21 square meters equals 2100 square decimeters.
Square meter: a metric unit of area, defined as the area of a square with a side length of 1 meter. In life, square meters are usually referred to as square meters or square meters for short.
Square decimeter: is the metric unit of area, which is defined as the area of a square with a side length of 1 decimeter.
An area unit is a unit that measures the size of an object's surface. The order from small to large is: square millimeters, square centimeters, square decimeters, square meters, hectares, square kilometers. In the International System of Units, the standard unit area is square meters, and the area is one meter long square area.