Gross floor area is the total amount of floor space in a building, usually one used for business. It is important to know the gross floor area for many reasons, such as acquiring building permits, determining rent and advertising with real estate companies. You can calculate gross floor area by performing some basic measurements and mathematical operations.

Measure the length and width of the inside of the building's walls.

Multiply the length and width measurements in order to find the square footage.

Multiply the square footage times the number of floors in the building.

Subtract the square footage of any elevator shafts, lobbies (other than on the first floor), or rooms that house only equipment used for the building's operation. The result is the gross floor area.

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Check with local ordinances before determining the gross floor area of a certain building. Some local governments do include items such as stairwells or atrium, while others do not.