But half of those 7.2 billion people live in only six countries: the United States, Brazil, Pakistan, India, China, and Indonesia, shown above on the map from the Pew Research Center.

China has the world's largest population — 1.36 billion people. Next comes India with 1.28 billion people. The U.S., Brazil, Pakistan, and India combined house about 702 million people.

The U.N. set the theme this year as "investing in young people." Today's 1.8 billion young people will continue to shape the social and economic status of countries. 

"On this World Population Day, I call on all with influence to prioritize youth in development plans, strengthen partnerships with youth-led organizations, and involve young people in all decisions that affect them. By empowering today's youth, we will lay the groundwork for a more sustainable future for generations to come," U.N. Secretary-General Ban Ki-moon said.