Frost line depth in Mackinac County, MI
How deep the ground freezes in Mackinac County, MI
53 in. Dig to 53 in, just below the frost. Nearest NOAA station: Engadine Mdot, 13 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.