There are different formulae for different shapes. And since you have not bothered to share any information about the shape, I cannot provide a more useful answer.
The area of Tapia de Casariego is 65.99 square kilometers.
The area of Torremocha de Jiloca is 33 square kilometers.
The area of Puygaillard-de-Lomagne is 7,080,000.0 square meters.
The area of Bejucal de Ocampo is 82 square kilometers.
The area of Tezontepec de Aldama is 133.6 square kilometers.
The area of Bárcena de Pie de Concha is 30.53 square kilometers.
The area of Hermandad de Campoo de Suso is 222.65 square kilometers.
The area of De Bilt is 67.1 square kilometers.
The area of De Lier is 8,920,000.0 square meters.
The area of De Wilp is 9,170,000.0 square meters.
The area of De Wolden is 226.35 square kilometers.
The area of De Klinge is 2,720,000.0 square meters.
The area of De Marne is 240.31 square kilometers.
The area of Ribera de Arriba is 21.98 square kilometers.
The area of Rubielos de Mora is 63 square kilometers.
Rio de Janeiro's area is 1,260km2
The area of Villa de Tututepec de Melchor Ocampo is 1,249 square kilometers.