Introduction
Illuminance is how much light falls on a surface, in lux (lx) or foot-candles (fc). Office desks need 300-500 lux for comfortable work. Direct sunlight is 100,000 lux. Full moon is 0.25 lux. The calculator below converts between lux and foot-candles (older US unit).
Why illuminance units exist and how they diverged
One lux = one lumen per square meter. One foot-candle = one lumen per square foot. 1 fc = 10.76 lx. Foot-candles survive in US building codes; lux dominates elsewhere. Photography light meters can show either or both.
How to convert illuminance
Foot-candles to lux: multiply by 10.76. Lux to foot-candles: divide by 10.76. Photography: incident light meter reads lux or fc; exposure maps to shutter/aperture/ISO.
Units supported by this illuminance calculator
- Lux (lx)
- Foot-candle (fc)
- Phot (ph)
- Nox
- Kilolux (klx)
- Millilux (mlx)
- Lumen/m²
- Lumen/ft²
Common illuminance conversion mistakes
- Lumens of the source ≠ illuminance at the surface. A 1,000-lumen bulb at the ceiling puts far fewer lumens per square meter on a desk than a 1,000-lumen task light a foot away.
- Lux from averages vs point measurements. Illuminance varies across a room. Averages hide dark corners.
- Foot-candles in old US code. Some US safety codes still specify illumination in fc. Always convert consistently.
Real-world illuminance examples
- Full moon: 0.25 lx.
- Street lighting: 5-20 lx.
- Living room: 50-100 lx.
- Office task lighting: 500 lx.
- Operating room: 1,000-5,000 lx.
- Overcast day: 1,000-10,000 lx.
- Clear day indirect: 10,000-25,000 lx.
- Direct sunlight: 100,000 lx.
Tips for accurate illuminance conversion
- For workspaces, 500 lx is comfortable for most reading and desk work.
- For retail, 750-1,000 lx highlights products; 100-300 lx is moody.
- Smartphone ambient light sensors estimate lux, but are imprecise. A dedicated light meter costs $30-100.
Related: Luminous Intensity Converter · Area Converter · Power Converter.