Lux to Candela Calculator
Convert lux (illuminance) to candela (luminous intensity) based on distance from the light source. This calculator uses the inverse square law to determine the required candela for a given lux level. See also our Candela to Lux Calculator and Lux to Lumen Calculator.
How to Convert Lux to Candela
Converting lux to candela requires knowing the distance between the light source and the illuminated surface. Lux measures how much light falls on a surface (illuminance), while candela measures how intense the light source is in a particular direction (luminous intensity). The relationship between them follows the inverse square law. Here is the step-by-step process:
- Measure or determine the lux level: Use a lux meter to measure the illuminance at the surface, or use the target lux level from lighting standards (e.g., 500 lux for office work, 300 lux for general areas).
- Measure the distance: Determine the distance from the light source to the measurement point in meters. If measured in feet, convert to meters by multiplying by 0.3048.
- Apply the formula: Multiply the lux value by the square of the distance in meters: Candela = Lux × Distance².
- Interpret the result: The candela value represents the minimum luminous intensity needed from the light source to achieve the specified lux level at that distance.
- Account for real-world factors: Add 20–30% margin for maintenance factor, surface angle, and light losses in real installations.
This conversion assumes a point source illuminating a surface perpendicular to the light direction. For angled surfaces, divide the result by cos(θ) where θ is the angle of incidence. For extended sources or diffuse lighting, the relationship becomes more complex and may require photometric software for accurate calculations.
Lux to Candela Formula
Candela = Lux × Distance²
Where:
Candela (cd) = Luminous intensity of the source
Lux (lx) = Illuminance at the surface (lumens per m²)
Distance (m) = Distance from source to surface in meters
Derived from the inverse square law:
Lux = Candela / Distance²
Therefore: Candela = Lux × Distance²
Unit conversions:
If distance is in feet: Distance_m = Distance_ft × 0.3048
If distance is in inches: Distance_m = Distance_in × 0.0254
For angled surfaces:
Candela = Lux × Distance² / cos(θ)
Where θ = angle between light direction and surface normalThe inverse square law states that illuminance decreases proportionally to the square of the distance from a point source. Doubling the distance reduces lux to one-quarter. This means achieving the same lux level at twice the distance requires four times the candela. This fundamental relationship governs all point-source lighting calculations and is the basis for photometric testing standards like IES LM-79.
Example Calculation
Problem: An office workspace requires 500 lux of illumination. The ceiling-mounted light is 2 meters above the desk. What candela rating is needed?
Step 1: Identify the values
Lux = 500 lx (office standard)
Distance = 2 meters
Step 2: Apply the formula
Candela = Lux × Distance²
Candela = 500 × 2²
Candela = 500 × 4
Candela = 2,000 cd
Step 3: Verify with reverse calculation
Lux = 2,000 cd / 2² = 2,000 / 4 = 500 lx ✓
Step 4: Check at other distances
At 1m: 500 × 1 = 500 cd needed
At 3m: 500 × 9 = 4,500 cd needed
At 5m: 500 × 25 = 12,500 cd needed
Answer: A light source with at least 2,000 candela intensity is needed to provide 500 lux at a 2-meter distance. In practice, specify a fixture rated at 2,400–2,600 cd to account for maintenance factor and aging.
Lux to Candela Reference Table
This table shows the candela required to achieve common lux levels at various distances. Use it for quick lighting design estimates.
