Frost line depth in Ogle County, IL
How deep the ground freezes in Ogle County, IL
36 in. Dig to 36 in, just below the frost. Nearest NOAA station: Polo 5nw, 14.7 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.