From: http://www.zo.utexas.edu/courses/THOC/land.html

(Apologies for miles instead of kilometers.)

> The total land surface area of Earth is about 57,308,738 square miles, of which about 33% is desert and about 24% is mountainous.

    In [1]: total_land_surface_area_square_miles = 57308738
       ...: total_land_surface_acres = total_land_surface_area_square_miles / 0.0015625
       ...: non_desert_non_mountain_acres = total_land_surface_acres * (1.0 - (0.33 + 0.24))
       ...: acres_per_human = non_desert_non_mountain_acres / 7000000000.0
       ...: print '%f' % acres_per_human
       ...:
    2.253052

So about two-and-a-quarter acres of "good" land per human. And...

> we have not allowed for any amenities such as highways, schools, hospitals, shopping malls, stadiums, agricultural fields, rivers, lakes, reservoirs, parks, golf courses, etc.