Shophouse Coruscant

SF writers who envision "world cities" seem to turn off their math brains when they do so.

Trantor

First to sin was Asimov, with Trantor, land (and maybe ocean basins too?) covered in multiple layers of steel arcology, for a population of 40 billion people.

Assuming the land area of Earth, 150 million km2, 40 billion people would be 266 people/km2. Basically one person per acre. If we ignore climate and rainfall (a big ignore), that could be subsistence agriculture.

Even if we assume (unlike Asimov) people living on only the nicest 40 million km2, that's 1000 people/km2, or suburban. Like half-acre lots for each house.

Worse, checking the web, Trantor seems to have had more land area, 194 or 200 million km2. One page also says "80 layers", which would mean like 80+ acres of floor space per person.

Coruscant

From Star Wars we have this capital world, official population varying over the years but I'll run with 2 trillion people. 50x Trantor, but likewise described as layers upon layers upon layers of city -- 5127 in the Timothy Zahn novels that first went into it, Wikipedia says. But assuming Earth-size, the density is 13,000 people/km2 -- not even Brooklyn (14,500), city of 4-story brownstones. If we assume using the best 40 million km2, we get 50,000/km2, which is like the density of central Hong Kong.

Shophouse Coruscant

But heck, we can almost do Coruscant as single-family homes! If we can do 5000 homes per km2, and 2.5 people/home, that's 12,500 people/km2, and 1.875 trillion people. Can we do 5000? Assume a lot size of 1500 sqft or 140 m2, that's over 7100 potential lots/km2. We need streets, but 70% of the land being 140 m2 lots hits 5000 lots/km2 exactly.

We do need things beyond housing, but that's where shophouses come in. 3 story buildings: ground level is a shop or office or day care or whatnot, upper stories are housing. In reality you would want more diversity of building form and concentration of uses, but as an order of magnitude of residential vs. non-residential floor space, it nearly works out. This source says that the US has about 30 m2 of "commercial" space per person, where "Commercial buildings include, but are not limited to, stores, offices, schools, places of worship, gymnasiums, libraries, museums, hospitals, clinics, warehouses, and jails."

So say that a shophouse lot is 7x20 meters, with 7x14 = 98 m2 of building footprint. Nearly 100 m2 of 'commercial' space, 200 of residential, 2.5 people, so 80 m2 of housing and 40 m2 of commercial per person. Even at 4 people per house, that's 50 and 25.

Behold, Coruscant!

But what about food?

Look, that was handwaved from the beginning. Trantor imported food from 20 or 25 other planets. Coruscant officially imports too.