Frost line depth in Sanilac County, MI
How deep the ground freezes in Sanilac County, MI
38 in. Dig to 39 in, just below the frost. Nearest NOAA station: Sandusky, 9.1 mi.
Your building department sets the legal frost line depth (IRC Table R301.2(1)); footings must go below it (R403.1.4.1). Call them before you dig.