The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom or Britain, is a sovereign country located off the north western coast of the European mainland. The United Kingdom includes the island of Great Britain, the north eastern part of the island of Ireland, and many smaller islands. Northern Ireland shares a land border with the Republic of Ireland. Otherwise, the United Kingdom is surrounded by the Atlantic Ocean, with the North Sea to the east, the English Channel to the south and the Celtic Sea to the southwest, giving it the 12th-longest coastline in the world. The Irish Sea separates Great Britain and Ireland. The total area of the United Kingdom is 94,000 square miles (240,000 km2). In 2017, the population was estimated to total 66.0 million inhabitants.