Gross domestic product (GDP) is the market value of all final goods and services from a nation in a given year. Countries are sorted by nominal GDP estimates from financial and statistical institutions, which are calculated at market or government official exchange rates. Nominal GDP does not take into account differences in the cost of living in different countries, and the results can vary greatly from one year to another based on fluctuations in the exchange rates of the country's currency.  Such fluctuations may change a country's ranking from one year to the next, even though they often make little or no difference in the standard of living of its population. 
Comparisons of national wealth are also frequently made on the basis of purchasing power parity (PPP), to adjust for differences in the cost of living in different countries. PPP largely removes the exchange rate problem, but has its own drawbacks; it does not reflect the value of economic output in international trade, and it also requires more estimation than nominal GDP.  On the whole, PPP per capita figures are less spread than nominal GDP per capita figures. 
The United States is the world's largest economy with a GDP of approximately $19.39 trillion, notably due to high average incomes, a large population,  capital investment, moderate unemployment,  high consumer spending,  a relatively young population,  and technological innovation.  Tuvalu is the world's smallest national economy, with a GDP of about $32 million, because of its very small population, a lack of natural resources, reliance on foreign aid, negligible capital investment, demographic problems, and low average incomes. 
The first list includes estimates compiled by the International Monetary Fund's World Economic Outlook, the second list shows the World Bank's data, and the third list includes data compiled by the United Nations Statistics Division. The IMF definitive data for the past year and estimates for the current year are published twice a year in April and October. Several economies which are not considered to be countries (the world, the European Union, and some dependent territories) are included in the lists because they appear in the sources as distinct economies. These economies are italicized and not ranked in the charts, but are listed where applicable.
- List of countries by GDP (PPP)
- List of countries by GDP (nominal) per capita
- List of countries by GDP (PPP) per capita