The US Census Bureau's online County Look-up Tool provides the unique 3-digit code for the Identification of Counties and Equivalent Entities of the United States, its Possessions, and Insular Areas.

Codes for the identification of the States, the District of Columbia, Puerto Rico, and the Insular Areas of the United States. Contains the 2-digit state FIPS code, postal abbreviation, and 8-digit Geographic Names Information Systems Identifier (GNISID)