As you design a room or redecorate it, finding the right amount of light to balance the decor and the natural light of the room is important. A complex series of mathematical formulae exist for calculating the light required for a specific point in the room. However, for general purposes, and for satisfying most homeowners' basic needs, a simple formula does the trick to help you find out how much light you need for a room.
Measure the dimensions of the room, including openings and nooks or areas that will be hidden or shadowed by overhead lighting. Generally, you need the length and width in metres of the room you wish to light.
Multiply the width times length, in metres, rounded up to the nearest whole or half number. Then take this total and multiply it by 1.5. This gives you the number of watts that it will take to light the room appropriately for general use. For example, a room that is 10 by 11.5 metres (33 by 38 feet) becomes a formula of 10 x 11.5 = 115. Then, 115 x 1.5 = 172.5 total watts to light the room.
Disperse the total watts needed over several light fixtures such as an overhead fan-light holding four 40 watt bulbs (to total 160 watts) and a table lamp with a 40-watt bulb. This actually gives you a total of 200 watts, more than your space technically requires for adequate lighting.
Increase the lighting of a work area or a desk nook by multiplying the room's total area by 2.5 instead of 1.5. This gives you more wattage, and you can concentrate some of that over the specific area you want to light more strongly. Because light disperses throughout the room, use a lamp or overhead light for that work area (such as a kitchen island) to increase the available light there.
This formula works for any room, though often bedrooms or living rooms don't need as much light as this formula provides.
If you are trying to light a particular section along a wall, use special features like track lighting and use a formula for calculating light level at a particular point.