The short answer
A nautical mile equals 1,852 metres exactly, or 6,076 feet. That is about 800 feet longer than a statute (land) mile of 5,280 feet. One nautical mile equals one minute of arc on a great circle of the Earth, which is the reason the unit exists at all: it makes celestial navigation arithmetic clean.
A knot is one nautical mile per hour. So a ship traveling at 25 knots is moving at 25 nautical miles per hour, equivalent to 28.8 statute mph or 46.3 km/h. Aviation uses the same units: a commercial jet cruising at 480 knots is moving at 552 mph or 889 km/h.
This guide explains where the nautical mile comes from, why ships and aircraft still use it, and how the knot speed unit works.
The latitude connection
A nautical mile is defined so that one nautical mile equals one minute of latitude along any meridian. A degree of latitude equals 60 minutes, so one degree of latitude equals 60 nautical miles. This connection is what makes the unit useful for navigation.
In old-school celestial navigation, a navigator would sight a star or the sun, calculate their latitude in degrees and minutes, and read the distance to a charted reference point directly in nautical miles. No conversion needed. A 5-minute change in latitude is exactly 5 nautical miles of north-south distance.
This worked because the Earth is roughly spherical. A circle around the equator (or any great circle) measures about 360 × 60 = 21,600 nautical miles, which gives an Earth circumference of 21,600 × 1.852 km = 40,003 km. The actual equatorial circumference is 40,075 km, so the nautical mile slightly under-states the true value (by 0.18 percent), an artifact of the Earth being an oblate spheroid rather than a perfect sphere.
The International Hydrographic Organization formally set the nautical mile at exactly 1,852 metres in 1929. Before that, different countries used slightly different values: the British nautical mile was 1,853.18 m, and the US nautical mile was 1,853.25 m. The 1929 standardization fixed the universal value.
Nautical vs statute miles
The nautical mile is longer than the statute mile that Americans use on the road:
| Unit | Length | Use |
|---|---|---|
| Statute mile | 1,609.344 m (5,280 ft) | US road distances, British road distances |
| Nautical mile | 1,852 m (6,076 ft) | Maritime and aviation navigation |
| Kilometer | 1,000 m (3,281 ft) | Most of the world for road and general distance |
The conversion factor:
For mental conversion: nautical miles to statute miles, add 15 percent. Statute miles to nautical miles, subtract about 13 percent. Or just remember: nautical miles are 15 percent longer.
A practical example: New York to Los Angeles is 2,451 statute miles by road, but about 2,129 nautical miles by great-circle air route. The air route is shorter because it cuts across the curve of the Earth.
The knot speed unit
A knot is one nautical mile per hour. So:
| Speed | Knots | Statute mph | km/h |
|---|---|---|---|
| Slow ship | 5 | 5.8 | 9.3 |
| Cruising sailboat | 10 | 11.5 | 18.5 |
| Modern cargo ship | 20 | 23.0 | 37.0 |
| Fast ferry | 40 | 46.0 | 74.1 |
| Cruise ship | 22-25 | 25-29 | 41-46 |
| Aircraft taxi | 10 | 11.5 | 18.5 |
| Light aircraft cruise | 100-130 | 115-150 | 185-241 |
| Regional jet cruise | 400 | 460 | 741 |
| Commercial jet cruise | 480-510 | 552-587 | 889-944 |
| Concorde cruise | 1,100 (Mach 2) | 1,265 | 2,037 |
The conversion: 1 knot equals 1.151 statute mph or 1.852 km/h. For pilots, the mental math is to multiply knots by 2 and subtract a bit to get km/h (1 knot is about 1.85 km/h, so multiply by 2 over-estimates by 8 percent; this is the rough field-day check).
The term “knot” comes from the 17th-century “common log” device. Sailors would attach a wooden float to a long line knotted at regular intervals (originally about 8 fathoms, or 47 feet, but the spacing was calibrated to a specific sand-glass timer). When the float was streamed behind the ship, they would count knots passing through their hands over a fixed time interval. The number of knots in that interval gave their speed in nautical miles per hour, which became simply “knots.” The unit was never plural-pluralized: a ship moves at 20 knots, not “20 knots per hour.”
Why ships and aircraft still use these units
The case for switching to kilometers and km/h is the same as for any unit migration: switching costs.
Maritime navigation has 500 years of established practice using nautical miles. Every nautical chart in the world is graduated in latitude and longitude, which makes the nautical mile the natural unit. Switching to km would require redrawing every chart and retraining every navigator. The cost is enormous and the benefit is purely cosmetic, because the modern GPS system internally uses metric coordinates and converts to nautical miles for display.
Aviation inherited the same system from maritime navigation when air traffic control was being designed in the 1920s and 30s. ICAO (the International Civil Aviation Organization) formalized nautical miles for distance and knots for airspeed in its Annex 5 standard. The same arguments apply: any cost of switching would dwarf any benefit.
There are a few specialized aviation uses where metric appears. Runway lengths are usually in meters globally (and feet in the US). Visibility is in meters (kilometres) in most weather reports. Altitude uses feet, as covered in our flight levels guide and why aircraft cruise at 36,000 ft.
How navigation actually works today
Modern maritime and aviation navigation uses GPS as the primary positioning system. GPS internally uses metric coordinates (WGS84 ellipsoidal latitude, longitude, and height). The display layer translates to nautical miles and knots for the user.
A ship’s chartplotter shows position in lat-long, distance to next waypoint in nautical miles, and speed in knots. The underlying GPS receiver is computing everything in metric and converting at the display. Same for aircraft navigation.
For long-distance route planning, the math is now done in metric in software but presented to the pilot or navigator in their familiar units. Great-circle route calculations, for example, are clean to do in metric (using spherical trigonometry on the Earth’s radius in meters) and then convert the result to nautical miles for the flight plan.
The compatibility with the existing chart catalog and the existing radio phraseology is what keeps nautical miles in operational use, even though the underlying technology no longer needs them.
A nautical-mile cheat sheet
For quick reference:
- 1 nautical mile = 1,852 m = 6,076 ft = 1.151 statute miles
- 1 knot = 1.151 mph = 1.852 km/h
- A great-circle route Los Angeles to Tokyo is about 4,750 nm = 5,470 statute mi = 8,800 km
- New York to London (great-circle) is about 3,000 nm = 3,450 statute mi = 5,560 km
- A 7-hour transcontinental flight at 480 knots covers about 3,360 nautical miles
For converting any specific value of meters to feet, use the calculator on the homepage or browse the precomputed conversion pages. For more on aviation units, see our flight levels guide.
Sources and further reading: