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How do you convert feet into square feet?

A single measurement in feet cannot be converted into square feet, because feet measure length and square feet measure area. You need at least two dimensions, length and width, and you multiply them. A room 12 feet long and 10 feet wide contains 120 square feet.

12 feet × 10 feet = 120 square feet

When one dimension includes inches, change the inches to decimal feet before multiplying. Six inches is 0.5 foot, so a room measuring 12 feet by 10 feet, 6 inches is 126 square feet: 12 × 10.5 = 126.

Irregular areas get divided into simple sections, with the results added. A triangular area is one-half of base × height.

Be clear about what you are measuring. Floor area, wall area, lot area, and building footprint are all expressed in square feet, but each uses different dimensions and different inclusion rules.