The total length of the Canada–United States border is 8,891 kilometers. It is the world’s longest international border between two countries and stretches across forests, plains, mountains, lakes, rivers, and coastal waters.
The figure includes Canada’s boundary with the contiguous 48 United States as well as its separate boundary with Alaska. Of the total, about 6,416 kilometers lie between Canada and the contiguous United States, while roughly 2,475 kilometers separate Canada from Alaska. That distinction explains why some sources give a much smaller figure when discussing only the lower-48 border.
The boundary’s modern shape developed through several agreements. The Treaty of Paris in 1783 established much of the early line after the American Revolutionary War. Later treaties—including the Treaty of 1818, the Webster–Ashburton Treaty of 1842, and the Oregon Treaty of 1846—settled different sections and disputes.
Another famous feature is the 49th parallel, which forms a long straight section of the boundary. The International Boundary Commission helps survey, mark, and maintain the border.