Illuminance Converter
Illuminance, the measure of light falling on a surface, is fundamental to lighting design, photography, safety standards, and countless practical applications. The SI unit is the lux (lx), defined as one lumen per square metre, adopted globally in science and engineering. North America and some older lighting specifications still use the foot-candle (fc), an imperial unit equal to approximately 10.764 lux. The phot (ph), a CGS unit equal to 10,000 lux, appears in historical scientific literature and specialized optics contexts. Beyond these, millilux and kilolux extend the range for extremely dim (twilight, starlight) and extremely bright (spotlight, industrial arc light) scenarios, while the nox, a rare unit used in specialized contexts, equals 0.001 lux. Converting between these units manually is cumbersome and error-prone, particularly when working across systems or translating between modern SI specifications and legacy imperial or CGS data. This calculator displays six illuminance units simultaneously: millilux, lux, kilolux, foot-candles, phot, and nox. Type a value into any field and all the others update instantly. Conversion factors are derived from SI definitions and NIST standards.
Illuminance conversion factors
All conversions below use standard SI-defined factors, with all units expressed relative to lux (lx).
| Unit | Symbol | Lux (lx) |
|---|---|---|
| Millilux | mlx | 0.001 |
| Lux | lx | 1 |
| Kilolux | klx | 1,000 |
| Foot-candle | fc | 10.76391 |
| Phot | ph | 10,000 |
| Nox | nox | 0.001 |
Illuminance converter: frequently asked questions
What is illuminance?
Illuminance is the amount of light falling on a surface, measured per unit area. The SI unit is the lux (lx), defined as one lumen per square metre. It differs from luminous intensity (candela) and luminous flux (lumen), which measure the light source itself rather than the light reaching a surface.
What is a lux and how is it used?
A lux is the standard unit of illuminance in the International System of Units (SI). One lux equals one lumen per square metre. It is widely used in lighting design, photography exposure calculations, and workplace safety standards. For reference, bright sunlight is roughly 100,000 lux, while office lighting is typically 300-500 lux.
What is a foot-candle?
A foot-candle (fc) is a traditional imperial unit of illuminance, defined as the light falling on a one-foot sphere from a one-candela source at one foot distance. One foot-candle equals approximately 10.764 lux. It is still used in North America, particularly in older lighting specifications and some photography applications.
How do foot-candles and lux relate?
One foot-candle equals approximately 10.764 lux. To convert foot-candles to lux, multiply by 10.76391. To convert lux to foot-candles, divide by 10.76391. This calculator performs all conversions instantly.
What is a phot?
A phot (ph) is a CGS (centimetre-gram-second) unit of illuminance, equal to one lumen per square centimetre, or 10,000 lux. The phot is rarely used in modern practice, having been superseded by the lux, but remains relevant in older scientific literature and specialized optics work.
Official sources
- SI Brochure (9th edition, 2019): The International System of Units. BIPM.
- NIST Special Publication 330: The International System of Units (SI).
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.