An acre is equal to 43,560 square feet or 4,840 square yards. A square mile contains 640 acres. The word acre comes from the Old English term "aecer" or "aker," which meant "open field."
Historically, an acre was measured as the area a pair of oxen, known as a yoke, could plough in one day. Typically, acres of this period were long and narrow rectangles, as it was easier to plow in long straight lines. Turning a plow was not an easy task. An acre was a furlong in length by a chain, or four rods, in width. A furlong was the length that a pair of oxen could pull a plow before resting.