NOAA's official value for the total length of the US shoreline is 95,471 miles.
As there is no reference that designates one specific shoreline as the “legal” shoreline, numbers for the length of the U.S. shoreline can vary depending on how the shoreline is defined.
The NOAA shoreline length calculation of 95,471 miles was determined by hand in 1939-40 with a recording instrument on the largest-scale charts and maps available at that time. Shorelines of outer coast, offshore islands, sounds, bays, rivers and creeks were included to the head of the tidewater or to a point where tidal waters narrow to a width of 100 feet. For the Great Lakes, the shoreline lengths were measured in 1970 by the International Coordinating Committee on Great Lakes Basic Hydraulic and Hydrologic Data.
The total length of tidal shoreline includes measurements of the coastal states as well as the outlying US territories and possessions.
Canada has the longest coastline in the world, at 152,100 miles. Australia's is 25,760km (a bit over 16,000 miles) while Monaco's is the shortest, at 3.5 miles.
Article and diagram courtesy of NOAA's National Ocean Service.