Every year, institutions in the United States dominate rankings of the best colleges in the world.

Of the top 10 best universities in the world, eight are located in the U.S. But despite having some of the best educational institutions on earth, the Organisation for Economic Co-operation and Development (OECD) ranks the U.S. sixth for adult education level.

The OECD defined a country's adult education level as the percentage of people between the ages of 25 and 64 who have completed some kind of tertiary education in the form of a two-year degree, four-year degree or vocational program.

Here are the 10 most educated countries: