Berlin is the capital and largest city of Germany by both area and population. Its 3,723,914 (2018) inhabitants make it the second most populous city proper of the European Union after London. The city is one of Germany's 16 federal states. It is surrounded by the state of Brandenburg, and contiguous with its capital, Potsdam. The two cities are at the center of the Berlin/Brandenburg Metropolitan Region, which is, with 6,004,857 (2015) inhabitants and an area of 30,370 square km, Germany's third-largest metropolitan region after the Rhine-Ruhr and Rhine-Main regions.