What Is a Nautical Mile?
Exactly 1,852 metres (~1.151 statute miles). One nautical mile equals one minute of latitude — travel 60 nautical miles due north and your latitude changes by exactly one degree. The nautical mile is the only length unit still defined by Earth’s geometry: it was chosen so that one minute of latitude equals one nautical mile on a spherical Earth. In practice the value was fixed internationally in 1929 at exactly 1,852 metres, ending earlier national variations.
What Is a Kilometre?
A kilometre is 1,000 metres — the metric unit for land distances worldwide. Unlike the nautical mile, it has no tie to Earth’s geometry: it descends from the metre, which is defined by the speed of light. Road distances, railway distances and most land maps use kilometres almost everywhere outside the United States and the United Kingdom.
Why Ships and Planes Don’t Use Kilometres
Navigators measure distance directly off a chart’s latitude scale with dividers — no calculation needed. Speed at sea is measured in knots (nautical miles per hour): 1 knot = 1.852 km/h. Aviation inherited the convention; ICAO standardized it worldwide. This convenience is no accident: Mercator-projection charts are drawn so that the latitude scale stretches exactly the way distances do, which makes the dividers method work worldwide. At the speeds involved — a jetliner cruises near 450 knots — a unit tied to the chart avoids the conversion errors that could put an aircraft or ship off course.
Exact Conversion Factors
| From | Equals |
|---|---|
| 1 nautical mile | 1.852 km (exact) |
| 1 nautical mile | 1.15078 statute miles |
| 1 kilometre | 0.539957 nautical miles |
| 1 knot | 1.852 km/h (exact) |
| 1 knot | 1.15078 mph |
When Is Each Unit Used?
Use nautical miles and knots for sea voyages, flight planning, marine charts and aviation — everywhere navigators work with latitude and longitude. Use kilometres for road travel, railway distances, cycling, running and almost all land distances. The two worlds meet at ports and airports: a ferry timetable may quote the crossing in nautical miles and the onward road journey in kilometres.
Frequently Asked Questions
Why is a nautical mile 1,852 metres?
Because it was defined as one minute of arc of latitude — and Earth’s circumference works out so that one minute equals roughly 1,852 metres. The value was fixed at exactly 1,852 metres by international agreement in 1929, ending earlier national definitions.
Is a knot faster than km/h?
A knot is one nautical mile per hour, which equals exactly 1.852 km/h — so a knot is almost twice as fast as one kilometre per hour. A 20-knot ferry is doing about 37 km/h.
Do pilots use kilometres?
Almost never. Pilots plan and report distances in nautical miles, speeds in knots, and altitude in feet; ICAO, the UN aviation body, standardised this worldwide. Some countries’ domestic general aviation uses kilometres, but international aviation is nautical.
Nautical mile vs mile?
A nautical mile (1,852 m) is about 15 percent longer than a statute mile (1,609.344 m). One nautical mile equals 1.15078 statute miles, and the two are used in completely different worlds: nautical miles at sea and in the air, statute miles on land.
Why not kilometres at sea?
Because the nautical mile ties directly to chart coordinates: navigators can read distance straight off the latitude scale with dividers, with no conversion. Kilometres would need constant conversion against latitude, adding w
Related Conversions
The 1929 Definition and Earth’s Circumference
The nautical mile was not chosen at random. A full circle of latitude (or any meridian) spans 360 degrees, and each degree contains 60 minutes of arc. Multiply 360 × 60 and you get 21,600 — so there are 21,600 nautical miles around the equator. Multiply 21,600 by 1,852 metres and you get 40,003,200 metres, remarkably close to the actual meridional circumference of about 40,007,860 metres. The small difference exists because Earth is slightly squashed at the poles, so one minute of latitude is marginally longer at the equator than at high latitudes. Before 1929, different nations used slightly different values — the British nautical mile was 1,853.18 metres (the Admiralty mile) and the US value 1,853.248 metres. The international agreement of 1929 fixed a single exact value of 1,852 metres for everyone, ending that confusion.
Worked Examples
Example 1 — Sailing distance: A yacht sails 12 nautical miles. In kilometres that is 12 × 1.852 = 22.224 km, so the crew can report the leg as 22.2 km to anyone on shore.
Example 2 — Ferry timetable: A route is listed as 8.5 nautical miles. Multiply 8.5 × 1.852 = 15.742, so the crossing is about 15.7 km. At 20 knots (37 km/h), the journey takes roughly 25 minutes.
Example 3 — Flight planning: A 1,240 km flight segment converts to 1,240 ÷ 1.852 = 669.5 nautical miles, the figure a pilot enters into the flight plan. At a cruising speed of 450 knots, it takes about 89 minutes.
More Frequently Asked Questions
How many nautical miles are in 100 km?
Divide by 1.852: 100 ÷ 1.852 = 53.9957, so 100 km equals roughly 54 nautical miles. The table above shows the full conversion factors in both directions.
Is a knot the same as mph?
No. A knot is one nautical mile per hour (1.852 km/h), while a mile per hour is one statute mile per hour (1.609344 km/h). One knot equals 1.15078 mph, so wind reported at 20 knots is about 23 mph.