Frost line depth in Gallatin County, MT
How deep the ground freezes in Gallatin County, MT
40 in. Dig to 40 in, just below the frost. Nearest NOAA station: Bozeman 6 W Exp Farm, 10.7 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.