Candela to Lux Calculator
Convert luminous intensity in candela to illuminance in lux using the inverse square law. Enter your candela value and distance to instantly calculate lux, foot-candles, and lumens per square meter. See also our Lux to Candela Calculator and Candela to Lumen Calculator.
How to Convert Candela to Lux
Converting candela to lux requires knowing the distance between the light source and the surface where illuminance is measured. Candela (cd) is the SI unit of luminous intensity, representing the amount of light emitted by a source in a specific direction. Lux (lx) is the SI unit of illuminance, measuring how much luminous flux is spread over a given area. The conversion relies on the inverse square law, which states that illuminance decreases proportionally to the square of the distance from the source. To perform the conversion, simply divide the candela value by the square of the distance in meters. This gives you the illuminance in lux at that specific distance from the source. The calculation assumes a point source radiating uniformly in the measured direction, which is a good approximation for most practical lighting scenarios when the measurement distance is at least five times the largest dimension of the light source.
Follow these steps to convert candela to lux:
- Determine the luminous intensity of your light source in candela (cd).
- Measure the distance from the light source to the surface in meters.
- If your distance is in feet, multiply by 0.3048 to convert to meters.
- Square the distance value (multiply it by itself).
- Divide the candela value by the squared distance to get lux.
- To get foot-candles, divide the lux result by 10.764.
Candela to Lux Formula (Inverse Square Law)
The candela to lux formula is a direct application of the inverse square law of light propagation. This fundamental photometric relationship describes how illuminance decreases as distance from a point source increases. The formula is straightforward and widely used in lighting design, photometry, and optical engineering.
Lux (lx) = Candela (cd) / Distance² (m²)
Where:
• Candela (cd) = luminous intensity of the light source
• Distance (m) = distance from light source to measurement surface
• Lux (lx) = illuminance at the surface
Additional conversions:
• Foot-candles (fc) = Lux / 10.764
• Lumens per square meter (lm/m²) = Lux (they are equivalent)
The inverse square law:
E = I / d²
Where E = illuminance (lux), I = intensity (candela), d = distance (meters)This formula assumes a point light source emitting uniformly in the direction of measurement. For real-world light sources with reflectors, lenses, or diffusers, the formula provides an accurate approximation when the measurement distance is sufficiently large relative to the source dimensions. The inverse square relationship means that doubling the distance reduces illuminance to one-quarter, and tripling the distance reduces it to one-ninth of the original value. This rapid decrease in light intensity with distance is why proper fixture placement is critical in lighting design.
Example Calculation
A spotlight has a luminous intensity of 1,000 candela. What is the illuminance at a distance of 2 meters from the source?
Given: Luminous Intensity = 1,000 cd, Distance = 2 meters
Formula: Lux = Candela / Distance²
Calculation: Lux = 1,000 / 2² = 1,000 / 4 = 250 lux
Foot-candles: 250 / 10.764 = 23.22 fc
Lumens/m²: 250 lm/m²
Result: At 2 meters from a 1,000 candela source, the illuminance is 250 lux. This is suitable for general office lighting, reading areas, and retail spaces where moderate illumination is required for comfortable visual tasks.
Another example: A car headlight rated at 20,000 candela illuminates the road at 25 meters distance. The illuminance would be 20,000 / 625 = 32 lux. This explains why headlights provide adequate visibility at typical driving distances but appear extremely bright at close range. At just 5 meters, the same headlight produces 20,000 / 25 = 800 lux, which is why oncoming headlights can cause temporary glare.
Candela to Lux Reference Table
The following table shows the illuminance in lux produced by a 1,000 candela light source at various distances. This demonstrates how rapidly illuminance decreases with distance according to the inverse square law.
| Distance | Lux (from 1,000 cd) | Foot-Candles | Typical Use Case |
|---|---|---|---|
| 0.5 m | 4,000.00 lx | 371.61 fc | Close-range task lighting |
| 1 m | 1,000.00 lx | 92.90 fc | Desk lamp illumination |
| 2 m | 250.00 lx | 23.23 fc | General office lighting |
| 3 m | 111.11 lx | 10.32 fc | Ambient room lighting |
| 5 m | 40.00 lx | 3.72 fc | Corridor or hallway lighting |
| 10 m | 10.00 lx | 0.93 fc | Outdoor pathway lighting |
Notice how the illuminance drops dramatically as distance increases. At 0.5 meters, the 1,000 candela source produces 4,000 lux, which is brighter than direct sunlight on a cloudy day. By 10 meters, the same source produces only 10 lux, barely enough for safe navigation in a corridor. This demonstrates why lighting designers must carefully consider mounting heights and fixture spacing to achieve uniform illumination across a space.
The Inverse Square Law of Light
The inverse square law is one of the most fundamental principles in physics and is central to understanding how light behaves as it travels through space. This law states that the intensity of light (or any other form of radiation from a point source) is inversely proportional to the square of the distance from the source. Mathematically, if you double the distance from a light source, the illuminance drops to one-quarter of its original value. If you triple the distance, illuminance drops to one-ninth.
