Everyone talks in footcandles. Here's what one actually is, how it relates to lux, and why NFPA sets egress requirements in footcandles.
The definition
A footcandle (fc) is one lumen of light falling on one square foot of surface. It's a measurement of illuminance — how much light is landing on a surface — not a measurement of how much light a source produces.
Lumens vs. footcandles
Lumens measure the total light a source emits. Footcandles measure the light that reaches a specific surface. One fixture producing 1,000 lumens will illuminate a surface directly below it very differently depending on mounting height, beam distribution, and room reflectance.
Metric equivalent
1 footcandle = 10.76 lux (10.8 rounded). Most of the world uses lux. The U.S. lighting industry, NFPA, and IES keep footcandles. Familiar reference points:
- Full sunlight: ~10,000 fc
- Overcast daylight: ~1,000 fc
- Office lighting: 30–50 fc
- Emergency egress minimum (NFPA): 1 fc average / 0.1 fc minimum
- Full moon: 0.01 fc (yes, 10× below NFPA's emergency floor)
Why NFPA picked 1 fc
At 1 footcandle average, a person with normal vision can see floor-level obstacles, read exit signs, and navigate a corridor without tripping. It's not bright — it's enough. The standard is set for the minimum competent visibility required to get people out safely.