@LizCallis,
My method of remembering this is from high school history--the homestead act stated that a township is 16 square miles and that a square mile is 640 acrea--it went on to state that so much of each township is commons (schools, township house, mail, etc.), but for some reason 640 acres in a square mile always stuck in my head. Now a mile is 5280 feet, so a square mile is (5280^2) ft^2 and this is 640 acrea--so one acre is (5280^2/640) ft^2.
Rap