One lux is 0.09290304 foot-candles. The factor is exactly the square-foot-to-square-metre conversion, because a lux is one lumen per square metre and a foot-candle is one lumen per square foot — the light is the same, only the area differs.
About the units
Illuminance measures how much luminous flux falls on a surface, and both units are a lumen divided by an area. The lux is the SI unit; the foot-candle is its American counterpart, and its name records its origin — the illuminance produced by one standard candle at a distance of one foot. That definition made it useful before photometers existed, since a candle at a measured distance was a reproducible reference. Because the area is the only difference between the units, the conversion factor is 0.09290304 exactly, the same number that converts square feet to square metres.
The exact factor
The exact value is 1 fc = 10.7639104167 lx, and 1 lx = 0.09290304 fc. Reference levels: full daylight 10,000 to 25,000 lx, or 930 to 2,300 fc; an overcast day 1,000 lx or 93 fc; a well-lit office 500 lx or 46 fc; a corridor 100 lx or 9 fc; and full moonlight 0.25 lx or 0.023 fc. The span from moonlight to direct sun is five orders of magnitude, which the human eye handles by adaptation rather than by any linear response.
Where you meet this conversion
Lighting design and workplace regulation. EN 12464-1, the European standard for indoor workplace lighting, specifies task illuminance in lux — 500 lx for office work, 100 for corridors and circulation areas, 750 or more for detailed inspection — while American codes use foot-candles for the same requirements. Photography light meters read in either. Horticultural lighting, film sets and museum conservation limits all specify illuminance, the last with strict maxima to limit fading of pigments and textiles. Anyone reading an American lighting specification against a European standard converts constantly.