A foot-candle is one lumen falling on one square foot. Divide the lumens by the area in square feet to find foot-candles.
What the terms mean
| Symbol | Meaning |
|---|---|
| fc | Illuminance, in foot-candles |
| Φ | Luminous flux, in lumens |
| A | Area, in square feet |
Worked example
3,500 lumens over a 100 ft² area gives 35 foot-candles.
See the Lux Formula and the Recommended Foot Candle Levels.
The foot-candle formula
A foot-candle is one lumen falling on one square foot, so the basic formula is foot-candles = lumens ÷ area (sq ft) for light spread over a surface. If 5,000 lumens reach 100 square feet, that’s 50 foot-candles average. For a directional source, the point method uses intensity instead: fc = candela ÷ distance² (distance in feet), the imperial form of the inverse-square law.
Foot-candles remain common in North American codes. To design a space, pick a target level for the task (say 30 fc for an office), multiply by the area to get the lumens needed on that plane, then divide by the delivered lumens per fixture to estimate fixture count — applying a light-loss factor so the level holds as fixtures age. To convert, 1 fc is about 10.76 lux.