From the Wikipedia article on
Ecological footprint:
In 2003, the average biologically productive area per person worldwide was approximately 1.8 global hectares (gha) per capita. The US footprint per capita was 9.6 gha, and that of Switzerland was 5.1 gha per person, whilst China's was 1.6 gha per person.<2> <3> The WWF claims that the human footprint has exceeded the biocapacity (the available supply of natural resources) of the planet by 25%.<4> Wackernagel and Rees originally estimated that the available biological capacity for the 6 billion people on Earth at that time was about 1.3 hectares per person, which is smaller than the 1.8 global hectares because it did not include bioproductive marin areas.<5>.
There is some dispute about the concept of the ecological footprint, and over the question of carrying capacity in general. the concepts are quite open to redefinition, and so are in some sense as much political as ecological statements.
I, for instance, think that our use of the one-time gift of fossil fuels has enabled humanity to overshoot the true carrying capacity of the planet by about three to four hundred percent. In other words, in the absence of fossil fuels the carrying capacity of the planet would be 1.5 to 2 billion people with an average lifestyle similar to Portugal. The story is worse than that, though, because the overshoot enabled by the use of fossil fuels has allowed us to degrade the Earth's underlying carrying capacity without being adversely affected by it (e.g. we have killed the oceans and destroyed the topsoil, but the use of synthetic fertilizers has sustained food production so we haven't noticed the damage). Given that, the carrying capacity of the earth once the oil/gas/coal goes away in another century or so may be on the order of one billion people as opposed to the 6.6 billion there are now.
But hey, that's just one man's opinion...