Frost line depth in Linn County, IA
How deep the ground freezes in Linn County, IA
36 in. Dig to 37 in, just below the frost. Nearest NOAA station: Cedar Rapids #1, 2 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.