The frost line—also known as frost depth or freezing depth—is most commonly the depth to which the groundwater in soil is expected to freeze. The frost depth depends on the climatic conditions of an area, the heat transfer properties of the soil and adjacent materials, and on nearby heat sources. For example, snow cover and asphalt insulate the ground and homes can heat the ground (see also heat island).

Alternatively, in Arctic and Antarctic locations the freezing depth is so deep that it becomes year-round permafrost, and the term "thaw depth" is used instead. Finally, in tropical regions, frost line may refer to the vertical geographic elevation below which frost does not occur.

Frost front refers to the varying position of the frost line during seasonal periods of freezing and thawing.

Building codes

Building codes sometimes take frost depth into account because of frost heaving which can damage buildings by moving its foundation. Foundations are normally built below the frost depth for this reason. Water pipes are normally buried below the frost line, or insulated to prevent them from freezing.

There are many ways to predict frost depth including factors which relate air temperature to soil temperature.