A knot is the standard unit of speed used in maritime and air navigation. One knot equals one nautical mile per hour.
Unlike kilometers per hour or miles per hour, the knot is directly linked to the Earth’s geometry, making it especially useful for navigation across charts.
Definition
The modern international knot is defined as:
1 knot = 1 nautical mile per hour
1 knot = 1.852 km/h = 0.514444 m/s
Because the nautical mile is tied to latitude, the knot naturally connects speed with position on the Earth’s surface.
Origin of the knot
The term “knot” comes from a practical method used by sailors to measure speed at sea before modern instruments.
A device known as a chip log was used. It consisted of a wooden board attached to a rope with knots tied at regular intervals.
The board was thrown into the water, and the rope was allowed to run freely for a fixed period of time, measured with a sand glass. The number of knots that passed through the sailor’s hands during that time gave the ship’s speed.
Why knots are still used
Despite the widespread use of the metric system, knots remain standard in maritime and aviation contexts.
The main reason is practical: nautical charts are based on latitude and longitude, and the nautical mile fits naturally into that system. Using knots allows navigators to relate speed directly to distance on charts without conversion.
International vs historical knots
Today, the international knot is based on the international nautical mile of exactly 1,852 meters.
Historically, British Admiralty charts used a slightly different nautical mile of 6,080 feet (1,853.184 meters). This produced a slightly different knot value:
International knot ≈ 0.514444 m/s
UK Admiralty knot ≈ 0.514773 m/s
The difference is small (about 0.06%), but it can accumulate over long distances or in precise historical calculations.
Knots and navigation
Because one knot corresponds to one nautical mile per hour, speed, time and distance are easy to relate:
- 10 knots for 1 hour ≈ 10 nautical miles
- 15 knots for 2 hours ≈ 30 nautical miles
- 20 knots for 30 minutes ≈ 10 nautical miles
This direct relationship is one of the reasons the knot has remained in use even as other measurement systems evolved.
Knots beyond the sea
Knots are also used in aviation. Aircraft speed is often measured in knots because air navigation, like maritime navigation, relies on geographic coordinates.
Using a shared unit simplifies calculations between air and sea navigation systems.