San Francisco is a city in, and the cultural, commercial, and financial center of, Northern California. San Francisco is the 13th-most populous city in the United States, and the fourth-most populous in California, with 883,305 residents as of 2018.It covers an area of about 46.89 square miles (121.4 km2), mostly at the north end of the San Francisco Peninsula in the San Francisco Bay Area, making it the second-most densely populated large US city, and the fifth-most densely populated U.S. county, behind only four of the five New York City boroughs.(Wikipedia)