Frost line depth in Middlesex County, NJ
How deep the ground freezes in Middlesex County, NJ
19 in. Dig to 19 in, just below the frost. Nearest NOAA station: New Brunswick 3 Se, 2.4 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.