Berlin is the capital and largest city of Germany by both area and population. Its population of around 3.8 million inhabitants makes it the most populous city of the European Union, according to population within city limits. One of Germany's 16 constituent states, Berlin is surrounded by the state of Brandenburg, and contiguous with Potsdam, Brandenburg's capital. Berlin's urban area has a population of around 4.5 million and is the second most populous urban area in Germany after the Ruhr. The Berlin-Brandenburg capital region has about six million inhabitants and is Germany's third-largest metropolitan region after the Rhine-Ruhr and Rhine-Main regions.