This relationship exists because light from a point source radiates outward in all directions, spreading over the surface of an ever-expanding sphere. The surface area of a sphere is 4πr², so as the radius (distance) doubles, the area over which the light is distributed quadruples. Since the total luminous flux remains constant, the flux per unit area (illuminance) must decrease by the same factor. This geometric spreading is the physical basis for the inverse square law.
Practical implications of the inverse square law:
- Moving a lamp from 1 meter to 2 meters away reduces light on your desk by 75%.
- A photographer moving twice as far from a subject needs 4× the flash power for the same exposure.
- Street lights must be spaced carefully because illuminance drops rapidly between poles.
- Stage lighting requires extremely high-intensity fixtures because of the large throw distances involved.
- Security cameras need brighter lights at greater mounting heights to maintain minimum lux requirements.
The inverse square law applies perfectly to point sources in free space. In real-world environments, reflections from walls, ceilings, and floors can increase the actual illuminance above what the formula predicts. Conversely, atmospheric absorption and scattering can reduce illuminance below predicted values over very long distances. For indoor lighting calculations, the contribution of reflected light (called inter-reflected light) can add 20-50% to the direct illuminance calculated from the inverse square law alone.
Recommended Lux Levels
Different environments and tasks require specific illuminance levels for safety, comfort, and productivity. The following table shows recommended lux levels based on international standards (ISO 8995, EN 12464-1) and common industry guidelines. Use these values with the candela to lux formula to determine the required luminous intensity for your fixtures at a given mounting distance.
| Environment | Recommended Lux | Notes |
|---|---|---|
| General office | 300–500 lx | Desk work, computer screens, meetings |
| Warehouse / storage | 100–200 lx | Navigation, reading labels, forklift operation |
| Surgery / operating room | 10,000–100,000 lx | Precision medical procedures |
| Outdoor parking lot | 20–50 lx | Vehicle and pedestrian safety |
| Retail showroom | 500–1,000 lx | Product display and customer comfort |
| Classroom | 300–500 lx | Reading, writing, whiteboard visibility |
| Hospital ward | 100–300 lx | Patient comfort and staff visibility |
| Industrial assembly | 500–1,000 lx | Detailed manual work and inspection |
| Residential living room | 100–300 lx | Relaxation and general activities |
| Sports stadium | 500–2,000 lx | Player visibility and broadcast quality |
To determine the candela rating needed for a fixture, multiply the required lux by the square of the mounting distance. For example, achieving 500 lux in an office with ceiling-mounted downlights at 2.5 meters requires fixtures producing at least 500 × 6.25 = 3,125 candela in the downward direction. Multiple fixtures are typically used to ensure uniform coverage across the workspace.
Frequently Asked Questions
How do you convert candela to lux?
To convert candela to lux, divide the luminous intensity in candela by the square of the distance in meters. The formula is: Lux = Candela / Distance². For example, a 500 candela source at 3 meters produces 500 / 9 = 55.56 lux. The distance must be in meters for the standard SI calculation. If your distance is in feet, first multiply by 0.3048 to convert to meters before squaring.
What is the inverse square law?
The inverse square law states that the illuminance (lux) from a point light source is inversely proportional to the square of the distance from that source. This means doubling the distance reduces illuminance to one-quarter, and tripling the distance reduces it to one-ninth. The law arises because light spreads over an increasingly larger spherical area as it travels outward. It applies to all point sources of electromagnetic radiation and is fundamental to lighting design, photography, and radiometry.
How many lux is 1 candela at 1 meter?
1 candela at 1 meter produces exactly 1 lux. This is because Lux = Candela / Distance² = 1 / 1² = 1 lux. This relationship is fundamental to the definition of these photometric units. At 2 meters, 1 candela produces 0.25 lux, and at 0.5 meters it produces 4 lux. The candela is one of the seven SI base units, and the lux is derived from it through this geometric relationship.
What is the difference between candela and lux?
Candela measures luminous intensity, which is the amount of light emitted by a source in a particular direction. It is a property of the light source itself and does not change with distance. Lux measures illuminance, which is the amount of light arriving at a surface per unit area. Lux depends on both the source intensity and the distance from the source. Think of candela as how bright the source is, and lux as how bright the surface appears when illuminated by that source at a given distance.
How far does 1000 candela reach?
How far 1,000 candela "reaches" depends on the minimum lux level you need. At 1 meter it produces 1,000 lux, at 3.16 meters it produces 100 lux (adequate for corridors), at 10 meters it produces 10 lux (minimum for outdoor walkways), and at 31.6 meters it produces 1 lux (equivalent to full moonlight). For practical purposes, a 1,000 candela source provides useful illumination for tasks up to about 3-5 meters and basic visibility up to about 10-15 meters.
What lux level do I need for my workspace?
The recommended lux level depends on the task. General office work requires 300-500 lux, detailed drafting or inspection needs 500-1,000 lux, warehouses need 100-200 lux, and surgical environments require 10,000-100,000 lux. For home offices, 300-500 lux at desk level is ideal. Use our calculator to determine what candela rating your light fixtures need based on their mounting distance to achieve these levels at your work surface.