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Nautical Miles Explained: Why Ships Use Different Units

June 14, 2025 โ€ข 4 min read

If you have ever looked at a marine chart or listened to an aviation broadcast, you have encountered the nautical mile. It is not simply an old-fashioned unit โ€” there is a very practical reason why sailors and pilots still use it today.

What is a Nautical Mile?

1 nautical mile = 1.852 km = 1.15078 statute miles

1 nautical mile = 1 minute of latitude

A nautical mile is defined as exactly 1,852 metres. But the reason it exists is far more elegant: it equals one minute of arc of latitude on the Earth's surface. This makes it incredibly useful for navigation on a sphere.

Why Not Just Use Regular Miles or Kilometres?

Navigation at sea and in the air involves working with latitude and longitude โ€” angular measurements on a sphere. Because one nautical mile equals one minute of latitude:

  • โ–ถ You can read distances directly from a chart's latitude scale
  • โ–ถ One degree of latitude always equals 60 nautical miles
  • โ–ถ Plotting a course becomes simple arithmetic rather than complex conversion
  • โ–ถ Speed in knots (nautical miles per hour) integrates naturally with chart distances

Nautical Miles vs Statute Miles vs Kilometres

  • โ–ถ 1 nautical mile = 1.852 km
  • โ–ถ 1 nautical mile = 1.151 statute miles
  • โ–ถ 1 statute mile = 0.869 nautical miles
  • โ–ถ 1 kilometre = 0.54 nautical miles

What About Knots?

A knot is simply one nautical mile per hour. The name comes from the old practice of measuring ship speed by throwing a knotted rope overboard and counting how many knots passed through a sailor's hands in a set time.

  • โ–ถ 1 knot = 1 nautical mile/hour = 1.852 km/h = 1.151 mph
  • โ–ถ Cruise ship: 20-24 knots (37-44 km/h)
  • โ–ถ Cargo ship: 12-16 knots (22-30 km/h)
  • โ–ถ Commercial aircraft: 450-550 knots (830-1,020 km/h)

Who Uses Nautical Miles?

  • โ–ถ Maritime navigation โ€” all shipping worldwide
  • โ–ถ Aviation โ€” all air traffic control and flight planning
  • โ–ถ Weather services โ€” for reporting visibility and wind at sea
  • โ–ถ International law โ€” territorial waters are defined in nautical miles (12 NM from shore)

The nautical mile is one of those rare units where the definition is not arbitrary โ€” it is baked into the geometry of our planet. That is why, even in an age of GPS, it remains the standard for anyone navigating Earth's surface over long distances.

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