- Living area for a single person: 100 m^2 (this would make it 400 m^2 for a 4-people family)
- Number of people you can fit in a km^2: 10,000 (because 1 km^2 = 1 million m^2)
- Land area of the Earth: 148 million km^2 (see Wikipedia)
- Say only a fifth of that area is available for people, that would be 30 million km^2. Multiplied by 10,000 this gives us 300 billion people.
300 billion people. With no many-stories buildings, no overcrowding a la Asimov's fiction (how on Earth could he estimate only 40 billion people for Trantor I cannot imagine) and no food problems ('cause we have four fifths of the land area for that - in fact, even if we remove 30% of the total area as desert it still leaves us with 70 million km^2 for animals and crops).
One other thing - what the overcrowding gang never mention is this: on average, people will necessarily produce more than they consume (because they need to produce for their own consumption, for the new population and to improve the living standard). More people means more resources, not less - well, unless governments interfere with the process, of course.
Finally - these calculations are useless for persuading anyone. If it's not population, it's food. (Malthus said that people grow exponentially while food grows arithmetically, so we're going to run out of food sooner or later. That he was an idiot is not necessarily a surprise; that there are millions of other idiots who repeat that without realizing that food is also biological, and therefore should also increase exponentially, is the really weird part.) If it's not those, it's water (I'd try calculating the amount of water in a single glacier but it would also be pointless) or energy or who knows what else.
So. I guess the conclusion is: we're all gonna die. Yay :P