One acre is a measure of area which is equal to 4840 sq yd or 1/640  square mile. It does not matter what shape it has.    Historically an acre was the amount of land … a set of oxen could  plough in 1 day.nswer by Ablex. Confidence votes 824. There are 43,560 square feet in one acre so take the length times the width in feet and divide by that figure to find the number or fraction of an acre. If the property is irregular, take length times width of each roughly rectangular chunk and add them together before dividing.