Frost line depth in Gladwin County, MI
How deep the ground freezes in Gladwin County, MI
40 in. Dig to 40 in, just below the frost. Nearest NOAA station: Gladwin, 5.1 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.