Foot-candles to Lux

How Many Lux in a Foot-candle?

To convert foot-candles to lux, multiply the foot-candle value by 10.764. The formula is lx = fc × 10.764. For example, 50 foot-candles equals approximately 538.2 lux. This conversion is essential for lighting professionals, architects, and engineers working on projects that span the US and international markets. American building codes and the Illuminating Engineering Society of North America (IESNA) specify lighting requirements in foot-candles, while the rest of the world uses lux. When importing lighting fixtures designed to international specifications, when working with multinational architecture firms, or when consulting scientific literature that uses SI units, converting foot-candles to lux ensures that your lighting design meets the correct illuminance targets. The conversion is also critical in horticulture, photography, and display technology where light levels must meet precise thresholds. It is equally common during retrofits, when an older US specification must be compared with a metric lux meter or an EN 12464 recommendation. Converting accurately prevents underlighting desks, classrooms, retail shelves, and inspection areas. It also keeps commissioning teams aligned when submittals and field readings use different unit systems.

How to Convert Foot-candle to Lux

1. Start with the illuminance value in foot-candles (fc).
2. Multiply the foot-candle value by 10.764 to get lux.
3. The result is the illuminance expressed in lux (lx).
4. Quick estimate: multiply by 11 for a fast approximation accurate to about 2%.
5. For mental math, multiply by 10 and add about 8% of the result.

Real-World Examples

History of Foot-candle and Lux

The foot-candle was one of the earliest photometric units, emerging in the 19th century when standardized candles served as reference light sources. The concept was straightforward: if you place a standard candle one foot away from a surface, the illumination on that surface is one foot-candle. This definition made it easy for gas lighting engineers and later electrical lighting pioneers to quantify and compare light levels in buildings. As electric lighting transformed cities and workplaces in the early 1900s, the foot-candle became embedded in American engineering practice. The Illuminating Engineering Society, founded in 1906, adopted foot-candles as the standard unit for all its recommendations. These recommendations eventually became incorporated into building codes, OSHA workplace safety standards, and architectural specifications throughout the United States. The international shift to lux began in earnest during the 1960s and 1970s as the SI system gained universal adoption. Today, virtually every country except the United States uses lux as its primary illuminance unit. Even in the US, many lighting manufacturers now provide specifications in both units, and the trend toward global harmonization of building standards makes the foot-candle-to-lux conversion increasingly important.

Common Mistakes to Avoid

• Dividing instead of multiplying. To convert foot-candles to lux, multiply by 10.764. Dividing goes the wrong direction and produces a value about 116 times too small.
• Confusing foot-candles with candlepower or candela. Foot-candles measure illuminance (light on a surface), while candela and candlepower measure luminous intensity (light emitted in a direction). They are different physical quantities.
• Assuming a linear relationship between distance and illuminance. Illuminance follows the inverse-square law — doubling the distance from a light source reduces illuminance to one-quarter, not one-half.
• Comparing maintained foot-candle requirements with initial lux readings without accounting for dirt, aging, or lumen depreciation. The unit conversion is straightforward, but the design basis still has to match the specification basis.

Frequently Asked Questions