In 2013, the four most populous countries worldwide were China (1.41 billion), India (1.25 billion), the United States (320 million), and Indonesia (250 million). But if you counted the total number of international migrants worldwide (231.5 million), they could make up a very populous country on their own -- ranking the fifth in the world. More than 3.2% of the world's 7.2 billion people were international migrants in 2013. For comparison, in 1990 the number of international migrants made up 2.9% (or 154.2 million) of the then world population of 5.3 billion.
Because governments collect statistics on international migrants residing within their countries' boundaries in varying ways, there is no consistent cross-country definition of this population. The UN Population Division provides the mid-year estimate of international migrants based on official statistics on the foreign born, i.e., people born outside of the country of current residence. However, for countries that do not collect data on place of birth but have data on citizenship, the estimated number of noncitizens is used instead. In both cases, migrant stock also includes refugees, some of whom may not be foreign born. Most of the data were obtained from population censuses; population registers and nationally representative surveys were also used for information on the number and composition of international migrants. For more information on the UN Population Division’s methodology to estimate the size of international migrants, click here: http://esa.un.org/unmigration/migrantstocks2013.htm?mdocs.
Migration Policy Institute tabulation of data from the United Nations, Department of Economic and Social Affairs (2013). Trends in International Migrant Stock: The 2013 revision (United Nations database, POP/DB/MIG/Stock/Rev.2013). Available here: http://esa.un.org/unmigration/migrantstocks2013.htm?mtotals. (Photo courtesy of Flickr user selva).