In the scifi I am currently writing one of the central planets is a fully urbanized planet; that is, it is an ecumenopolis. I have my fair share of ideas on how to explain many of the logistical and ecological problems that such a setup brings, but I'm a bit stumped when it comes to estimating the population. It's definitely in the trillions, but beyond that I find it hard to estimate, the reason being that I can't find any reliable overviews of the ratio of zones in current cities. That is, the ratio of residential to commercial to agricultural to industrial development. Do we use twice as large an area for agricultural development versus residential? Three times much residential as agricultural? Half as much commercial? And so on. I'm specifically interested in the total amount of area (that is, not just the ground floor of a multi-story building but the total area of its floor), to get a good grip on just how much space we're using for various purposes. My planet would be covered with ca 1km tall buildings (on average) and, knowing how much space we use for industrial vs. commercial vs. agricultural vs. residential purposes. it should be easy to extrapolate (let's say, 30% is residential, then 300m of the the buildings would be used for residential purposes; with 10m floors that would be 30 floors, multiplied by the planet's area, divided by the average residential area one person occupies, to get a very rough estimate which is, in the very least, not off by orders of magnitude). Why not simply use, say, the population density of dense urban areas for reference? Well, areas such as Manhattan are, obviously, mostly commercial and residential. Its commercial areas takes more people than its residential area can provide (so a planet with that area would not have enough workers for its commercial areas by far), made up for by people working in the city living outside the city (to a certain degree doable on my planet). But then, what about all the industries that you need space for, and the agriculture needed to sustain all those residents? Obviously if you'd look at a dense urban area and include all the area needed to support it, the actual density would be far lower. You could stack it all vertically on Manhattan and you'd probably have a one-two kilometer high city with quite a lot more residents as well, but what's that population density look like? I can ultimately just handwave it all; there are many inhabitants living offworld, industrial and agricultural imports and such may alleviate space requirements fore those practices (or it's mostly selfsustained in which case the residential area is comparatively small) , so I can always say "it's got 15 trillion inhabitants" and then maybe a realistic number would be closer to 150 trillion or 1.5 trillion depending on how the logistics are solved ultimately, but it'd work. I've been reading about fictional ecumenopolises and serious looks at them usually gives estimates on 10-1000 trillion depending on various factors but there's also a lot of ifs and buts and unreliable factors concerning them, so, yeah, asking here in case new ideas or thoughts might provide some answers or at least better estimates.