San Francisco, officially City and County of San Francisco and colloquially known by its initialism SF, 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 U.S. city, and the fifth-most densely populated U.S. county, behind only four of the five New York City boroughs. San Francisco is the 12th-largest metropolitan statistical area in the United States, with 4,729,484 people in 2018. With San Jose, it forms the fifth most populous combined statistical area in the United States, the San Jose–San Francisco–Oakland, CA Combined Statistical Area (9.67 million residents in 2018).