The United States paid France $15 million for the Louisiana Purchase in 1803.
The agreement transferred approximately 828,000 square miles of territory to the United States. The purchase stretched west of the Mississippi River and included land that later became all or part of 15 present-day U.S. states, as well as small portions of two Canadian provinces.
President Thomas Jefferson authorized the negotiations, initially hoping to secure access to New Orleans and the Mississippi River. French leader Napoleon Bonaparte offered the entire Louisiana Territory instead, partly because France’s plans in North America had changed and Napoleon needed funds for European warfare.
The $15 million figure refers to the agreed purchase price in 1803 dollars. The transaction was not simply a purchase of the modern state of Louisiana; it acquired a much larger territory and roughly doubled the size of the United States.