The depth of footings is determined by the depth of the frost line. The top of the footing must be poured below the frost line to maintain stability.
The depth of the frost line can vary by location. In northern states, the frost line may be as deep as 60 inches or more. Southern locations may have a frost line of less than 20 inches. The poured depth of the footing should be equal to the thickness of the wall that it supports. The width of the footing must be at least twice the width of the wall that it supports.