San Francisco is the most dense of large cities (population > 200,000) in California, and is second to New York nationally. The population numbers given most reflect the city of San Francisco in the 1990s. According to the Office of Financial Management, the most common unit of population density is expressed as number of people per square mile of land area, with the land area measurements excluding any bodies of water located within. Population density is used to determine how crowded a piece of land (or a city) is which is helpful when the goverment evaluates what services are needed in a given area.
With the given numbers, the calculation to achieve population density expressed in people per acre looks like this:
750,000 people / 49 sq. mi. = 15,306 people/ sq. mi.
15,306 people / sq. mi. X 1 sq. mi. / 640 Ac = 23.92 people / acre
Therfore the population density of San Francisco at this given time is 23.92 persons per acre