An acre is a unit of area. 640 acres equal a square mile. Hence, it is 1/640 of a square mile. Since a square mile contains 27,878,400 square feet, an acre is 27,878,400 divided by 640, which is 43,560 square feet.
Remember: an acre can be converted to SQUARE feet, not plain (linear) feet.
Square feet and
feet are not the same units. A
foot is unit of distance. A
square foot is a unit of area.