Claim: San Francisco is densely populated with 18,581 people per square mile.

San Francisco -LRB- initials SF -RRB- -LRB- -LSB- sæn_frənˈsɪskoʊ -RSB- , Spanish for Saint Francis ; Spanish : -LSB- san franˈsisko -RSB- -RRB- , officially the City and County of San Francisco , is the cultural , commercial , and financial center of Northern California . Located at the north end of the San Francisco Peninsula , San Francisco is about 47.9 sqmi in area , making it the smallest county -- and the only consolidated city-county -- within the state of California . With a density of about 18,581 people per square mile -LRB- 7,174 people per km2 -RRB- , San Francisco is the most densely settled large city -LRB- population greater than 200,000 -RRB- in California and the second-most densely populated major city in the United States after New York City . San Francisco is the fourth-most populous city in California , after Los Angeles , San Diego , and San Jose , and the 13th-most populous city in the United States -- with a census-estimated 2016 population of 870,887 . The city and its surrounding areas are known as the San Francisco Bay Area , and are a part of the larger OMB-designated San Jose-San Francisco-Oakland combined statistical area , the fifth most populous in the nation with an estimated population of 8.7 million .   San Francisco was founded on June 29 , 1776 , when colonists from Spain established Presidio of San Francisco at the Golden Gate and Mission San Francisco de Asís a few miles away , all named for St. Francis of Assisi . The California Gold Rush of 1849 brought rapid growth , making it the largest city on the West Coast at the time . San Francisco became a consolidated city-county in 1856 .  After three-quarters of the city was destroyed by the 1906 earthquake and fire , San Francisco was quickly rebuilt , hosting the Panama-Pacific International Exposition nine years later . In World War II , San Francisco was a major port of embarkation for service members shipping out to the Pacific Theater . It then became the birthplace of the United Nations in 1945 . After the war , the confluence of returning servicemen , massive immigration , liberalizing attitudes , along with the rise of the `` hippie '' counterculture , the Sexual Revolution , the Peace Movement growing from opposition to United States involvement in the Vietnam War , and other factors led to the Summer of Love and the gay rights movement , cementing San Francisco as a center of liberal activism in the United States . Politically , the city votes strongly along liberal Democratic Party lines .   A popular tourist destination , San Francisco is known for its cool summers , fog , steep rolling hills , eclectic mix of architecture , and landmarks , including the Golden Gate Bridge , cable cars , the former Alcatraz Federal Penitentiary , Fisherman 's Wharf , and its Chinatown district . San Francisco is also the headquarters of five major banking institutions and various other companies such as Levi Strauss & Co. , Gap Inc. , Salesforce.com , Dropbox , Reddit , Square , Inc. , Dolby , Airbnb , Weebly , Pacific Gas and Electric Company , Yelp , Pinterest , Twitter , Uber , Lyft , Mozilla , Wikimedia Foundation , Craigslist and Weather Underground . It has several nicknames , including `` The City by the Bay '' , `` Fog City '' , `` San Fran '' , and `` Frisco '' , as well as older ones like `` The City that Knows How '' , `` Baghdad by the Bay '' , `` The Paris of the West '' , or simply `` The City '' . , San Francisco is ranked high on world liveability rankings .