Convert candela and distance into lux, with inverse-square reference rows showing how illuminance changes at common distances. Use it to test different inputs quickly, compare outcomes, and understand the main factors behind the result before moving on to related tools or deeper guidance.
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Illuminance from intensity and distance Lux depends on how much luminous intensity reaches a surface at a given distance. This
worksheet keeps the inverse-square relation visible so you can see how quickly illuminance
falls as distance increases.
Candela to lux calculator: convert luminous intensity and distance into illuminance
A candela to lux calculator is only useful if it keeps the distance effect visible. Candela measures luminous intensity in a direction, while lux measures illuminance falling on a surface. This page converts candela and distance into lux with the inverse-square relationship, then adds reference distances so you can see how quickly illuminance changes as a light source moves closer to or farther from the target surface.
What candela and lux measure
Candela and lux are related lighting units, but they answer different questions. Candela measures the luminous intensity of a source in a direction. Lux measures illuminance, which is the amount of luminous flux arriving on a surface per square metre. That is why a candela to lux conversion needs distance information instead of just another unit factor.
In practice, this means a source with the same candela can produce very different lux levels depending on how far the surface is from the source. Close surfaces receive much higher illuminance, while the lux level falls off quickly as distance increases.
How the candela-to-lux formula works
For a point-source style approximation with normal incidence, illuminance in lux equals luminous intensity in candela divided by the square of the distance in metres. Doubling the distance does not cut lux in half; it reduces illuminance to one quarter because the distance term is squared.
That is why a useful candela to lux calculator should keep the inverse-square law visible. The real interpretation comes from seeing both the intensity and the distance together rather than treating lux as if it could be inferred from candela alone.
Uses the inverse-square relationship to convert directional intensity into illuminance at a surface.
If distance doubles, lux becomes one quarter
Shows why illuminance falls off quickly with distance under the inverse-square model.
Worked examples and reference distances
If a source has an intensity of 100 candela at a distance of 1 metre, the illuminance is 100 lux. At 2 metres, the same source produces 25 lux because the distance squared is 4. At 5 metres, it produces only 4 lux because the distance squared is 25.
These examples show why distance dominates interpretation. The same candela value can represent a brightly lit nearby surface or a dimly lit distant surface. The reference sheet on this page is intended to make that falloff obvious at a glance.
This page converts between candela, distance, and lux only. It does not calculate total lumens, beam angle, luminous efficacy, room reflectance, or multi-source lighting design. Those are separate lighting problems that need additional geometry and photometric information.
It also assumes a simplified point-source style relationship and does not account for beam shape, off-axis measurement, reflector losses, or real-world fixture photometry. Use it as a transparent unit-and-relationship worksheet, not as a full lighting-design simulator.
Further reading
BIPM — SI base units — Official SI reference page covering the candela as an SI base unit.
NIST — SI Units — Official NIST overview of SI base and derived units.
Frequently asked questions
Can I convert candela to lux without distance?
No. Lux depends on how far the illuminated surface is from the source. Candela alone is not enough because illuminance changes with the square of the distance.
Why does lux fall so quickly when distance increases?
Because the inverse-square relation divides intensity by distance squared. If distance doubles, lux falls to one quarter. If distance triples, lux falls to one ninth.
Is this the same as converting candela to lumens?
No. Candela to lumens uses solid angle and total luminous flux. Candela to lux uses distance and illuminance at a surface. They are related lighting calculations, but they answer different questions.
Does this calculator work for every real-world fixture?
It is best used as a simplified worksheet. Real fixtures can have beam-shape effects, off-axis distribution, reflector losses, and other photometric details that are not captured by the ideal inverse-square model alone.
