Many people approximate the answer to 100 calories per mile. However this is likely inaccurate for the majority of us.
True calorie burn will depend on a number of factors such as how much you weigh, how fast you walk and the type of terrain you cover so there is no one-size-fits-all answer. Heavier people, walking faster over hilly ground will burn more calories than lighter people, walking slower over flat ground.
Aside from burning calories walking has a host of other health benefits making it an important daily habit and perfect supplement to intermittent bouts of high intensity exercise.