How much light you need in an area is partly a matter of personal taste, especially in your home. For a work environment, it's a legal requirement to maintain proper lighting levels so that everyone can see to perform their required tasks safely. The metric measure of lighting level or intensity is the lux. Total illumination is measured in lumens. Once you've worked out the lighting level you need, calculating the total illumination necessary is a fairly simple matter.
Determine the lighting intensity level you need. Most people prefer lower lighting levels in their homes except for work areas like kitchens. For an office or factory work environment, 500 lux per metre square is the recommended light level.
Find the area to be illuminated in square metres. Measure the length and width of a room and multiply the measurements together. For example, a 15 by 20 metre room has an area of 300 square metres.
Calculate the lumens needed to achieve the required lighting level. Multiply the intensity in lux per square metre by the area of the room in square metres. For example, if you want an office 300 metres square to have a lighting level of 500 lux per square metre, you need 300 square metres times 500 lux for a total of 150,000 lumens.
Determine how many light bulbs you need. For example,13 watt compact fluorescent lights produce about 825 to 900 lumens each. To produce 150,000 lumens you will need 167 of these bulbs. You can find the rating in lumens for a light bulb on the package.