Determining lighting levels for a room is partly a matter of comfort and partly one of efficiency. Most people prefer considerably lower light levels at home in a living room or bedroom than required in an office environment. Of course you usually want brighter lighting in work areas like around a kitchen counter. Once you decide how bright you want the lighting, calculating lumens for a room is a straightforward task.
Measure the length and width of the room with a tape measure, and multiply the two to determine the area of the room in square metres (or square feet). For example, a 3 by 4 m room is 12 m squared or 10 by 15 foot room has 150 square feet.
Examine the area to determine the lighting level you need. For instance, the Illuminating Engineering Society of North America suggests 320 to 540 lux (30 to 50 foot candles) for offices. (See the IESNA Handbook in the Resources below).
Multiply the area of the room by the light level desired in lux (or foot candles -- fc) to calculate lumens needed for a room. A 12 m square office with a moderate lighting level of 400 lux would require 4800 lumens (or a 150 square foot office with a lighting level of 40 fc would require 6000 lumens).
Refer to the lumen rating on light bulb/lamp packages to decide the size and number of light tubes or bulbs needed.
Lux is the metric equivalent of a foot candle, and one foot candle is equal to 10.76 lux (the number of square feet in a square meter). To convert foot candles to lux, multiply foot candles by 10.76, or to convert lux to foot candles, divide lux by 10.76.