If you have ever followed a flight tracker, read a shipping report, or watched a sailing race, you will have met the nautical mile. It looks like an ordinary mile, and it is close in size, but it is a different unit with a different job. Where a land mile is an arbitrary length agreed by history, a nautical mile is tied directly to the size and shape of the Earth itself — and that connection is exactly why it survives in navigation long after most countries have gone metric on land.

This guide explains the definition in plain terms, shows where the figure of 1,852 metres comes from, and works through conversions to kilometres, statute miles, feet and metres, so you can move between them without guessing.

What Is a Nautical Mile?

A nautical mile is a unit of distance used in sea and air navigation. Its modern, internationally agreed definition is simple and exact: one nautical mile equals 1,852 metres. It is not an SI unit, but it is accepted for use with the SI system, and it remains the standard distance unit on nautical charts, in aviation, and in international maritime law.

The symbol used for the nautical mile is M in international maritime usage, although nmi is common in aviation and in general writing, and nm is also seen. Take care with that last one: in science, nm usually means nanometre, so nmi is the clearer choice outside a navigation context. On charts you may also see distances marked simply in minutes of latitude, which, as we shall see, amounts to the same thing.

The defining idea is older than the exact figure. A nautical mile was originally defined as one minute of arc of latitude — that is, one sixtieth of one degree measured along a meridian, the imaginary line running from pole to pole. There are 360 degrees in a full circle and 60 minutes in each degree, so there are 21,600 minutes in a full meridian circle. In other words, the circumference of the Earth measured through the poles is, by this definition, 21,600 nautical miles. That neat relationship between distance and position on a chart is the whole point of the unit.

Where the Number 1,852 Comes From

The Earth is not a perfect sphere. It bulges slightly at the equator and is flattened at the poles, so the length of one minute of latitude is not quite the same everywhere: it is about 1,843 metres near the equator and about 1,862 metres near the poles. Early navigators therefore found that a minute of latitude measured slightly differently depending on where they were sailing, which made a single fixed value desirable.

In 1929, the International Hydrographic Organization adopted 1,852 metres as the international nautical mile — a rounded value close to the average length of a minute of latitude. The United States adopted the same figure in 1954, and the United Kingdom followed in 1970, replacing its older Admiralty mile of 6,080 feet (about 1,853.18 metres). Since then, 1,852 metres has been the one figure used worldwide, so a nautical mile means the same thing on every chart, in every country, with no local variation to allow for.

That link to latitude is also why the nautical mile is so practical at sea. On a nautical chart, one minute of latitude on the vertical scale equals one nautical mile on the water. A navigator can measure a distance with dividers against the latitude scale at the side of the chart and read off the distance directly, with no conversion at all. Try doing that with kilometres or statute miles, and you need a scale bar and some arithmetic. The nautical mile, in short, was designed for the chart, and the chart was designed around the Earth.

The Conversion Formulas

Because 1,852 metres is exact, every conversion from the nautical mile is exact too — any rounding happens only when you shorten the answer for display. A statute mile, the ordinary mile used on roads in the United States and the United Kingdom, is 1,609.344 metres, so dividing 1,852 by 1,609.344 gives 1.15078 statute miles per nautical mile. A nautical mile is therefore about 15% longer than a statute mile, which is a difference worth getting right: confusing the two over a long voyage or flight adds up to a serious error.

For feet, divide 1,852 metres by 0.3048 metres per foot to get 6,076.12 feet per nautical mile. For yards, the figure is 2,025.37 yards. These are the numbers used when a distance first given in nautical miles has to be expressed in the imperial units still common in aviation in some countries, for example when altitudes and runway lengths are discussed alongside distances.

Worked Examples

The table below converts a range of distances from nautical miles into kilometres, statute miles and feet. Each row uses the same three multiplications: × 1.852 for kilometres, × 1.15078 for statute miles, and × 6,076.12 for feet.

Converting in the other direction is just as straightforward. To turn kilometres into nautical miles, divide by 1.852: 100 km ÷ 1.852 = 53.9957 nautical miles, and 10 km ÷ 1.852 = 5.3996 nautical miles. To turn statute miles into nautical miles, divide by 1.15078: 10 statute miles ÷ 1.15078 = 8.6898 nautical miles. The most common slip is to multiply when you should divide, so it helps to check the sense of your answer first: a distance in nautical miles should always be a smaller number than the same distance in kilometres, because the nautical mile is the longer unit.

Nautical Mile vs Statute Mile vs Kilometre

The three units answer three different questions. A statute mile (1,609.344 metres) is the land mile of the imperial and US customary systems, used for road distances and land measurement in the United States and, alongside metric units, in the United Kingdom. A kilometre (1,000 metres) is the metric land unit used almost everywhere else. A nautical mile (1,852 metres) is the navigation unit, used where position on the globe matters more than distance along a road.

In size order, the kilometre is the shortest of the three, the statute mile sits in the middle, and the nautical mile is the longest: 1 nautical mile = 1.852 km = 1.1508 statute miles. Put another way, 1 kilometre is 0.54 nautical miles, and 1 statute mile is 0.869 nautical miles. Keeping that order in mind is a quick sanity check for any conversion — if your answer breaks the order, the arithmetic has gone wrong somewhere.

Want it instant? Our nautical mile vs kilometer converter converts between nautical miles and kilometres as you type, so you can check any of the examples above in seconds.

What Is a Knot?

Speed at sea and in the air is measured in knots, and one knot is one nautical mile per hour. The name comes from the old chip log, in which sailors paid out a rope knotted at regular intervals and counted how many knots passed in a fixed time to judge their speed. The word stuck, even though the rope is long gone.

One knot equals 1.852 km/h, or 1.15078 miles per hour. So a ship making 20 knots is travelling at 20 × 1.852 = 37.04 km/h, which is 20 × 1.15078 = 23.02 mph. A commercial aircraft cruising at 480 knots is travelling at about 889 km/h. Note that a knot is already a speed, so saying “knots per hour” is a mistake — it would mean nautical miles per hour per hour. The correct phrasing is simply “20 knots”.

Knots also appear in weather forecasts for wind speed and in aviation for airspeed, for the same reason the nautical mile itself does: when distances are measured in nautical miles, speeds in knots keep the arithmetic clean. A ship travelling at 12 knots covers 12 nautical miles in one hour, 6 nautical miles in half an hour, and 1 nautical mile in five minutes — no conversion factor needed.

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Guide published by LengthConversion.online. One nautical mile is exactly 1,852 metres by international agreement.