It's nice to see that they're taking non-human life into account according to their FAQ; most calculators of this type don't (or at least, they didn't last time I looked), which leads to rather meaningless estimates. I'm still a bit wary about their assumption that all of the factors can be expressed in terms of common units, however.

It's also a shame that they don't take population density into account (as far as I know--the FAQ doesn't mention it directly); the U.S. is at 31 people per square km compared to the world average of 48. Overpopulation is one of the fastest routes to unsustainability and so is the key factor if we're going to try to project these numbers into the future.

But all in all, a solid effort based on credible source material. It puts me at 18 acres.

Don't forget political action on the list of things we can/should do. Things like raising fuel efficiency standards and supporting renewable energy will have effects that go far beyond individual actions. Likewise, most of the complaints about GM crops go away if we can get government support for them and this could drastically increase the amount of land available (say, by creating crops that grow in the desert) and the value of the land (say, by creating crops that can survive in higher density conditions).

Political action is definitely essential. This website mentioned how many of the factors are outside of our direct control - thinks like local, state and national policies. But we can still influence them. One very effective way is to educate and influence your local city officials to begin implementing more "green" requirements in local building codes and the like. This is just starting to happen here in our area (a woman from our church was appointed to head an environmental advisory committee for our city council) and I'm excited to see what comes of it.