# Foot-candle

> A foot-candle is an imperial unit of illuminance (one lumen per square foot, about 10.76 lux); like lux, it measures eye brightness, not plant-usable light.

- Canonical: https://genetixbay.com/knowledge/glossary/foot-candle.html
- Also written: fc, foot candle, footcandle, ft-c, foot-candles
- Glossary category: Environment and light · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

A foot-candle (fc) is one lumen per square foot, equal to 10.764 lux. It is the [illuminance](https://genetixbay.com/knowledge/glossary/lux.md) 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](https://genetixbay.com/knowledge/glossary/blue-light.md) 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](https://genetixbay.com/knowledge/glossary/hps.md) era, when nearly everyone used the same spectrum, so a rough conversion held. They do not transfer to [LED](https://genetixbay.com/knowledge/glossary/led.md) 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](https://genetixbay.com/knowledge/glossary/ppfd.md).

## 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](https://genetixbay.com/knowledge/glossary/photosynthesis.md) value.

## Related terms

[Lux](https://genetixbay.com/knowledge/glossary/lux.md), [PPFD](https://genetixbay.com/knowledge/glossary/ppfd.md), [PAR](https://genetixbay.com/knowledge/glossary/par.md)

## Read more

- [PPFD and DLI by stage, with the evidence](https://genetixbay.com/knowledge/environment/ppfd-dli-by-stage.md)