How deep is the frost line where I live?
Type your county and get how far down the ground freezes in a normal winter, in inches, worked out from NOAA weather records for 3,144 US counties. Then the depth to dig for a footing, post or water line.
You get the frost depth in inches, and a dig depth with the 12 in code minimum already applied.
- 5 in Seattle King County, WA
- 16 in Manhattan New York County, NY
- 22 in Denver Denver County, CO
- 28 in Chicago Cook County, IL
- 34 in Milwaukee Milwaukee County, WI
- 42 in Burlington, VT Chittenden County, VT
- 48 in Minneapolis Hennepin County, MN
- 64 in Fargo Cass County, ND
This is the physics. Your building department has the legal number.
Each county figure here is an estimate of how deep frost actually reaches, worked out from NOAA's 1991–2020 daily temperature normals with the Modified Berggren equation, for a sandy silt under snow. It is not copied from a code table, because no national table of adopted frost depths exists.
The depth an inspector holds you to is the one your local building department writes into IRC Table R301.2(1). Footings go below it (R403.1.4.1) and never shallower than 12 in (R403.1.4). If their number is deeper than ours, theirs wins. A two-minute call before you rent the auger is cheaper than a footing that heaves.