What is the formula used?

The inverse square law for point sources: E (lux) = I (candela) / d² (meters²). This assumes the light source is a point emitting uniformly in all directions.

Does this formula apply to all lights?

It applies accurately to point sources or when the measurement distance is much greater than the size of the light source. For large diffuse panels or non-point sources, the formula is an approximation.

What is the difference between candela and lux?

Candela (cd) measures luminous intensity — the power of light emitted per unit solid angle from the source. Lux (lx) measures illuminance — the total luminous flux received per unit area at a surface. Lux depends on both the source intensity and the distance.

Why does lux decrease with the square of distance?

As light travels outward from a point source, it spreads over an ever-larger spherical surface. The area of a sphere grows as r², so the same luminous flux is spread over 4× the area when the distance doubles, giving ¼ the illuminance.

What lux level is recommended for reading?

Typical recommendations range from 300–500 lux for reading and office work. Direct task lighting often targets 500–750 lux at the work surface.

Can I convert lux back to candela?

Yes. Rearranging the formula: I (cd) = E (lux) × d² (m²). The reverse calculator tab on this tool performs that conversion.

Candela to Lux Converter

Enter luminous intensity (cd) and distance (m) to instantly calculate illuminance in lux using the inverse square law.

Candela to Lux

Luminous Intensity (cd)

Candela — how bright the source is in this direction

Distance (m)

Distance from source to the lit surface in meters

Incident Angle (°) — optional

Angle between light ray and surface normal (Lambert cosine law). Leave at 0 for perpendicular incidence.

Enter values to see the result

Typical Illuminance Levels

Setting	Lux
Moonlit night	0.1
Street lighting	10
Corridor / hallway	100
Office / reading	300–500
Surgery / inspection	10 000+
Full sunlight	100 000

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Summary

Enter luminous intensity (cd) and distance (m) to instantly calculate illuminance in lux using the inverse square law.

How it works

Enter the luminous intensity of the light source in candela (cd).
Enter the distance from the light source to the surface in meters.
The tool calculates illuminance using the formula: lux = candela / distance².
Results update instantly as you type.
Use the reverse mode to find required candela for a target lux level.

Use cases

Determine illuminance at a given distance from a spotlight or lamp.
Verify compliance with workplace lighting standards (lux requirements).
Plan stage or architectural lighting setups.
Calculate reading light intensity at desk height.
Estimate outdoor lighting coverage from floodlights.
Cross-check photometric data from manufacturer spec sheets.

Frequently Asked Questions

Last updated: 2026-09-22 · Reviewed by Nham Vu