Area conversion calculator for metric and imperial area units and includes several commonly used old measurement systems units.
In metric system (SI), the area unit (derived unit not a base unit) is square metre ("meter" in american spelling). Common imperial and U.S. customary area units are square feet and square yard.
Area is amount of surface within two dimensional quantity (square, rectange, triangle etc). It is calculated by different formulas depending on the shape, as can be found here: area calculator.