Illuminance Converter
Same value in every unit
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Lumens leave the bulb, lux lands on the desk
A lumen is the total light a source emits. A lux is one lumen falling on one square metre. That distinction is the whole subject: a 1,000 lumen bulb produces 1,000 lux on a surface one metre square directly beneath it, but spread the same light over ten square metres and each one gets a tenth as much.
Which is why buying bulbs by lumens tells you about the bulb, and measuring lux tells you about the room.
Reference points
- 100,000+ lux — direct summer sunlight
- 10,000–25,000 lux — full daylight, no direct sun
- 1,000 lux — an overcast day
- 500 lux — a well-lit desk for reading
- 300 lux — general office lighting
- 50 lux — a dim living room
- 0.25 lux — full moon on a clear night
Foot-candles
The same idea in customary units: one lumen per square foot. Since a square metre holds 10.76 square feet, one foot-candle is 10.76 lux. American lighting specifications are usually written in foot-candles and European ones in lux, so converting between the two is mostly a matter of reading someone else's standard.
Why the eye is a poor judge
Human vision adapts across a range of about a million to one, so a room that feels adequately lit can measure at a tenth of the recommended level. That adaptation is also why moving from bright sunlight into a shop makes it look almost dark for a few seconds.
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Frequently Asked Questions
A lumen measures how much light a source puts out in total. A lux measures how much of it lands on a surface — one lumen spread over one square metre. The same bulb gives a high lux reading close up and a low one across a room, while its lumen rating never changes.
Broadly: 100 to 200 lux for a corridor, 300 for general office work, 500 for reading and detailed tasks, 750 or more for fine work such as sewing or drawing. Those are working figures from lighting practice, not legal requirements, and the standard that applies to a workplace will be local.
One lumen per square foot, the customary unit in the United States. One foot-candle is about 10.76 lux, which follows directly from the number of square feet in a square metre.
With a light meter, or with a phone app using the ambient light sensor — good enough for comparing one part of a room against another, not for anything that has to be certified.
Direct summer sunlight is over 100,000 lux, an overcast day around 1,000, twilight about 10, and a full moon roughly 0.25. The range the eye copes with is enormous, which is why a room that feels bright may measure far less than you expect.
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