| Lux Level | At 1m (cd) | At 2m (cd) | At 3m (cd) | At 5m (cd) | At 10m (cd) |
|---|---|---|---|---|---|
| 50 lx | 50 | 200 | 450 | 1,250 | 5,000 |
| 100 lx | 100 | 400 | 900 | 2,500 | 10,000 |
| 200 lx | 200 | 800 | 1,800 | 5,000 | 20,000 |
| 300 lx | 300 | 1,200 | 2,700 | 7,500 | 30,000 |
| 500 lx | 500 | 2,000 | 4,500 | 12,500 | 50,000 |
| 750 lx | 750 | 3,000 | 6,750 | 18,750 | 75,000 |
| 1,000 lx | 1,000 | 4,000 | 9,000 | 25,000 | 100,000 |
| 2,000 lx | 2,000 | 8,000 | 18,000 | 50,000 | 200,000 |
Notice how candela requirements increase dramatically with distance. Achieving 500 lux at 10 meters requires 50,000 cd — 100 times more than at 1 meter. This is why high-bay warehouse lighting and stadium floodlights need extremely high-intensity fixtures, while desk lamps with modest candela ratings work well at close range.
Common Lux Standards by Application
Understanding required lux levels helps you determine the candela needed for your specific application. International standards (ISO 8995, EN 12464) specify minimum illuminance levels for various tasks and environments. Here are the most common requirements:
| Application | Required Lux | Standard |
|---|---|---|
| Parking areas | 50–100 lx | EN 12464-2 |
| Corridors, hallways | 100–150 lx | EN 12464-1 |
| Living rooms | 150–300 lx | General recommendation |
| Classrooms | 300–500 lx | EN 12464-1 |
| Office workstations | 500 lx | ISO 8995 / EN 12464-1 |
| Retail displays | 500–1,000 lx | IESNA |
| Detailed assembly work | 750–1,000 lx | EN 12464-1 |
| Surgical operating rooms | 10,000–100,000 lx | IEC 60601 |
Frequently Asked Questions
What is the difference between lux and candela?
Lux measures illuminance — the amount of light arriving at a surface per unit area (lumens per square meter). Candela measures luminous intensity — how bright a light source is in a specific direction. Lux depends on both the source intensity and the distance, while candela is a property of the source alone. Think of candela as the "sending power" and lux as the "receiving amount." The same candela source produces different lux values at different distances following the inverse square law.
How does distance affect lux?
Lux decreases with the square of the distance from a point source (inverse square law). Doubling the distance reduces lux to 1/4, tripling reduces to 1/9, and so on. For example, a light producing 1,000 lux at 1 meter will produce 250 lux at 2 meters, 111 lux at 3 meters, and only 40 lux at 5 meters. This is why ceiling height is critical in lighting design — higher ceilings require proportionally more powerful fixtures to maintain adequate desk-level illumination.
Can I convert lux to candela without knowing distance?
No, distance is essential for the lux-to-candela conversion. Lux is a distance-dependent measurement — the same light source produces different lux values at different distances. Without knowing the distance, there is no unique candela value that corresponds to a given lux reading. You must either measure the distance directly or know the mounting height and geometry of the installation to perform this conversion.
How many candela is a typical LED downlight?
A typical residential LED downlight produces 500–3,000 candela peak intensity depending on beam angle and lumen output. A 10W LED downlight with 800 lumens and a 40° beam angle produces approximately 1,700 cd peak. A 20W commercial downlight with 2,000 lumens and 60° beam produces about 2,400 cd. Narrow beam (15°) accent lights can reach 10,000–20,000 cd despite modest total lumen output because the light is concentrated into a small cone.
What is the inverse square law in lighting?
The inverse square law states that the illuminance (lux) from a point source is inversely proportional to the square of the distance: E = I/d². This occurs because light spreads out over an increasingly larger area as distance increases. At twice the distance, the same light covers 4× the area, so each unit area receives 1/4 the light. This law applies accurately to point sources and small fixtures at distances greater than 5× the fixture's largest dimension. For large area sources at close range, the relationship is more complex.
How do I measure lux?
Lux is measured using a lux meter (also called an illuminance meter or light meter). Place the sensor at the surface where you want to know the light level — typically desk height (0.75m) for offices or floor level for corridors. Point the sensor upward toward the light source. For accurate readings, turn off other light sources, allow the meter to stabilize for 10–15 seconds, and take multiple readings across the area. Smartphone lux meter apps can provide rough estimates but are not accurate enough for professional lighting design.