Light Unit Converter
Convert brightness between lux, foot-candles, kilolux, and phot.
What it does
Light on a surface is measured as lumens per area. The SI unit is lux, or lumens per square meter. American plans often use foot-candles, or lumens per square foot. The two differ only by the area unit.
The tool above converts among lux, kilolux, foot-candles, and phot. The defaults convert 500 lux to foot-candles, so the page opens showing 46.4515 fc. Pick new units to update the result and the full list.
The link is exact. One square foot is 0.09290304 square meters, so one lumen per square foot equals 1 / 0.09290304 = 10.7639104 lux. One phot is one lumen per square centimeter, which is 10,000 lux. Kilolux is simply 1000 lux.
Use lux for codes, datasheets, and sensors. Use foot-candles for US lighting layouts and real estate listings. Use phot mainly in older optics texts. Keep lumens (total light) apart from lux (light per area).
This page shows the factors, one full hand checked example at 500 lux, a reference table, and practice checks that run through the tool.
How to use
- Type the brightness number, for example 500.
- Pick the starting unit in From unit, for example Lux.
- Pick the target unit in To unit, for example Foot-candle.
- Read the main result with four decimals and unit name.
- Scan the note line for the same light in all four units.
- Check plan notes: US rooms often target 10 to 50 fc, or 100 to 500 lux.
Formula
- lux
- Lumens per square meter. SI unit.
- fc
- Lumens per square foot. 1 fc = 10.7639104 lux.
- ph
- Lumens per square centimeter. 1 ph = 10,000 lux.
Factors use the exact foot, 0.3048 m. One square foot is 0.09290304 square meters. Kilolux is 1000 lux by prefix.
Worked example
Example: 500 lux to foot-candles
A desk measures 500 lux. Express it in foot-candles for a US plan.
- Write the inputs: 500, from lux, to fc.
- Recall 1 fc = 10.7639104167 lux.
- Divide: 500 / 10.7639104167 = 46.451520.
- Round to four decimals: 46.4515 fc.
- Cross check: 46.4515 x 10.7639104167 = 500.00 lux.
- Read siblings: 0.5 klx and 0.05 ph.
Reference table
| Unit | Lux per unit | Use |
|---|---|---|
| lux | 1 | SI, sensors and codes |
| klx | 1000 | Daylight levels |
| fc | 10.7639104 | US layouts |
| ph | 10000 | Older optics texts |
Limits
- Illuminance only, not total lumens or brightness as seen.
- Does not model color, glare, or room reflectance.
- Four decimals suit field work. Sensor noise often exceeds the last digit.
- Phot is rare today. Confirm the unit before quoting it.
Common mistakes
- Multiplying by 10.7639 when converting lux to fc, instead of dividing.
- Confusing lumens with lux. Lumens total light, lux spreads it per area.
- Using square feet for length instead of area in the factor.
- Reporting kilolux as lux, off by 1000 times.
- Quoting phot values without noting 1 ph equals 10,000 lux.
Try it yourself
Quick checks. Answers sit tucked under each question.
1000 lux in fc?
1000 / 10.7639104167 = 92.903 fc.
10 fc in lux?
10 x 10.7639104167 = 107.6391 lux.
Questions
How many lux in 1 foot-candle?
10.7639104 lux.
How many fc in 500 lux?
46.4515 fc.
What is a phot?
1 lumen per square centimeter, or 10,000 lux.
Lux or lumens?
Lumens total light from a lamp. Lux light landing on each square meter.
What light suits a desk?
Many guides suggest near 300 to 500 lux, or about 30 to 50 fc. Task and age shift the target.
Why four decimals?
Enough to round trip small rooms without hiding conversion error.