The frost line depth varies by geographical location, but frost lines in the contiguous United States range from 6 inches to 6 feet. Local government building officials can provide the frost line depth in a specific location.
To find a frost line depth, the temperature of the ground is taken until a depth is reached in which 32 degrees Fahrenheit registers on the thermometer. Climatic conditions, such as air temperature, snow cover, soil type and moisture, affect the depth of a frost line in a particular area. The frost line in one area can vary over many decades, but an average of those frost lines helps scientists determine the general frost line depth.