Lumen Calculator
Free calculate luminous flux, illuminance, and luminous intensity for lighting design. Get instant, accurate results with our easy-to-use calculator.
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What is a Lumen?
A lumen (lm) is the SI unit of luminous flux - the total amount of visible light emitted by a light source. It measures the perceived power of light as seen by the human eye, weighted by the eye's sensitivity to different wavelengths.
Illuminance (measured in lux, lx) is the luminous flux per unit area: E = Φ/A. One lux equals one lumen per square meter. Luminous intensity (measured in candela, cd) is the luminous flux per unit solid angle: I = Φ/Ω.
These units are essential in lighting design, allowing engineers to calculate how much light is needed for different applications, from reading (300-500 lux) to surgery (10,000+ lux).
Lumen Formulas
Luminous Flux
Illuminance
Luminous Intensity
Where: Φ = flux (lm), E = illuminance (lx), A = area (m²), I = intensity (cd), Ω = solid angle (sr)
How to Calculate
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1
Convert units to SI
Convert footcandles to lux (1 fc = 10.764 lx), ft² to m², ensure consistent units.
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2
Calculate luminous flux
Φ = E × A (flux = illuminance × area). This gives total light output needed.
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3
Calculate illuminance
E = Φ / A (illuminance = flux / area). This gives light level on a surface.
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4
Calculate luminous intensity
I = Φ / Ω (intensity = flux / solid angle). For full sphere: Ω = 4π sr.
Practical Examples
Example 1: Room Lighting
Room: 10 m², desired illuminance: 300 lux. How many lumens needed?
Solution:
Φ = E × A = 300 lx × 10 m²
Φ = 3,000 lm (need 3,000 lumen light source)
Example 2: Light Bulb Illuminance
800 lm bulb illuminating 5 m² area. What illuminance?
Solution:
E = 800 lm / 5 m²
E = 160 lux (good for general lighting)
Applications
Interior Lighting
Designing lighting for homes, offices, and commercial spaces to meet recommended illuminance levels for different tasks.
Architectural Lighting
Calculating light requirements for buildings, ensuring adequate illumination for safety and functionality.
Product Design
Specifying LED lumens, designing light fixtures, and ensuring products meet lighting standards and regulations.
Education
Teaching photometry, understanding light measurement, and learning about human vision and lighting science.
Frequently Asked Questions
What is the difference between lumens and watts?
Lumens measure visible light output (brightness as perceived by humans). Watts measure power consumption. Modern LEDs produce 80-100+ lumens per watt, while incandescent bulbs produce ~10-15 lm/W.
How many lumens do I need for a room?
Depends on room size and use. General lighting: 300-500 lux. Reading: 500-800 lux. Kitchen/work: 500-1000 lux. Calculate: Lumens = Lux × Area(m²).
What is the relationship between lux and footcandles?
1 footcandle = 10.764 lux. Footcandles use ft², lux use m². Since 1 m² = 10.764 ft², the conversion factor is the same: 1 fc = 10.764 lx.
How does distance affect illuminance?
For point sources: E = I/r² (inverse square law). Doubling distance reduces illuminance by 4×. For extended sources, the relationship is more complex.
What is a typical lumen value for light bulbs?
Typical values: 40W incandescent ≈ 450 lm, 60W ≈ 800 lm, 100W ≈ 1600 lm. LED equivalents: 6W ≈ 450 lm, 9W ≈ 800 lm, 15W ≈ 1600 lm (much more efficient!).