Illuminance Conversion Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/10/2026
To convert an illuminance, it is reduced to the lux (the SI base unit), then converted into the new unit. One footcandle equals exactly 10.7639 lux, so a typical office illuminance of 1,000 lux equals about 92.9 footcandles.
Explanation
Illuminance measures the amount of light received by a surface, not to be confused with the brightness of a light source itself. The lux (lx), the SI unit, is defined as one lumen per square meter and is the reference in nearly all modern European lighting standards. The footcandle (fc), inherited from the imperial system, remains the standard used in the United States in building and professional lighting design: it corresponds to one lumen per square foot, and its exact conversion factor (10.763910416709722 lux) follows directly from the official definition of the international foot (0.3048 meter exactly since 1959), not from an approximate physical measurement. The phot, an older CGS unit defined as one lumen per square centimeter, is rarely encountered today outside some historical photometry texts, but is still occasionally quoted in older standards or technical publications — its conversion to lux (10,000 lux exactly) simply follows from the ratio between square centimeters and square meters. These three units all measure the same physical quantity under different historical conventions, much like the pressure conversion between pascals, bars, and psi: the choice of unit depends mainly on region and industry tradition, not on a real physical difference.
Example: converting 1,000 lux to footcandles
Inputs
Value: 1,000. Starting unit: lux. Target unit: footcandle.
Calculation
1,000 lx ÷ 10.763910416709722 (footcandle factor) ≈ 92.90304 fc.
Result
An illuminance of 1,000 lux equals about 92.9 footcandles — a typical level for a well-lit office workstation.
Frequently asked questions
Why is the footcandle conversion factor this precise number (10.7639...)?
Because it follows directly from the official definition of the international foot, fixed at exactly 0.3048 meter by a 1959 international agreement. One square foot is therefore exactly 0.09290304 square meter, and one footcandle (one lumen per square foot) is consequently exactly 1 ÷ 0.09290304 ≈ 10.7639 lux — not a measured or rounded value, but an exact arithmetic consequence of the definition of the foot.
What is the difference between illuminance and brightness?
Illuminance (measured in lux or footcandles) describes the amount of light RECEIVED by a given surface, for example a desk or a page of a book. Brightness (or luminous intensity, measured in candelas) instead describes the amount of light EMITTED by a source in a given direction, independent of the distance or the surface receiving it. The same spotlight can therefore produce a very different illuminance depending on the distance of the lit surface — a relationship precisely described by our inverse-square law of light intensity calculator — while its intrinsic brightness stays unchanged.
Is the phot still used today?
Very rarely in everyday practice: the phot belongs to the CGS (centimeter-gram-second) system, largely abandoned in favor of the International System since the mid-20th century. It nonetheless still appears occasionally in some older scientific texts or technical standards, which justifies being able to convert it with certainty rather than having to guess its lux equivalent.