How to calculate lumens per square foot

Updated June 18, 2018

Lumens per square foot, also known as a footcandle, is the measurement used to determine how much light an area is receiving. To calculate it, you need to calculate the area and the total amount of light. It is commonly used in homes to determine how much light is needed in different areas. For example, while 7.5 lumens per square foot might be sufficient for a hallway, you would probably want 35 lumens per square foot for a kitchen.

Determine the square footage of the area that you are lighting. For rectangular areas, multiply the length times the width. 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.

Determine the total lighting power in the area by adding the number of lumens for each bulb. The number of lumens will be listed in the manufacturer's information for the light bulb.

Divide the number of lumens generated, found in Step 2, by the square footage of the area lit, found in Step 1, to calculate lumens per square foot. For example, if you had a 135 square foot bedroom lit by a total of 2,025 lumens, you would have 15 lumens per square foot.


Do not use watts when calculating the light output. Watts represents how much power is being used, not how much light is being generated. For example, according to the Department of Energy, it takes 100 watts for an incandescent bulb to generate about 1,600 lumens, but it only takes between 23 and 30 watts for a compact fluorescent bulb to generate the same amount of light.

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About the Author

Mark Kennan is a writer based in the Kansas City area, specializing in personal finance and business topics. He has been writing since 2009 and has been published by "Quicken," "TurboTax," and "The Motley Fool."