I won't get into the details here—visit the Urban One site for the full story—but I'll tease it with this chart, showing the current population of a number of U.S. cities, in addition to the population if those cities grew to the same density as Manhattan (70,000 people per square mile), and how many times more people would need to live in each city to get there:
The full post—again, find that here—has another chart showing how many years it would take for these cities to fully Manhattanize. (Hint: for most cities its at least a few centuries, longer than the entire history of our country.) It also has a map comparing population growth to physical expansion, because not all cities are growing by increasing density. Instead, places like Phoenix and Austin are mainly growing through annexation, with density relatively unchanged.
There's a lot of interesting facts to be pulled from the data, so check it out and let me know what you think! And feel free to share the charts and map as you like (with attribution, of course)!