How to Calculate Light Fixtures

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A room with lighting fixtures that are too bright or too dim can be uncomfortable as well as making work and other activities harder. Lighting levels are partly a matter of personal preference, especially in your home. In work areas, proper lighting helps improve efficiency and may be essential to safety. Lighting intensity is measured in foot-candles per square foot. The overall amount of illumination that lighting fixtures must emit to provide a given lighting intensity is measured in lumens.

Things You'll Need

  • Tape measure
  • Light fixture specifications
  • Measure the area of the room you wish to illuminate. Use a tape measure to find the length and width in feet. Multiply the length by the width to find the square footage of the room.

  • Determine the lighting level you need. The Illumination Engineering Society of North America Handbook recommends lighting levels for office spaces of 30 to 50 foot-candles per square foot. For the home, most people prefer 10 to 20 foot-candles per square foot except in the kitchen, bathroom and a few other areas.

  • Multiply the maximum light intensity you need by the number of square feet to find the total lumens required. For example, if you want 20 foot-candles per square foot in a 250-square-foot living room, you need 5,000 lumens.

  • Look on the packages of lighting fixtures and bulbs to find the rating in lumens. Divide the number of lumens a bulb will produce into the total lumens needed to calculate the number of light fixtures required. For example, suppose you are considering light fixtures with 13-watt CFL (compact fluorescent light) bulbs. These bulbs will produce about 825 to 900 lumens each. For total illumination of 5,000 lumens, you have 5,000 divided by 900, which equals 5.55 light fixtures. Round that up to six fixtures.

Tips & Warnings

  • Occasionally you may run into a specification for lighting stated in Lux. The Lux is simply the metric system equivalent of the foot-candle. One Lux is the same light intensity as one foot-candle per square foot and is equal to the intensity for one square meter. Put another way, one Lux equals 10.76 foot-candles.

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References

  • Photo Credit lamp image by Irena Hnidkova from Fotolia.com
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