The industry standard for general lighting in a room is 70 foot candles per square foot. To figure out how many lights you need to achieve this level of lighting, you must complete an equation for lumens. To use the formula, you need the square footage of the room, the lumens per fixture, the co-efficient of usage, and finally the light loss factor. For example, use an 18-by-20-foot room with two 40-watt light bulbs in each fixture.
Measure the length and the width of the room with a tape measure and multiply the two numbers together to get the square footage of the room. For an 18-by-20 room, you obtain a square footage of 360 square feet.
Multiply the square footage of 360 by the 70 lumens needed per square foot to get the total lumens needed for the room. In the example, you need a minimum of 25,200 total lumens of light in the room.
Multiply the total lumens needed (25,200) by 1.40 (to factor in a light loss factor of .75 and a co-efficient of utilization of .65) and then add that answer to 25,200. You need 60,480 lumens.
Multiply 3,150 by 2 to achieve the lumen rating for one fluorescent light fixture with two 40-watt bulbs (3,150 is the standard lumens for one fluorescent tube). This gives you a lumen rating of 6,300 per fixture. Divide the total lumens needed (60,480) by 6,300 to get the number of fixtures you need, in this case 9.6. Round up to get 10 fluorescent fixtures. Given a room size of 20-foot by 18-foot, position the fixtures in two rows of five fixtures.
Measure across the 20-foot span of ceiling with the tape measure and secure a fixture every two feet. This will give you five fixtures in one row. Place each end of the fixture approximately 1/2 foot away from the adjacent wall.
Go to the other end of the room and measure across the 20-foot span of ceiling with the tape measure and secure a fixture every two feet. Again, this will give you five fixtures in one row. Place each end of the fixture approximately 1/2-foot away from the adjacent wall.