Frost line depth in Charles County, MD
How deep the ground freezes in Charles County, MD
13 in. Dig to 13 in, just below the frost. Nearest NOAA station: Quantico Mcas, 15.5 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.