Fast Facts: Nautical Miles and Knots

Speed and distance on the water are very different than on land. Miles and miles per hour do not exist at sea. When you travel on a vessel across the ocean, a nautical mile is the measurement used to determine the distance traveled through the water. A nautical mile is slightly longer than a mile on land, equaling 1.1508 land-measured (or statute) miles. The nautical mile was determined by the Earth’s longitude and latitude coordinates, with one nautical mile equaling one minute of latitude.

But what is the benefit of using a different measurement system for marine navigation? Using latitude and longitude coordinates is more functional for long-distance travel, where the curvature of the Earth becomes a factor in accurate measurement. Nautical charts use latitude and longitude, so it’s much easier for mariners to measure distance with nautical miles. Air and space travel also use latitude and longitude for navigation and nautical miles to measure distance.

The word “mile” might prompt you to question if there is a “nautical kilometer” as well— there’s not. The international nautical mile is used worldwide. The measurement was officially set at exactly 1.852 kilometers in 1929 by the International Hydrographic Organization. The U.S. and the United Kingdom used slightly different measurements after that, but later the U.S. adopted the international nautical mile in 1954 and the U.K. in 1970.

On the other hand, knots are used to measure speed. One knot equals one nautical mile per hour or approximately 1.15 statute mph. The term knot dates back to the 17th century when sailors measured their ship’s speed using a “common log.” The common log was a rope with knots at regular intervals attached to a piece of wood shaped like a piece of pizza. Mariners would drop the wood piece into the water and let it float freely behind the ship for a specific amount of time. They often timed this with an hourglass, and when the time was up, they would count the knots between the ship and the piece of wood, and that number gauged their speed.