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 Monestier-de-Clermont is 5,000,000.0 square meters.
The area of Villaviciosa de Odón is 68.17 square kilometers.
The area of Vallfogona de Balaguer is 27 square kilometers.
The area of Puygaillard-de-Lomagne is 7,080,000.0 square meters.
The area of Torremocha de Jiloca is 33 square kilometers.
Es "CaF2".
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 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 De Wilp is 9,170,000.0 square meters.
The area of De Lier is 8,920,000.0 square meters.
The total area of De Biesbosch National Park in the Netherlands is around 91 square kilometers.
The area of Rubielos de Mora is 63 square kilometers.
The area of Ribera de Arriba is 29.64 square kilometers.