Nautical Miles vs. Statute Miles: Aviation and Maritime Measurements
Why do planes and ships use knots and nautical miles instead of kilometers or standard miles? Discover the historical and practical reasons behind maritime conversions.
The Geometry of the Earth
If you have ever been on an international flight and checked the seatback screen, you probably noticed the plane’s speed and distance were measured in something called Nautical Miles (NM) and Knots, rather than standard miles or kilometers.
Why do the aviation and maritime industries stubbornly refuse to adopt the Metric system, or even the standard Imperial miles used on highways?
The answer has nothing to do with tradition, and everything to do with the physical geometry of planet Earth.
What is a Nautical Mile?
A standard “Statute Mile” (the kind used on US road signs) was originally defined by the Romans as 1,000 paces. It is arbitrary. A kilometer is exactly 1,000 meters, which was originally defined as one ten-millionth of the distance from the equator to the North Pole.
But a Nautical Mile is fundamentally tied to the Earth’s coordinates.
The Earth is a sphere (roughly), and its surface is mapped using 360 degrees of Latitude and Longitude. Each degree is divided into 60 “minutes.”
One Nautical Mile is defined as exactly one minute of latitude.
Because of this brilliant definition, a navigator on a ship or an airplane doesn’t need a complex calculator to figure out where they are. If they travel exactly 60 nautical miles directly North, they know they have moved exactly one degree of latitude on the map. It perfectly syncs distance traveled with global coordinates.
The Math: Converting Nautical Miles
Because the Earth is so massive, one minute of latitude is quite a long distance.
- 1 Nautical Mile = 1.15078 Statute Miles
- 1 Nautical Mile = 1.852 Kilometers
If an airplane is flying from New York to London (a distance of roughly 3,000 nautical miles), it is traveling about 3,450 standard highway miles.
What is a Knot?
Once you understand a Nautical Mile, understanding a Knot is incredibly simple.
A Knot is simply one Nautical Mile per hour.
If a ship is traveling at 20 knots, it is covering 20 nautical miles every single hour. In standard highway terms, 20 knots is roughly 23 miles per hour (mph), or 37 kilometers per hour (km/h).
The Origin of the “Knot”
Hundreds of years ago, sailors used a device called a “chip log” to measure their speed. They tied a piece of wood to a long rope that had knots tied in it at perfectly spaced intervals. They threw the wood overboard and used a sandglass to count how many knots slipped through their hands as the ship pulled away from the wood.
If 10 knots slipped through their hands in a set amount of time, the ship was traveling at “10 knots.” The name stuck, and it is still used by GPS satellites today!
Seamlessly Navigating Conversions
Unless you are a pilot or a ship captain, you rarely need to convert nautical miles in your head. But if you are tracking a flight, studying weather patterns, or taking a sailing course, translating these coordinate-based distances into distances you understand can be tricky.
Instead of multiplying by 1.15078 in your head, you can instantly translate any aviation or maritime distance using our Length Conversion Tool, or translate Knots into MPH or KM/H using our Speed Conversion Tool.