Washington DCformally the District of Columbia and commonly referred to as "Washington", "the District", or simply "D.C.", is the capital of the United States. The signing of the Residence Act on July 16, 1790, approved the creation of a capital district located along the Potomac River on the country's East Coast. Washington had an estimated population of 672,228 as of July 2015. Commuters from the surrounding Maryland and Virginia suburbs raise the city's population to more than one million during the workweek. The Washington metropolitan area, of which the District is a part, has a population of over 6 million, the sixth-largest metropolitan statistical area in the country.