Nautical Measurements: Knots And Fathoms And Leagues, Oh My!

nautical measurements

Stanley Compass
License: Creative Common


Nautical Knowledge

For the majority of us, nautical measurements exist only in the realm of naval movies and pirate literature. But for those sailing the seas (sans GPS), these terms are essential for maritime navigation. Here I’ll explain the meaning and history behind the geographic lexicon of the seas so you too can start speaking the nautical lingo.

Nautical Mile

A nautical mile is a standard measure of maritime distance, and has been historically defined as the length of one minute (1/60) of a degree of the earth. The distance of a nautical mile varied between nations for years, which led to quite a few navigational translation problems, until it was determined at the International Extraordinary Hydrographic Conference held in Monaco in 1929 that the international definition of a nautical mile would be 1852 meters. This number was a compromise between the value calculated from historic standards and the value calculated using standards set by the International Union of Geodesy and Geophysics. Today, a nautical mile is still used to estimate distances as it works much better with the physical dimensions of our planet.


A knot is the standard measurement of speed for boats, and is defined as one nautical mile per hour. The term originated from the old practice of measuring a boat’s speed wherein a weighted wooden panel called a “chip log” was cast out from the boat. Knots were placed in the attached rope every 47 feet 3 inches, and were allowed to slip through a sailor’s fingers as the weighted log floated perpendicular to the water surface. Another crewman used a sand glass to time the process for 30 seconds. The count of knots that passed through the sailor’s fingers was then reported as a measurement of the boat’s speed. The current measurement of a knot is approximated at 1.151 miles per hour, which differs from the traditional measure by less than 0.02%.


A fathom is used to measure the depth of water, and originally was measured as the length from one middle finger to the other with arms outstretched. The fathom evolved to a more standardized unit of measure, and is defined internationally and by the United States as a unit of measure equaling two yards (6 feet). The British fathom, however, is defined as one one-thousandth of a nautical mile, or 6.08 feet.


A league, in nautical terms, was historically defined as the distance an observer of average height can see when standing at sea level. This measurement has been standardized in much of the English-speaking world to three miles. The term league, though, was also used on land and was a measurement of the distance a person could walk in an hour. Unlike many other units of measurement, the league was never internationally standardized and thus has many different interpretations in different nations.

These terms may not be used frequently in modern times, but it is still fascinating to learn about how sailors used them to navigate the vast and dangerous oceans without the measurement technology we have today. If anything, the origins and evolution of these terms tell us a lot about the nautical history of the world.

About the Author:

A big thanks to my guest writer for a very informative article.  This article was composed by Steve Herring, who is a boating enthusiast and contributing blogger for Manitou Boats, a top manufacturer of pontoon boats.

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