A square foot is the area of a square whose sides are each one foot long. It is the usual unit for floor space, room sizes and lot sizes in the United States. Written ft², sq ft or sqft, it is exactly 144 square inches (12 × 12) and 1/9 of a square yard.
Formula
area (ft²) = length (ft) × width (ft)
square meters = square feet × 0.09290304
square feet = square meters × 10.7639
NIST lists the factor 0.09290304 m² per square foot as exact, which follows from the foot being exactly 0.3048 m (0.3048² = 0.09290304).
Example
A room 12 ft by 15 ft has 12 × 15 = 180 ft², or 180 × 0.09290304 = 16.72 m². A 43,560 ft² area is one acre.
Common mistakes
Square feet are not "feet squared in length": a room 10 ft long is not 10 ft². Also, 1 ft² is 144 in², not 12 in².
Use the area calculator for other shapes, square feet to square meters to convert, and square feet to acres for land.