Convert between knots and other major units of velocity.

A unit of velocity used for ships and aircraft, 17th century – present, currently = 1 international nautical mile per hour = 1852 meters per hour, about 1.1508 miles per hour. Previous versions include the Admiralty knot, based on the Admiralty measured mile of 6080 feet, and one based on the U. S. nautical mile of 6080.20. In concept, the nautical mile is 1 minute of latitude (see nautical mile). Symbol, kt.

The term “knot” arose from the method formerly used to measure a ship’s speed at sea. A sea anchor was thrown off the stern and the line to it allowed to play out freely for an interval of time measured with a sand glass, usually 30 seconds. The line had knots at regular intervals. By counting the number of knots that passed over the stern, and knowing the distance between knots and the length of time, the speed of the ship could be calculated. Actually, no calculation was involved, simply counting knots. A 17th century source says the knots were 50 feet apart, so the number of knots passing over the stern in 30 seconds was the speed of the vessel in knots (since 50 feet times 120 30-second intervals in an hour = 6000 feet, close enough for the technique). An 18th century source says the distance between knots was chosen to be 1/120 of a nautical mile, which would make the measurement even more precise.

For how knots were actually measured on shipboard in 1795, see this description.


Synonym for the nautical mile. This meaning is more recent and not uncommon, but it is not approved by mariners. The phrase “knots per hour” is particularly objectionable.


An unit of length used on shipboard to describe the distance between knots on the logline.

Eugene E[dward] O'Donnell.
Manual for the Guidance of Apprentices on Training Ships.
Boston: Issued by the Recruiting service, Sea Training Bureau, 1918.

“The only occasion then in which it is permissible to use the word knot as the equivalent of a length is in tracing the knots on the log line, and then by a familiar tendency in language, the distance between two knots is abbreviated in speech to the length of a knot.”


Various units of length used for yarn, 16th century – present?. used by spinners, a certain number of turns on the reel, marked by a knot.

In England, 17th century, a knot was 18 fathoms of thread.

in Massachusetts, a “thread” was one turn around the reel (74 inches) and a knot was 40 threads.

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