Frost line depth in Sheridan County, MT
How deep the ground freezes in Sheridan County, MT
66 in. Dig to 67 in, just below the frost. Nearest NOAA station: Plentywood, 5 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.