You should have read Gershon's reply above you before you replied. When you buy a 40 of land it is 1/4 mile by 1/4 mile on a 2-d map (or aerial projection). If you walk your entire boundary it will be farther than 1 mile and the surface area of your 40 will be more than 40 acres. In deed I once looked into purchasing a piece of land. It was a 40 with a small chunk cut out where somebody built a house. So it was less than a proper 40 but it was advertised as 42 acres.