The rod or perch or pole is a surveyor’s tool and unit of length exactly equal to ​5 1⁄2 yards, 16​1⁄2 feet, ​1⁄320 of a statute mile or one-fourth of a surveyor's chain. The rod is useful as a unit of length because whole number multiples of it can form one acre of square measure. The 'perfect acre' is a rectangular area of 43,560 square feet, bounded by sides of length 660 feet and 66 feet or, equivalently, 40 rods and 4 rods. An acre is therefore 160 square rods.