Nautical Converter

Nautical units are used in maritime and aviation navigation.

Distance conversions:

• 1 nautical mile = 1.852 km (exact, by international agreement)
• 1 nautical mile = 1.15078 statute miles
• 1 nautical mile = 6,076.12 feet
• 1 nautical mile = 1 minute of latitude (by definition)

Speed conversions:

• 1 knot = 1 nautical mile per hour
• 1 knot = 1.852 km/h (exact)
• 1 knot = 1.15078 mph
• 1 knot = 0.51444 m/s

Common references:

• Cruise ship: 20-24 knots (37-44 km/h)
• Container ship: 14-24 knots (26-44 km/h)
• Commercial aircraft: 450-550 knots (833-1,019 km/h)
• Sailboat: 5-8 knots (9-15 km/h)

Why nautical miles? They relate directly to Earth’s geometry. 1 nautical mile = 1 arc minute of latitude. This makes navigation calculations much simpler.

Because of that, 60 nautical miles equals one degree of latitude, and a navigator can step off distance against the latitude scale on the edge of a chart with no conversion at all. No land-based mile has that property, which is why ships and aircraft never gave it up.

A nautical mile runs about 15 percent longer than the statute (land) mile, so the two aren’t interchangeable, and a speed in knots is faster than the same number in mph. The word “knot” itself is a leftover from how speed was once measured: sailors trailed a rope with knots tied at fixed intervals and counted how many slipped overboard in a set time. The unit kept the name long after the rope disappeared.

Latitude works as a ruler, longitude does not. One minute of latitude is one nautical mile everywhere on Earth, so the scale printed up the side of a chart is a distance scale you can step off with dividers. The scale along the top is not. A minute of longitude is a full nautical mile only at the equator and shrinks by the cosine of the latitude: 0.87 NM at 30 degrees, 0.71 at 45, 0.50 at 60, and nothing at all at the pole. Measuring distance against the top edge of a chart is a classic beginner’s error and it under-reads badly in northern waters.

The 1,852 metres is a committee’s rounding, not a measurement. The original definition was one minute of arc along a meridian, but the Earth is not a sphere, so that arc is about 1,843 m near the equator and 1,862 m near the poles. The 1929 international conference split the difference at 1,852 m exactly and everyone eventually adopted it, the United States last, in 1954. The old US nautical mile of 1,853.248 m turns up on charts printed before then.

Distances at sea are quoted several ways and they are not the same thing. Cables are a tenth of a nautical mile, 185.2 m, still used for short ranges in pilotage. Fathoms are 6 feet, 1.8288 m, and measure depth rather than distance. A league is 3 nautical miles, which is why “twenty thousand leagues under the sea” describes a journey rather than a depth: 20,000 leagues is 60,000 nautical miles, or about 111,000 km, which is nearly three times around the planet.

Aviation kept the nautical mile too, for the same reason: charts, flight plans and air traffic control all work in nautical miles and knots, while altitude stays in feet almost everywhere outside Russia and China. An airliner cruising at 480 knots is covering 8 nautical miles a minute, which is the mental arithmetic every pilot uses for descent planning.