An acre is a unit of land area, and a mile is a unit of length. To determine how many acres are in a square mile, you can multiply the length of a mile by itself (since a square mile is a mile long and a mile wide). There are 43,560 square feet in an acre, and 5,280 linear feet in a mile. Therefore, a square mile contains 6,400 acres. This SEO description is optimized for search engines and includes the key phrase “how many acres in a mile.