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Multiple conversions
To convert from Foot-candle (ft-cd) to Lux (lx), use the following formula:
Lux (lx)
= 10.76391× Foot-candle (ft-cd)
With 10.76391 is the ratio between the base units Lux (lx) and Foot-candle (ft-cd).
A foot-candle (ft-cd) is the standard unit of illuminance, or light intensity, in the imperial system.
It's a fundamental concept used in lighting design to ensure spaces are adequately lit.
A foot-candle measures the amount of light that falls onto a specific surface.
To be precise, one foot-candle is the illuminance on a one-square-foot surface from a uniform light source of one candela that is exactly one foot away.
This direct relationship makes the foot-candle an intuitive unit for creating lighting plans in the imperial system.
It's important not to confuse foot-candles with lumens. While lumens measure the total light output from a source (like a lightbulb), foot-candles measure how much of that light actually reaches a surface.
The foot-candle is the imperial counterpart to the lux (lx), which is the standard unit for illuminance in the SI or metric system.
Since many lighting specifications are international, understanding how to convert between them is essential.
The conversion is straightforward:
For quick reference, here's a simple comparison table:
| Feature | Foot-candle (ft-cd) | Lux (lx) |
|---|---|---|
| System | Imperial | SI / Metric |
| Definition | 1 lumen / square foot | 1 lumen / square meter |
| Conversion | 1 ft-cd ≈ 10.764 lx | 1 lx ≈ 0.0929 ft-cd |
Using foot-candle measurements ensures that different environments are lit correctly for safety, comfort, and function.
Lighting standards often provide recommendations for various spaces. Here are a few common examples:
Lighting design is one of the last fields where two unit systems are used side by side in the same document, so the conversion is worth carrying in your head.
The awkward number is simply the number of square feet in a square metre. A foot-candle is one lumen per square foot; a lux is one lumen per square metre. Spread the same lumen over the larger metric area and you get a smaller number, which is why lux values always look about eleven times bigger than the same light expressed in foot-candles.
For rough fieldwork many designers divide or multiply by 10 and accept the 7% error, but that shortcut does not belong in a specification that has to be verified with a meter.
The foot-candle has been formally superseded by the lux almost everywhere, yet it remains the working unit of North American lighting practice for a practical reason: the standards are written in it.
Recommended illuminance levels published by the Illuminating Engineering Society (IES), along with a great deal of building code, energy code and utility rebate documentation, are expressed in foot-candles. So are the calibration scales on much of the installed base of light meters. An engineer who converts everything to lux still has to convert back to demonstrate compliance.
There is also a convenient coincidence that helped the unit stick. At a distance of one foot, a one-candela source produces exactly one foot-candle — the definition is directly readable from the name, which makes it easy to reason about point sources without a calculator.