Lux to footcandle conversion
To convert illuminance from lux (lx) to foot‑candle (fc, also abbreviated ft·cd), an exact conversion factor based on the relationship between the square meter and the square foot is used. One lux equals 0,09290304 foot‑candle, while one foot‑candle equals 10,7639104167 lux.
Conversion Formula
Section titled “Conversion Formula”The formula to obtain illuminance in foot‑candle from the value in lux is:
Efc = Elx × 0,09290304
| Variable | Meaning | Unit |
|---|---|---|
| Efc | Illuminance in foot‑candle | fc |
| Elx | Illuminance in lux | lx |
| 0,09290304 | Exact conversion factor (1/10,7639104167) | fc/lx |
It is also possible to express the conversion as division by the constant 10,7639104167:
Efc = Elx / 10,7639104167
Calculation Example
Section titled “Calculation Example”Calculate how many foot‑candle are equivalent to 500 lux.
- Take the value in lux: 500 lx.
- Apply the formula: Efc = 500 lx × 0,09290304 = 46,4515 fc.
- Result: 500 lux correspond approximately to 46,45 foot‑candle.
For a quick estimate with ±10 % accuracy, it is enough to remember that 1 foot‑candle ≈ 10 lux.
Quick Conversion Table
Section titled “Quick Conversion Table”| Lux (lx) / Foot‑candle (fc) |
|---|
| 1 lx / 0,0929 fc |
| 5 lx / 0,4645 fc |
| 10 lx / 0,9290 fc |
| 50 lx / 4,6452 fc |
| 100 lx / 9,2903 fc |
| 200 lx / 18,5806 fc |
| 500 lx / 46,4515 fc |
| 1 000 lx / 92,9030 fc |
| 5 000 lx / 464,5152 fc |
| 10 000 lx / 929,0304 fc |
| 50 000 lx / 4 645,1520 fc |
| 100 000 lx / 9 290,3040 fc |
Typical Illuminance Values
Section titled “Typical Illuminance Values”Outdoor Illuminance
Section titled “Outdoor Illuminance”| Condition | Illuminance (lx / fc) |
|---|---|
| Full direct sunlight | 107 640 lx / 10 000 fc |
| Full daylight (no direct sun) | 10 764 lx / 1 000 fc |
| Cloudy day | 1 076 lx / 100 fc |
| Very dark day | 108 lx / 10 fc |
| Twilight | 10,8 lx / 1 fc |
| Deep twilight | 1,08 lx / 0,1 fc |
| Full moon | 0,108 lx / 0,01 fc |
| Quarter moon | 0,0108 lx / 0,001 fc |
| Starry night | 0,0011 lx / 0,0001 fc |
| Cloudy night | 0,0001 lx / 0,00001 fc |
Recommended Indoor Illuminance
Section titled “Recommended Indoor Illuminance”| Activity | Illuminance (lx / fc) |
|---|---|
| Public areas with dark surroundings | 20–50 lx / 1,9–4,6 fc |
| Simple orientation (short visits) | 50–100 lx / 4,6–9,3 fc |
| Corridors, stairs, warehouses | 100–150 lx / 9,3–13,9 fc |
| Waiting rooms, archives, cinemas | 150 lx / 13,9 fc |
| Break rooms, technical facilities | 200 lx / 18,6 fc |
| Simple office work | 250 lx / 23,2 fc |
| Classrooms | 300 lx / 27,9 fc |
| Normal offices, laboratories, kitchens | 500 lx / 46,5 fc |
| Supermarkets, mechanical workshops | 750 lx / 69,7 fc |
| Normal drafting, operating rooms | 1 000 lx / 92,9 fc |
| Detailed drafting, precision electronics | 1 500–2 000 lx / 139–186 fc |
| Low contrast and very small size tasks | 2 000–5 000 lx / 186–465 fc |
| Very prolonged and demanding tasks | 5 000–10 000 lx / 465–929 fc |
| Extremely special visual tasks | 10 000–20 000 lx / 929–1 860 fc |
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the difference between lux and foot‑candle?
Section titled “What is the difference between lux and foot‑candle?”The lux is the SI unit that measures illuminance, defined as one lumen per square meter. The foot‑candle is the US customary unit, equivalent to one lumen per square foot. The difference lies in the reference area: 1 m² vs. 1 ft².
How much is 1 foot‑candle in lux?
Section titled “How much is 1 foot‑candle in lux?”1 foot‑candle is exactly equal to 10,7639104167 lux. In daily practice it is often rounded to 10,76 lux or even to 10,8 lux if high precision is not required.
How many lux are 200 foot‑candle?
Section titled “How many lux are 200 foot‑candle?”Applying the inverse of the formula: 200 fc × 10,7639104167 ≈ 2 152,8 lux. Therefore, 200 foot‑candle correspond to about 2 153 lux.
What illuminance level is suitable for an office?
Section titled “What illuminance level is suitable for an office?”According to indoor lighting standards, an office with normal PC work and reading requires around 500 lux (approximately 46,5 fc). For detailed drafting tasks, between 1 000 and 2 000 lux (93–186 fc) are recommended.
Can 10 lux = 1 foot‑candle be used as a rule of thumb?
Section titled “Can 10 lux = 1 foot‑candle be used as a rule of thumb?”Yes, for quick estimates with an error below 10 % it can be assumed that 10 lux ≈ 1 foot‑candle. This simplified equivalence should never be used in lighting design calculations that require precision.
Why does the United States still use foot‑candle instead of lux?
Section titled “Why does the United States still use foot‑candle instead of lux?”The foot‑candle is part of the US customary system of units. Although lux is the internationally adopted unit, professionals in lighting, architecture, museum design, and horticulture in the United States continue to use the foot‑candle due to legacy standards and compatibility with local measuring equipment.