A square is a rectangle with all sides equal. A square with side a has perimeter 4 a and area a2.

The square is used as the unit of area; that is, a figure's area is expressed as the number of equal squares of some standard, such as square inches or square meters, that the figure can contain. In Greek geometry, the area of a figure was determined by converting the figure to a square of the same area. This is easily accomplished for triangles, rectangles, and other polygons, but is often impossible for other figures, such as circles.