Area

Area[reg] gives the area of a two-dimensional region.

Examples

Area of a disk:

Area[Disk[{0, 0}, r]]
(* Pi r^2 *)

Area of a polygon:

Area[Polygon[{{0, 0}, {1, 0}, {1, 1}, {0, 1}}]]
(* 1 *)

Please visit the official Wolfram Language Reference for more details.