.
What does 100% received mean?
All emitted lumens reach the entered area. Use a lower measured or estimated fraction when light is lost elsewhere.
Is this the lux at every point?
No. It is an area average, excluding ambient light; beam shape, distance and obstructions affect local readings.
Sources
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