An acre is equal to 0.0015625 square miles. This means that 640 acres make up one square mile. In other words, one acre is exactly 1/640th of a square mile.

To visualize, if an acre was shaped as a square, each side would be about 208.7 feet, which is less than 0.04 miles per side. Alternatively, an acre can be represented as a strip of land one mile long and about 8.25 feet wide.