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How to calculate the daylight factor

Updated February 21, 2017

The daylight factor is used to measure interior light. Specifically, it is a ratio of how much light strikes a given spot in an interior, as compared to how much light would strike an exterior spot at the same moment under identical conditions. The daylight factor is useful in figuring lighting requirements for interior use, especially if you seek to maximise natural light for economical or environmental reasons. There is a basic formula for calculating the daylight factor of a room without a light meter.

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  1. Measure the length and width of your chosen room.

  2. Multiply the dimensions together. This yields the area of the room. For instance, if the room is 17 feet by 19 feet, the area is 323 square feet.

  3. Measure the area of the windows. For example if you have three windows in the room that each are 3 feet by 5 feet, the total is 45 square feet.

  4. Divide the window area by the room area. Here, 45 divided by 323 is 0.139.

  5. Express your result in Step 4 as a percentage. In this calculation, 0.139 expressed as a percentage is 13.9 per cent.

  6. Multiply the percentage figure by 0.1 to get the daylight factor. Here, 13.9 times 0.1 is 1.39.

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Things You'll Need

  • Measuring tape
  • Calculator

About the Author

Bill Brown has been a freelance writer for more than 14 years. Focusing on trade journals covering construction and home topics, his work appears in online and print publications. Brown holds a Master of Arts in liberal arts from St. John's University and is currently based in Houston.

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