Convert total lumens spread over an area into average illuminance (lux).
Worked example: 8000 lm over 20 m² → 400 lux average.
This converter divides the total lumens by the area in square meters to give the average illuminance in lux. For example, 8,000 lumens spread over 20 square meters gives 400 lux on average. It is a quick way to check whether a room has about the right amount of light for the task.
The result is an average under ideal conditions. In a real room, some light is absorbed by the walls and ceiling, some is lost in the fixture, and the light is not spread evenly, so the level at the work plane is lower than the simple answer, and it is highest directly below the fixtures. Designers apply a utilization factor and a maintenance factor for this reason.
To find the lumens you need, rearrange the formula by multiplying the target lux by the area, and then divide by the utilization and maintenance factors. For a single point source, such as a spotlight, use the candela and the distance instead of the average over the area.
Handy conversions: 1,000 lumens over 10 square meters is 100 lux, 5,000 lumens over 25 square meters is 200 lux, and 20,000 lumens over 40 square meters is 500 lux. Divide the lumens by the area in square feet to get foot candles instead, and remember that the real level at the work surface will be lower than the average because of losses.