Candela to Lux Calculator

Enter a light source's luminous intensity and distance to calculate illuminance in lux using the inverse-square law.

How to Use the Candela to Lux Calculator

Set Luminous Intensity to the source's cd rating and Distance to how far away you're measuring — the Illuminance card updates the instant either number changes.

Illuminance falls off with distance, but not proportionally. This is the inverse-square law, and it's the reason a lamp moved twice as far away doesn't look half as bright — it looks dramatically dimmer.

lux = candela ÷ distance²

Run the calculator's own defaults — 500 cd at 3 m — and you get 55.6 lux. Push the same source out to 6 m, twice the distance, and illuminance doesn't halve to roughly 28 lux. It drops to 13.9 lux, a quarter of the original, because distance is squared in the denominator.

Why Doubling Distance Doesn't Just Halve the Light

Light from a point source spreads outward over an expanding spherical surface. Double the radius of that sphere and its surface area quadruples, so the same total flux is spread across four times the area — hence a quarter of the illuminance, not half. It's the same geometry behind why a flashlight held at arm's length looks so much dimmer than one held an inch from a wall.

Security-light and stage-light spec sheets often publish candela at a fixed throw distance for exactly this reason. A fixture rated at 5,000 cd sounds intense until the math runs: at 10 m it only delivers 50 lux, about what a bright hallway gets, nowhere near the harsh glare the raw candela number implies on its own. This only holds cleanly for something close enough to a true point source — a bare bulb or a small LED, not a long fluorescent tube or a wall of light panels, where the source's own size distorts the falloff at short range.