Illuminance measures how much light actually lands on a surface, and it's expressed in different units depending on the region, industry, and era of the measurement. This calculator converts between the four most common illuminance units — lux, foot-candles, phot, and lumens per square meter — so you can compare a light meter reading, a lighting design spec, or an old textbook figure on the same scale.
How the Illuminance Converter works
Every unit here converts through lux, the SI unit of illuminance defined as one lumen per square meter. The calculator converts your input value into lux first, then converts that lux figure into whichever unit you selected as the target. The key constants are fixed by definition: 1 foot-candle = 10.7639 lux (because a square foot is smaller than a square meter, the same lumen output packs more densely), 1 phot = 10,000 lux (phot uses square centimeters instead of square meters), and 1 lumen per square meter is identical to 1 lux — they are the same physical quantity written two different ways.
Inputs and what they mean
Enter any positive numeric value and select the unit it's currently measured in, then pick the unit you want to see it in. The Reference tab shows your current value in all four units simultaneously, and the Common Levels tab lists typical illuminance readings — from full moonlight (0.1 lux) to direct sunlight (~50,000 lux) — so you can gauge whether a reading is unusually dim or bright for its context.
Limits and edge cases
This calculator performs a pure unit conversion — it does not account for how illuminance falls off with distance from a light source (the inverse-square law), nor does it convert luminous intensity (candela) or luminous flux (lumens) directly, since those require knowing the beam angle or surface area involved. If you're designing a lighting layout, use this tool to translate a target illuminance level between units, then consult a lighting designer or fixture spec sheet for the lumen output and placement needed to achieve it.