How to Calculate Floor Area Ratio

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Floor area ratio is a measure of the total area a building occupies. Different cities have different laws regarding the maximum floor area ratio a building can occupy. It is important to look up the square footage rules for your city and calculate the ratio for the building before finalizing your construction plans.

Calculate the area of the overall site. The area of the parcel equals the length of the site multiplied by the width of the site. For example, suppose that a company is planning to build a three-story building on a construction site that is 10,000 square feet, or 100 feet by 100 feet.

Calculate the area of each story of the building. In the example, if two stories were 80 feet by 80 feet and one story was 60 feet by 70 feet, the area would be 6,400 square feet for each of the first two stories and 4,200 square feet for the third story.

Add the total square footage of the building's stories. In the example, 6,400 square feet plus 6,400 square feet plus 4,200 square feet equals 17,000 square feet.

Divide the total square footage of the building's stories by the square footage of the site to calculate floor area ratio. In the example, 17,000 square feet divided by 10,000 square feet equals a ratio of 1.7.