It is no simple matter to measure the length of a coastline. What is known as the ‘coastal paradox’ states that a coastline does not have a well-defined length. The smaller the scale interval (meaning the more detailed the measurement), the longer the coastline will be. A "magnifying" effect comes into play for convoluted coastlines more than for relatively smooth ones.

Of the countries and continent listed the one with the longest coastline is Japan with 29,751 kilometres (18,486 miles). Australia is second longest with 25,760 kilometres (16,007 miles), ahead of the U.S.A. and Antarctica with 19,924 kilometres (12,380 miles) and 17,968 kilometres (11,165 miles) respectively.

These numbers are small in comparison with the 'big two' coastline countries which are Canada (202,080 kilometres and 125,567 miles) and in second place, Norway (58,133 kilometres and 36,122 miles).

Data marked "The World Factbook" (TWF) covers 198 countries and 55 territories, from the book published by the U.S. Central Intelligence Agency. In addition to coastline lengths, this is the source of the land area used to calculate the "coast/area ratio" for the TWF coastline measurement. This ratio measures how many metres of coastline correspond to every square kilometre of land area. The ratio illustrates the ease of accessibility to the country's coast from every point in its interior.

More Info: en.wikipedia.org