Manhattan, according to Wikipedia, contains 59.5 square kilometers of land area, or 14,700 acres. By the 100 person per acre rule, this should hold about 1.47 million people. The population of Manhattan is 1.626 million.
The City of Detroit, according to Wikipedia, contains 359.36 square kilometers of land area, or 88,800 acres. By the 100 person per acre rule, this should hold about 8.88 million people. The population of Detroit is a suburban-like 681,000 -less than a tenth of what it could be by the rule.
The City of Los Angeles, according to Wikipedia, contains 1214 square kilometers of land area, or about 300,000 acres. By the 100 person per acre rule, this should hold about 30 million people. The population of Los Angeles is 3.884 million.
The most densely inhabited incorporated place in the U.S. has only just under 92 persons per acre. A typical U.S. school building has a daytime population density of very close to 500 students per acre. The most densely populated modern capital city in the world, the City of Manila, where over 1.6 million people are packed into some 38.55 square kilometers of land area, or 9526 acres, has a population density of roughly 173 persons per acre. The Old City of Jerusalem, with 216.41 acres, has nearly thirty-seven thousand people crammed into its walls, giving a population density of nearly 171 persons per acre, or around 203 persons per acre when only the Temple Mount is excluded. The Old City, by the way, used to follow the 100 person-per-acre rule in the 19th century, then having a population of 21 or 22 thousand people. The Old City is clearly more population-dense today than it was a century ago:
According to Wikipedia, the 20th century population of the Old City of Constantinople (Fatih) peaked at 627012 persons in 1975. As its area (eastern citadel included) is about 3810 acres, that city’s peak population density was about 165 persons per acre. It is important to realize, however, that Fatih is surrounded by water on three sides, leaving little room for extensive light-density urban development. The situation is not comparable with that of such ancient cities as Rome and Cordoba, for which premodern populations and population densities are often wildly exaggerated.
Thus, we should consider the premodern urban maximum population density to be somewhere a little above 200 persons per acre. Premodern urban population densities were likely closer to this maximum when cities’ suburbs were already overcrowded and when cities were surrounded by geographic barriers to expansion, such as bodies of water, mountains, and valleys.
On the subject of the previous post, the maximum population of Rome was likely around 400,000 or half a million (the number of slaves in the city is the largest unknown variable relating to this figure) either sometime in the reign of Nero (before the Great Fire of Rome) or during the middle of the period of the Five Good Emperors. The first city in the world to reach one million inhabitants was not Rome or Constantinople, but probably Tang Dynasty Chang’an, or, less likely, Sui Dynasty Daxing (same location as Chang’an, but slightly earlier). Hypothetically, the city could hold nearly three million by the 100 person-per-acre rule.