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 based on purchasing power parity (PPP), to adjust for differences in the cost of living in different countries. Other metrics, nominal GDP per capita and a corresponding GDP (PPP) per capita, are used for comparing national standard of living. On the whole, PPP per capita figures are less spread than nominal GDP per capita figures.

The rankings of national economies have changed significantly over time. For instance, the United States overtook the British Empire around 1916; Japan rose rapidly in the post-World War II period to become the world’s second-largest economy by the 1970s; China moved from ninth place in 1978 to second in 2010 following market reforms; and more recently India has climbed into the top five. These shifts reflect long-term changes in global economic output.

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's definitive data for the past year and estimates for the current year are published twice a year in April and October. Non-sovereign entities (the world, continents, and some dependent territories) and states with limited international recognition (such as Kosovo and Taiwan) are included in the list where they appear in the sources.