Candela to Lux Calculator
Convert luminous intensity in candela to illuminance in lux at a given distance (inverse-square). Runs locally in your browser. Try it free.
Trust summary Engine tested · Specification checked · v1.1.0
- Input interpretation
- Enter values to calculate.
- Result
- —
- Model
- Illuminance from luminous intensity and distance, or intensity from illuminance and distance.
- Scope
- Point source
- Verification
- Engine tested · Specification checked · v1.1.0
- Named expert review
- Optional · Not performed
- Specification basis
- Inverse-square law: E = I / d² for SI units
- 1 m² = 10.76391 ft²
Formulas
Core equations used by this calculator.
How to use
Choose direction and units
cd → lx or lx → cd, with distance in meters or feet.
Enter candela (or lux) and distance
Use the intensity toward the point and the source-to-surface distance.
Read illuminance or intensity
Result follows the inverse-square law for a point source on a facing surface.
Example calculations
Common configurations with formula and result.
500 cd · 3 m
Default (meters)
500 cd · 10 ft
Default (feet)
1 cd · 1 m
Definition check
10,000 cd · 5 m
Strong spotlight
lx → cd
500 lx · 3 m
Typical illuminance (context)
Common values at a glance.
| Scene | Approx. lux | Notes |
|---|---|---|
| Moonlight | ~1 lx | Very dim outdoor night |
| Overcast day | ~100 lx | Diffuse daylight |
| Office desk | 300–500 lx | Common task lighting |
| Full daylight | 10,000–25,000 lx | Outdoors, shade varies |
| Direct sun | ~100,000 lx | Bright clear day |
Candela to Lux calculator specification
Version 1.1.0 · Engine tested
- Engine tested 4 published cases
- Named expert review Not performed
- Calculation version 1.1.0
- Definition
- Candela (cd) is luminous intensity in a direction; lux (lx) is illuminance on a surface (lm/m²). For a point source on a surface facing the light, E = I / d². With d in meters: lx = cd ÷ d². With d in feet: lx = 10.76391 × cd ÷ d(ft)² (because 1 m² = 10.76391 ft²).
- What it calculates
- Illuminance from luminous intensity and distance, or intensity from illuminance and distance.
- Inputs
- Candela (cd) or lux (lx)
- Distance in meters or feet
- Outputs
- Lux (cd → lx) or candela (lx → cd)
- Formula
E=I/d² (m); E=10.76391·I/d² (ft)- Assumptions
- Point source
- Surface facing the source (normal incidence)
- Units
- cd, m → lx
- cd, ft → lx
- Boundary conditions
- d = 0 → undefined / treated as 0 result in UI
- Example
- 500 cd, 3 m → ≈ 55.56 lx
- Validation cases
4 published on this page
- 500 cd, 3 m → ≈ 55.5556 lx
- 500 cd, 10 ft → ≈ 53.8196 lx
- 1 cd, 1 m → 1 lx
- 500 lx, 3 m → cd → 4500 cd
- Specification basis
- Inverse-square law: E = I / d² for SI units
- 1 m² = 10.76391 ft²
- Calculation version
- 1.1.0
Background
Interpretation and common distinctions.
Convert candela (cd) to lux (lx) at a known distance using the inverse-square law.
E((lx)) = (I((cd)))/(d((m))²) E((lx)) = 10.76391 × (I((cd)))/(d((ft))²)
Default: 500 cd · 3 m → ≈ 55.56 lx.
Candela vs lux vs lumens
| Unit | Measures |
|---|---|
| Candela (cd) | Luminous intensity in a direction |
| Lux (lx) | Illuminance on a surface (lm/m²) |
| Lumen (lm) | Total luminous flux from the source |
Candela stays the same if you only change viewing distance; lux on the target drops as 1/d². Narrow beams raise candela (and lux on axis) without needing more total lumens.
Notes
- Assumes a point source and a surface facing the lamp. Oblique incidence multiplies by cosα.
- Extended luminaires and non-uniform beams need photometry for design-critical work.
- Typical context: moonlight ~1 lx; offices often 300–500 lx; bright daylight tens of thousands of lux.
Related tools
Other calculators in this family: Lux to Candela, Candela to Lumen, Lumen to Lux, Footcandles to Lux .
Frequently asked questions
Key distinctions behind the calculation.
How do I convert candela to lux?
Divide intensity by distance squared: lx = cd ÷ d² with d in meters. With d in feet, use lx = 10.76391 × cd ÷ d(ft)².
What is the difference between candela and lux?
Candela is source intensity in a direction. Lux is how bright a surface is (illuminance). Lux falls with distance; candela does not change just because you move farther away.
Why is there a 10.76391 factor for feet?
Lux is defined per square meter. One square meter equals 10.76391 square feet, so the feet form of the inverse-square law multiplies by that factor.
Is 1 cd at 1 m equal to 1 lx?
Yes, for a point source illuminating a surface normal to the ray: E(lx) = I(cd) / d(m)² → 1 cd / 1 m² = 1 lx.
Does beam angle matter for cd → lx?
Not in this formula — you already have intensity (cd) toward the point. Beam angle matters when converting between candela and total lumens.
Why do flashlight specs list lux at a distance?
Illuminance drops with distance. A lux figure without distance is incomplete for directional lights; manufacturers often quote peak lux (or candela) at a stated range.
Is 5000 lux the same as 5000 lumens?
No. Lumens are total output; lux is lumens per square meter on a surface. 5000 lm over 1 m² is 5000 lx; over 10 m² it is 500 lx.
Can I convert lux back to candela?
Yes — use lx → cd: cd = lx × d² (meters), or cd = lx × d(ft)² ÷ 10.76391.