Lux to Foot-Candles Converter
Convert a light-meter reading between lux and foot-candles, check it against US and European lighting requirements, and see the matching exposure value for photography.
fc = lux × 0.09290304 · lux = fc × 10.7639
Everyday Light Levels in Lux and Foot-Candles
Typical ranges, not targets. Outdoor light swings by a factor of a million between a moonlit night and noon sun, which is why light meters work across so many decades.
| Scene | Lux | Foot-candles |
|---|---|---|
| Full moon on a clear night | 0.05–0.3 | 0.005–0.028 |
| Street lighting on a residential road | 5–15 | 0–1 |
| Living room in the evening | 50–150 | 5–14 |
| Very dark overcast day | 100–1,000 | 9–93 |
| Office desk | 300–500 | 28–46 |
| Overcast daylight | 1,000–10,000 | 93–929 |
| Daylight, not in direct sun | 10,000–25,000 | 929–2,323 |
| Direct midday sunlight | 32,000–100,000 | 2,973–9,290 |
Where 10.764 Comes From
Both units are lumens per unit area. A lux is one lumen per square metre and a foot-candle is one lumen per square foot, so the factor is just the ratio of the two areas:
The foot is defined as exactly 0.3048 m
So, exactly
And the other way
The common shortcut "divide lux by 10" overstates foot-candles by about 7.6%. That's enough to make a reading that just fails a requirement look as if it passes, as the next example shows.
Worked Example: A Site Office Against OSHA's 30 fc
A safety walk-through on a US construction site uses a light meter that reads in lux. On the desk in the site office trailer it shows 280 lux. OSHA 1926.56 requires at least 30 foot-candles in offices. (This is the Load Example.)
Step 1: Convert the reading
Step 2: Compare with the minimum
Step 3: The target in lux
The office is about 13% short. The divide-by-10 shortcut would have given 28 fc: still a fail, but it hides how far off the reading is. A reading of 300 lux makes the difference clearer: the shortcut says 30 fc and a pass, while the true figure is 27.9 fc.
US Rules in Foot-Candles, European Standards in Lux
US regulations and the IES lighting handbook historically quote foot-candles, while European and international standards use lux. Converting is often the first step when a spec from one side of the Atlantic meets a meter from the other.
OSHA construction minimums (29 CFR 1926.56)
| Area | fc | lux |
|---|---|---|
| General construction areas, excavation, access ways, loading platforms | 3 | 32 |
| General construction area lighting | 5 | 54 |
| Indoor warehouses, corridors, hallways and exitways | 5 | 54 |
| Tunnels, shafts and general underground work | 5 | 54 |
| Shops, plant rooms, locker rooms, mess halls, indoor toilets | 10 | 108 |
| First aid stations, infirmaries and offices | 30 | 323 |
EN 12464-1 indoor workplaces (maintained illuminance)
| Task or area | lux | fc |
|---|---|---|
| Corridors and circulation areas | 100 | 9.3 |
| Classrooms in primary and secondary schools | 300 | 27.9 |
| Writing, typing, reading, data processing | 500 | 46.5 |
| Technical drawing | 750 | 69.7 |
OSHA's figures are legal minimums for construction sites. EN 12464-1's are design values the lighting must never fall below over its life, so new installations are usually brighter.
Lux and Exposure Value for Photographers
An incident light meter is really an illuminance meter, so a lux reading converts to an exposure value. With the usual calibration constant of 250 at ISO 100:
Bright sun at 100,000 lux gives EV 15.3, which is the "sunny 16" rule (f/16 at 1/125 s and ISO 100 is EV 15). A 500-lux office is EV 7.6. Each stop doubles the light, so 1,000 lux is exactly one stop brighter than 500. The converter shows the EV under every result. Meters with a flat diffuser and those with a dome use slightly different constants, so expect up to a third of a stop of difference.
Why Lux Is a Poor Guide for Grow Lights
Lux and foot-candles weight light by how bright it looks to the human eye, which peaks in the green. Plants mainly use red and blue, so growers measure PPFD in µmol/m²/s instead. For sunlight, dividing lux by about 54 gives a rough PPFD. A purple LED grow light looks dim to a lux meter while delivering plenty of usable light, so the same ratio doesn't hold.
A lux or foot-candle reading is still useful for comparing two spots under the same lamp, or for checking a windowsill against the shade-plant guidance in older gardening books, which is often written in foot-candles.
Lux, Foot-Candles and Lumens
Lumens measure a lamp's total output; lux and foot-candles measure how much of it lands on a surface. If an 800-lumen bulb's light fell evenly on 4 m² of floor, that floor would get 200 lux (18.6 fc). In practice some light hits walls and ceiling and the spread is uneven, so real readings are lower and vary from spot to spot.
Illuminance from a small source also falls with the square of distance: move a meter from 1 m to 2 m away and the reading drops to about a quarter. Always note where and how the meter was held when you record a reading.
Frequently asked questions
Is 1 foot-candle equal to 10 lux?
Not quite. 1 fc is 10.764 lux. Using 10 is fine for a quick sense of scale but is 7.6% off, which matters when you are checking a reading against a minimum.
Is a foot-candle the same as a lumen per square foot?
Yes. Both name the same unit, one lumen spread over one square foot.
How many lux is direct sunlight?
Roughly 32,000 to 100,000 lux (about 3,000 to 9,300 fc), depending on the sun’s height, season and haze.
Can a phone app measure lux?
It reads the phone’s ambient light sensor, which is designed for screen brightness, not measurement. Expect errors of 10–20% or more. Use it to compare spots, not to sign off a requirement.
Sources
- NIST SP 811, Guide for the Use of the SI: foot-candle = 1.076 391 × 10¹ lux.
- OSHA 29 CFR 1926.56, Illumination: Table D-3 minimum foot-candles for construction areas.
- EN 12464-1:2021, Light and lighting: lighting of work places, Part 1: indoor work places: maintained illuminance by task.
- ISO 2720:1974, General purpose photographic exposure meters: incident-light calibration constants used for the EV conversion.
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