[Bartweiss]

Precisely, thanks. I can get an upper bound on Earth's carrying capacity with (volume of earth / volume of a human), but it's going to be way harder to calculate the number that will actually matter.

[dredmorbius]

Joel E. Cohen has done some of the more substantive, and range-based, estimates that I'm aware.

http://science.sciencemag.org/content/269/5222/341.long

(Numerous other articles and books.)