Frost depth calculator
In cold places the ground can freeze several feet down. Water lines, footings and fence posts set above that depth can burst, crack or get pushed up by frost heave. This estimates how deep frost goes in an average winter and a cold one, for your climate, surface and soil.
You need two numbers: the average temperature of your coldest month and of your warmest month. They are in the climate table for your town (on Wikipedia or your weather service). In the mountains, use a station at a similar elevation: temperatures drop about 3.5 °F per 1,000 ft (6.5 °C per 1,000 m) as you go up.
Why frost depth varies so much
- Cold, and how long it lasts. What matters is the freezing index: how far below freezing it is, added up over every day of winter. A long, steady cold freezes deeper than a short, sharp one.
- Snow. A foot of fluffy snow insulates about as well as a couple of inches of foam board. Plowed driveways and shoveled paths lose that blanket and freeze deepest.
- Soil and moisture. Water gives off a lot of heat as it freezes, which slows the frost. Dry gravel and rocky fill freeze much deeper than wet clay.
- Warm ground below. Where summers are warm, the ground stores heat that pushes back against the frost from below.
About this estimate
This uses the same heat-flow physics as engineering frost-depth methods, with typical soil properties for each soil type. It is an estimate for an average and a colder-than-average winter, not a design value. Soil properties alone can shift the answer by about a quarter. Building codes set frost depths from the coldest winters in decades, so follow your local code for footings.
For more control (freezing index, density, water content and surface factors), use the frost depth tool for researchers.
References
Carslaw, H.S., Jaeger, J.C. (1959). Conduction of Heat in Solids, 2nd ed. Oxford University Press. Aldrich, H.P., Paynter, H.M. (1953). Analytical Studies of Freezing and Thawing of Soils. Arctic Construction and Frost Effects Laboratory, Technical Report 42. Kersten, M.S. (1949). Laboratory Research for the Determination of the Thermal Properties of Soils. University of Minnesota Engineering Experiment Station. US Army (1988). Arctic and Subarctic Construction: Calculation Methods for Determination of Depths of Freeze and Thaw in Soils. TM 5-852-6.