There are a variety of coordinate systems in use in the State of California. One of the most common is the State Plane Coordinate System. As implemented using the North American Datum of 1983, this system has 6 zones in California, with counties assigned as follows:
ZONE I
Del Norte, Humboldt, Lassen, Modoc, Plumas, Shasta, Siskiyou, Tehama, and Trinity Counties
ZONE II
Alpine, Amador, Butte, Colusa, El Dorado, Glenn, Lake, Mendocino, Napa, Nevada, Placer, Sacramento, Sierra, Solano, Sonoma, Sutter, Yolo, and Yuba Counties
ZONE III
Alameda, Calaveras, Contra Costa, Madera, Marin, Mariposa, Merced, Mono, San Francisco, San Joaquin, San Mateo, Santa Clara, Santa Cruz, Stanislaus, and Tuolumne Counties
ZONE IV
Fresno, Inyo, Kings, Monterey, San Benito, and Tulare Counties
ZONE V
Kern, Los Angeles, San Bernardino, San Luis Obispo, Santa Barbara, and Ventura Counties
ZONE VI
Imperial, Orange, Riverside, and San Diego Counties
This system (NAD 83/SPCS) is designated EPSG:102641 through 102646 in QGIS. The measurement units are U.S. survey feet (3,937′ = 1,200 m). Note that there is an earlier implementation of the California SPCS based on the North American Datum of 1927, in which Los Angeles County gets its own zone, zone VII.