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The various methods for calculating the lighting illumination needs for a room range from simple to complex. For general purposes and basic lighting needs, the two most commonly used are the wattage and lumens formulas. Watts per square foot measures how much light you need to properly illuminate a room. Lumens per square foot, known as foot-candle or lux, is the measurement for determining how much light an area is receiving.
Calculate by Wattage
Measure the length and width of the room. Round up the numbers to the next whole integer.
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Multiply the length times the width for rectangular areas to get the square footage of the room. For example, a 12-by-16-foot room has 192 square feet. For circular areas, divide the diameter by two to find the radius, then square the radius and multiply it by pi, which is about 3.14. For example, with a room with a diameter of 14 feet, divide by two to get seven, square that to get 49 and multiply that by 3.14 to reach 154 square feet.
Multiply your square footage number by 1.5 for general lighting needs, such as for watching television and listening to music. In our example above, multiply 192 square feet by 1.5 and get 288. This number is the minimum wattage you need to properly illuminate the room. For our circular room, multiply 154 times 1.5 to get 231.
Increase the lighting level of a work room by multiplying the square footage by 2.5 to provide more wattage for activities such as reading or craft-making. Using our above examples, 192 square feet multiplied by 2.5 yields 480, or 154 multiplied by 2.5 equals 385. Light disperses throughout the room, so you might want to use an overhead light or table lamp for the task area to increase the available light even more.
Calculate by Lumens
Measure the area that you are lighting with a measuring tape and use the formulas above.
Add the lumens of each bulb in the room. The bulb lumens can be found on the packaging or on the bulb manufacturer's website.
Divide the number of total lumens from the light bulbs by your square footage found in Step 1. This number is the lumens per square foot for the area. If you had a 192-square-foot bathroom lit by a total of 2,400 lumens, 2,400 divided by 192 equals 12.5. You would have 12.5 lumens per square foot. This number is too low, because bathrooms need 75 lumens per square foot. If you multiply 192 by 75, you'll see that you need 14,400 lumens to reach the recommended total. A 100-watt incandescent bulb has 1,800 lumens, so you'll need eight 100-watt bulbs to properly light the bathroom (14400 divided by 1800 equals 8).