A section of land contains 640 acres. A section of land is 1 square mile or 27,878,400 square feet. An acre is 43,560 square feet. By division, a section contains 640 acres.
One reason for surveying land into sections of 640 acres is the ease of breaking the land into halves and quarters using whole acre measurements. A quarter section of land, or 160 acres, could be broken down into quarters, or a quarter-quarter section of 40 acres, relatively easily without a great grasp of mathematics. In 1832, a quarter-quarter section of land was the smallest unit of land that could be acquired.