Nautical and Aviation Units: Knots, Nautical Miles, and Why They're Different
Ask a pilot how fast they're flying and you'll get an answer in knots, not mph. It's not tradition for tradition's sake, the nautical mile and the knot are built directly around the shape of the Earth, which makes them genuinely more useful for navigation than land-based units. Having spent time around both military and civilian navigation systems, this is one of those conventions that seems odd until you see the geometry behind it.
What a Nautical Mile Actually Is
A nautical mile is defined as exactly 1,852 meters, standardized internationally in 1929 at the First International Extraordinary Hydrographic Conference in Monaco. What makes it different from a regular mile is what it's tied to: one nautical mile equals one minute of arc along a line of longitude on the Earth's surface. Since a circle has 360 degrees and each degree splits into 60 minutes, the Earth's circumference works out to almost exactly 21,600 nautical miles. That direct link between distance and position is what makes nautical miles genuinely useful for chart-based navigation, a distance on a nautical chart translates straight into a coordinate difference.
What a Knot Is
A knot is simply one nautical mile per hour. The name traces back to old sailing ships, which measured speed by throwing a knotted rope with a floating log tied to the end over the stern and counting how many knots passed through a sailor's hands in a fixed time. The method is centuries obsolete, but the name and the unit stuck. Saying "knots per hour" is technically redundant, since a knot already means nautical miles per hour.
Why Aviation Adopted Knots and Nautical Miles
In 1947, the International Civil Aviation Organization (ICAO) moved to standardize units across global aviation around the metric system, but explicitly carved out an exception: nautical miles and knots, along with feet for altitude, were kept in permanent use because switching them too fast risked confusion in the skies. That exception, recorded in ICAO Annex 5, has never had an end date set. Since air routes are plotted using the same latitude/longitude coordinate system as marine navigation, and aircraft frequently coordinate with ships (search and rescue, for instance), sharing the same distance and speed units avoids a layer of conversion error in situations where mistakes matter.
Converting Knots to MPH and KM/H
That factor comes directly from dividing the exact nautical mile (1,852 m) by the exact statute mile (1,609.344 m). A 20-knot wind, for example, is about 23 mph. For the metric side, 1 knot equals 1.852 km/h exactly, since the nautical mile is defined in meters. Try it on the knots to mph, km/h to mph, and mph to km/h converters, or the full Speed converter.
Frequently Asked Questions
Do commercial airline passengers ever see speed in knots?
Usually not directly, in-flight displays and pilot announcements to passengers typically convert to mph or km/h depending on the airline and region, since that's what most passengers are used to. The cockpit instruments and air traffic control communications, however, run in knots.
Is a nautical mile the same everywhere on Earth?
The modern international nautical mile (1,852 m) is a single fixed value used globally today. Before 1929 standardization, different countries used slightly different nautical mile definitions based on their own geodetic measurements, which is part of why an international standard was adopted.