A foot-candle (fc) is one lumen per square foot, equal to 10.764 lux. It is the illuminance unit used in older North American horticulture texts and building lighting codes. It shares the flaw of lux: it is weighted to the human eye, which sees green and yellow best and undervalues the red and blue light plants rely on.
In practice
Older grow guides quote targets such as "5,000–10,000 fc for flowering". These numbers date from the HPS era, when nearly everyone used the same spectrum, so a rough conversion held. They do not transfer to LED fixtures, whose spectra vary widely. A red-heavy LED can deliver more usable light at a lower foot-candle reading than an HPS.
If you only have a foot-candle meter, use it to check uniformity under one fixture type: walk it across the canopy and look for weak corners. For anything else, use a quantum sensor and work in µmol/m²/s.
Units and formula
- 1 fc = 1 lm/ft² = 10.764 lux
- 1,000 fc ≈ 10,760 lux ≈ 200 µmol/m²/s in sunlight (sunlight factor about 54 lux per µmol/m²/s; varies by source)
Don't confuse with
- Lux: the same quantity in metric units.
- PPFD: the plant-relevant photon count, in µmol/m²/s.
- Foot-lambert: a unit of luminance (light leaving a surface), not illuminance.
Myth check
"Lux and foot-candles are fine for grow lights" is a Myth once you mix lamp types. Brightness to your eye is not photosynthetic value.
