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# Knot (speed)

A knot is a non SI unit of speed equal to one nautical mile per hour. Knot is usually abbreviated kt.

The knot is now usually 1.852 kilometres per hour precisely (approximately 1.15 miles per hour or 0.514 metres per second). See nautical mile for various earlier definitions.

The knot is widely used in air and sea navigation, even though it is a non-metric unit. It is listed by the BIPM among the units "currently" accepted for use with SI (table).

Because a knot is already a measure of speed, the expression "knots per hour" is a solecism. Taken literally (nautical mile/hour²), it would be a measure of acceleration.

## Origin

In some sailing ships, speed was measured by casting the log from the stern. The log was relatively immobile, and attached by line to a reel. Knots placed at a distance of 47 feet 3 inches (14.4018 m) passed through a sailor's fingers, while another used a 28 second sandglass to time the operation. The knot count would be reported and used in the sailing master's dead reckoning and navigation. This method gives a value for the knot of 20.25 in/s, or 1851.66 m/hr. The difference from the accepted value today is a bit less than 0.02%.