640 acres is the size of a chunk of land. A square mile of land is equal to 27,878,400 square feet. 43,560 square feet equals one acre. A section is divided into 640 acres.
One argument for surveying land into 640-acre pieces is the simplicity with which halves and quarters can be divided using whole-acre measurements. Without a strong grasp of arithmetic, a quarter section of land, or 160 acres, could be divided into quarters, or a quarter-quarter section of 40 acres. In 1832, the lowest unit of land that could be purchased was a quarter-quarter piece.
