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), also hectare is a non-SI unit but accepted in SI since commonly used in everyday life. Common imperial and U.S. customary area units are square feet and square yard. Acre is mainly used in U.S. and a few other countries.
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.