US Population

United States of America is the third most populous country in the world trailing only China and India with current population of 321.8 million persons or 4% of the total world's population. Moreover, US is one of only two developed economies (the second is Japan) being listed in top-10 countries by population. Large population and correspondingly large workforce are not the least factors behind the leading positions of United States in the world economy.
United States, as the major part of developed world, refer to the low-fertility countries where each woman gives, on average, birth to less than 2 children. So, starting from 2010 total fertility rate in US is below 1.9. Though such countries generally have low population growth rates, in 2010-2015 they still accounted for 46 percent of the world's population.
Concerning future population projections, United States alongside other 8 countries is expected to account for more than a half of the world population increase in 2015-2050. Population division of the UN expects the US population to exceed 400 million people in 2058. However, US is going to lose some positions in the list of the world's most populated countries. Thus, population of Nigeria, currently the seventh most populated country in the world, is projected to surpass that of the US by 2049 as a consequence of Nigeria's population growth rate, which is currently the highest among 10 largest countries in the world.

See also: Top 50 Cities in the US by Population, Top 100 Cities in the US by Population, Top 1000 Cities in the US by Population