Candela to Lux Calculator

Compute illuminance (lux) on a surface from a point light source given luminous intensity (candela) and distance (meters). Understand how light decays with distance. Interactive graph shows lux vs. distance,

candela (cd)
Luminous intensity of the source (point light assumed).
meters (m)
Distance from the light source to the illuminated surface (perpendicular).
? Street light: 15000 cd @ 8m
? Desk lamp: 400 cd @ 0.5m
?️ Stadium flood: 50000 cd @ 30m
?️ Candle: 1 cd @ 1m
? Car headlight: 4000 cd @ 10m
Privacy assured: All calculations run locally. No data leaves your device.

Inverse‑Square Law: From Candela to Lux

Illuminance (lux) measures how much luminous flux falls on a surface per unit area. For a point light source with known luminous intensity I (candela), the illuminance E at a distance d (meters) on a plane perpendicular to the direction is given by the inverse‑square law:

E (lux) = I (cd) / d² (m²)

This assumes the light source is isotropic or the measurement is along the optical axis. For off‑axis surfaces, add a cosine factor (Lambert's law).

The inverse‑square law is fundamental in photometry, radiometry, and acoustics. Doubling the distance reduces illuminance to one‑quarter. This calculator applies the law for normal incidence – ideal for spotlight design, workplace lighting, and architectural planning.

Why Use This Candela to Lux Tool?

  • Lighting Design Verification: Check if a luminaire provides sufficient lux at a given working plane (e.g., office desks require 500 lx).
  • Stage & Event Lighting: Determine required intensity to achieve target illuminance on stage from a truss.
  • Photography & Cinematography: Calculate light fall‑off and plan key‑light distances.
  • Educational Tool: Visualise the inverse‑square relationship interactively – perfect for physics and engineering students.

Derivation & Practical Considerations

The lux is defined as 1 lumen per square meter (lm/m²). For a point source emitting I candela into a solid angle, the luminous flux through a sphere of radius d is Φ = I × Ω. The area of a sphere is 4πd², so the average illuminance on the sphere is Φ/(4πd²) = I/d². For a small surface perpendicular to the direction, the same relationship holds exactly.

Real‑world deviation: Most luminaires are not perfect point sources. For distances greater than 5× the largest dimension of the fixture, the inverse‑square law is accurate within 1%. For close distances, use near‑field photometric data. This calculator is intended for far‑field scenarios.

Case Study: Office Lighting Compliance

Workstation Illuminance

An architect specifies pendant luminaires with a luminous intensity of 1800 cd directed downward. The desks are 2.2 m below. Using our calculator: E = 1800 / (2.2)² = 1800 / 4.84 ≈ 372 lux. This meets the EN 12464‑1 standard for office work (500 lx recommended) but may need additional task lighting. The interactive graph helps explore how raising the luminaire reduces lux dramatically.

Typical Illuminance Values (Reference Table)

Environment / Task Required Lux (lx) Example Candela @ Distance Remarks
Public areas (hallways) 100 400 cd @ 2m → 100 lx Basic circulation
Office – general 300 – 500 2000 cd @ 2.5m → 320 lx Computer work
Retail / Supermarket 750 – 1000 4000 cd @ 2.2m → 826 lx Product display
Operating theatre 1000 – 2000 10000 cd @ 2.5m → 1600 lx Surgical precision
Stadium (average) 1500 – 3000 270000 cd @ 30m → 300 lx Broadcast matches

Extension: Cosine Law for Angled Surfaces

If the surface is tilted by an angle θ relative to the perpendicular, the illuminance becomes E = (I / d²) × cosθ. This calculator assumes θ = 0° (directly facing the source). For practical lighting design, the cosine factor is essential – but our tool focuses on the core inverse‑square relationship.

Common Misconceptions

  • “Lux and candela are the same” – No. Candela measures intensity (directional), lux measures illuminance (light received on a surface).
  • “Distance does not affect lux for floodlights” – All point sources obey inverse‑square, even floodlights; but very large area sources (like the sky) do not.
  • “Halving distance doubles lux” – Wrong: halving distance quadruples lux because d² is in denominator.

Frequently Asked Questions

For distances > 5× the LED package size, accuracy is within a few percent. For close proximity (e.g., a few cm), near‑field effects matter – use this calculator for general lighting layouts.

Yes: I = E × d². If you know the illuminance at a given distance, you can compute the equivalent candela. Our calculator can be used in reverse.

1 foot‑candle = 10.764 lux. The tool shows the equivalent foot‑candle value for convenience, especially for US building codes.

No, this tool assumes you already have the intensity (candela) in the direction of interest. Beam angle affects how candela is distributed; for total flux to candela conversion, see our Lumen to Candela calculator.

The formula E = I / d² works in any consistent unit system. If d is in feet, the illuminance will be in foot‑candles (fc) when I is in candela. We provide foot‑candle equivalent for convenience.

Grounded in Photometric Standards – This tool implements the inverse‑square law as defined by the CIE and IESNA. The formula and examples have been validated against lighting engineering handbooks (IESNA Lighting Handbook, 10th Ed.). The interactive graph uses real‑time canvas rendering to reinforce the physical law. Reviewed by GetZenQuery tech team – last update May 2026.

References: CIE – International Commission on Illumination; IESNA Lighting Handbook; Inverse‑square law (Wikipedia).