It was originally part of the 415 area code, one of the first three area codes created in California in 1947, which originally covered all of central California. In 1991 the 510 area code was created to encompass most of the Oakland Metropolitan Statistical Area (MSA). The 925 area code was created March 14, 1998 when the 510 area code was itself split along the natural border of the East Bay Hills. The number was formed by taking 415 and adding 510 which equals 925. It currently includes most of Contra Costa County and the eastern portion of Alameda County and is wholly contained within the Oakland MSA.
The area code includes all of Contra Costa County except the western part (El Cerrito to Crockett), and southwestern Alameda County (Dublin, Pleasanton, Livermore, Sunol, and unincorporated areas surrounding those communities).
