Lumen Calculator
Convert lumens to candelas and lux with this free online light unit converter. Get instant photometry results with interactive charts and formula breakdowns.
About This Calculator
The Lumen Calculator is a versatile photometry tool that converts between the three most important units of light measurement: lumens, candelas, and lux. Whether you are a lighting designer, architect, photographer, engineer, or physics student, this calculator helps you understand and convert light quantities quickly and accurately. It supports three conversion modes — Candela to Lumen, Lumen to Lux, and Candela to Lux — each with its own conversion parameter.
The calculator uses well-established photometric formulas. The Candela to Lumen mode applies the solid-angle formula lm = cd × 2π × (1 − cos(θ/2)), where θ is the apex angle in degrees. The Lumen to Lux mode uses lux = lm / area, dividing luminous flux by surface area. The Candela to Lux mode applies the inverse-square law: lux = cd / d². Each result includes a breakdown of all parameters and interactive sensitivity charts that visualize how the output changes with the conversion parameter — helping you develop an intuitive understanding of these photometric relationships.
Understanding Light Units
Lumens (lm) measure the total amount of visible light emitted by a source — the luminous flux. A standard 60W incandescent bulb outputs about 800 lumens. Candelas (cd) measure luminous intensity — the light emitted in a particular direction, per unit solid angle. A standard candle emits roughly 1 candela in all directions. Lux (lx) measures illuminance — how much light falls on a surface, equal to one lumen per square meter. These three quantities are related through geometry: luminous intensity × solid angle = luminous flux, and luminous flux ÷ area = illuminance.
Regional Notes
Lumens, candelas, and lux are SI (International System) units used universally in science, engineering, and industry worldwide. In the United States, footcandles (1 fc = 10.764 lux) are sometimes used in architectural lighting specifications, but the SI units are the global standard for photometry. The calculator's results apply identically across India, the US, the UK, and all regions using the metric system.
Frequently Asked Questions
What is the difference between lumens, candelas, and lux?
Lumens (lm) measure total luminous flux — the total amount of visible light emitted by a source in all directions. Candelas (cd) measure luminous intensity — light emitted in a particular direction per unit solid angle. Lux (lx) measure illuminance — the amount of light received per unit area on a surface. One lux equals one lumen per square meter.
How do you convert candelas to lumens?
To convert candelas (cd) to lumens (lm), multiply by the solid angle: lm = cd × Ω. For a light source with apex angle θ, the solid angle is Ω = 2π × (1 − cos(θ/2)). For a source emitting uniformly in all directions, Ω = 4π, so 1 cd = 4π ≈ 12.57 lm.
How do you convert lumens to lux?
To convert lumens (lm) to lux (lx), divide lumens by the surface area in square meters: lux = lm / area. For example, 1000 lumens spread over 10 m² gives 100 lux. Conversely, to convert lux to lumens, multiply by area: lm = lux × area.
How do you convert candelas to lux?
To convert candelas (cd) to lux (lx) at a given distance, divide candelas by the squared distance: lux = cd / d². This follows the inverse-square law of light propagation. For example, a 100 cd source at 3 meters produces 100/9 ≈ 11.11 lux. To convert lux to candelas, multiply by distance squared: cd = lux × d².
What is a typical apex angle for common light sources?
A standard incandescent bulb emits roughly uniformly, corresponding to an apex angle of 360° (Ω = 4π). An LED spotlight typically has a narrow apex angle of 30–60°, while a floodlight may have 90–120°. A candle has an apex angle close to 360°, and a laser pointer has an extremely narrow beam of 1–5°.
What are typical lux levels for common situations?
Typical illuminance levels include: direct sunlight ≈ 100,000 lux, overcast day ≈ 1,000 lux, office lighting ≈ 300–500 lux, living room ≈ 100–200 lux, street lighting ≈ 10–50 lux, full moon ≈ 0.1–0.3 lux, and starlight ≈ 0.0001 lux. In the US and UK, similar values apply since lux is an SI unit used worldwide.
What is the relationship between footcandles and lux?
Footcandles (fc) are the Imperial unit of illuminance, used primarily in the United States. One footcandle equals the illuminance on a surface one foot from a standard candle. The conversion is 1 footcandle = 10.764 lux. To convert footcandles to lux, multiply by 10.764. To convert lux to footcandles, divide by 10.764.
How is luminous efficacy related to lumens and watts?
Luminous efficacy measures how efficiently a light source converts electrical power into visible light, expressed in lumens per watt (lm/W). Incandescent bulbs have about 15 lm/W, halogen around 23 lm/W, CFL about 60 lm/W, and LED bulbs typically 80–150 lm/W. Higher efficacy means more light output for less energy consumption.