With 1.393 billion people living within its borders (as of July 1, 2014), China is the most populous country on the planet. On average, China's population density is 148 people per square kilometer.
However, much of China's population is unevenly distributed, with the highest density along the eastern coast of the country. Compared to the United States' mere 318.9 million people, China's population is over four times larger and equivalent to 19 percent of the total world population.
If the entire population of the United States were contained within China proper, it might look something like this (click image to enlarge):
[Image via vividmaps.com]