There are numerous ways to measure distance within the imperial and metric systems, one of which is the nautical mile. It's only been within the last 70 years that this unit of measurement has officially been used in the US and UK, despite being invented nearly 400 years ago.

Being able to navigate the sea and sky is important if you don't want to be left drifting through vast areas of empty waters or sky. It's also essential to know the exact distance that you will need to travel so that you are properly equipped with fuel and other necessities.

The nautical mile is different from the distance measurements that are used on solid ground. It isn't based on the other units used in the metric or imperial systems. Instead, it's inspired by the shape of the Earth and how this affects distance traveled.

In this guide, we'll look at why there is a different unit of distance measurement for nautical and aviation travel and the conversion rate between nautical miles and units of measurement used on land.

A nautical mile (nm) has a standard length of 6080 feet (1.151 land miles or 1853 meters). It is used for navigation both in the air and on the sea but isn't commonly used for land travel. This unit of measurement is used around the world, both in countries that use the metric system and countries that use the imperial system.

Nautical miles are based on the Earth's longitude and latitude. The Earth isn't a perfect sphere, as it is slightly flattened at the south pole and north pole. This means that the curvature of the Earth affects the measurement traveled over a long distance. It's more accurate to use latitude and longitude coordinates when planes and ships travel long distances than units based on measuring distance across land.

Kilometers and miles can typically be used when traveling for short journeys when the Earth's curvature won't have a massive impact on distance. Nautical miles are essential, however, when sailing or flying long distances. The nautical mile is also used in connection to the speed at which nautical vessels and aircraft travel.

One knot, which is used to measure ship speed at sea and aircraft when flying, is one nautical mile per hour. It originated in the 17th century when sailors wanted to see how fast their ships traveled. They would tie multiple knots in a piece of rope, tie it to their ship and count the knots in the unraveled rope between the ship and their starting point in a timed period. This would give them a rough idea of how fast the ship could sail in certain sea conditions.

British mathematician Edmund Gunter is often considered the early inventor of the nautical mile. In the early 1600s, Gunter set out to try and find ways to measure the speed and position of ships at sea. He was able to develop a navigational tool that could measure latitude, which he believed could be used to create a new unit of distance measurement at sea.

Gunter proposed that a nautical mile would be one minute or one-sixtieth of one degree of latitude. He then used the circumference of the Earth (believed to be 24,024 statute miles) to define a single nautical mile as 6,080 feet (1853 meters). This used the basis that the length of one minute of arc was at 48 degrees latitude.

However, the nautical mile didn't have an official definition until a few hundred years later, in 1929. During this year, Monaco’s first International Extraordinary Hydrographic Conference accepted a nautical mile as 6,076 feet (1,852 meters), only slightly less than Gunter had originally proposed it as.

The US used slightly different measurements until 1954, when they officially adopted the international nautical mile. It wasn't until 1970 that the UK also adopted the nautical mile in the English measurement system as their official unit of measurement at sea and in the air.

Nautical miles are based on the Earth's latitude and longitude coordinates. If you were to cut the Earth in half and look at the circle of the Earth's circumference around the equator, the circle can be divided into equal portions that measure 360°. One of these degrees can be divided into 60 minutes. One minute of arc around Earth is considered one nautical mile. To sail around the Earth's equator, you would have to travel 21,600 nautical miles (24,857 miles or 40,003 kilometers).

You can roughly work out how long your boat will take to arrive at your destination if you know the distance and speed that you will travel at. The equation you will need to use is Time = Distance/Speed.

For example, if the route you are planning to take is 50 nautical miles and you plan to travel at a consistent speed of 10 knots, the equation would be:

T = 50 ÷ 10 = 5

As one knot is equal to one nautical mile per hour, it would take you approximately five hours to reach your destination.

If you wanted to calculate the time it would take you to travel via sea, you could use the same equation in a different format. It would look like D = T x S. To calculate the speed you are traveling at, you would need to know how far you have traveled and how long it has taken you. The formula for this equation is S = D/T

Nautical charts show a section of the sea and nearby coastal regions, along with a specified scale so that you know what the map's relation is to real-life measurements. For example, if the map's scale is listed as 1:30,000, it means that one inch on the map represents 30,000 inches in actual size. Therefore, an inch on the map is equivalent to approximately 0.4 nautical miles. Reversing the conversion means that one nautical mile in real life is approximately 2.5 inches on the map.

Before you set off on a long journey by air or sea, it's a good idea to look at a map so that you see how many nautical miles you are likely to travel. This can help you calculate how long it will take you to reach your destination when weather and sea conditions are taken into consideration.

The standard international conversion rate is one nautical mile to 1.852 kilometers and 0.539957 nautical miles to one kilometer. Therefore, to convert between the two, you would need to multiply the given number of nautical miles by 1.852 or the given number of kilometers by 0.539957 nautical miles.

For example, if a stretch of water measures 70km, you would have to do the following calculation to work out how far that is in nautical miles:

70 x 0.539957 = 37.79699 nm

Therefore, 70km is approximately equal to 37.79699

A distance of 40 nm to km would be:

40 x 1.852 = 74.08 km

The table below shows the conversion rate between nautical miles and different units of length:

Unit (single)Conversion to nautical miles (nm)

If you wanted to travel from Dover, UK, to Calais, France, you would travel 27 nautical miles. This would take you approximately two and a half hours if your ship was moving at 10 knots. The journey from the port of Portland, Oregan, to the port of Astoria, Oregan, is approximately 75 nautical miles, which would take approximately seven and a half hours.

The table below shows a sample of distances between one port to another and how long it would take to sail the journey when traveling at 10 knots.

(FROM) Port #1(TO) Port #2Distance (nm)Days at sea
Los Angeles, USSeattle, US12745.3
Houston, USNorfolk, US20578.6
Portsmouth, UKHamburg, Germany5712.4
Algeciras Bay, SpainPort de Lyon Edouard Herriot, France9003.8
Oakland, USSavannah, US578224.1

A 'mile' originates from the Latin 'mille passus', which translates as one thousand paces. This is the average distance that Roman soldiers would travel with a stride being two paces. However, a nautical mile is based on the Earth's longitude and latitude. It is a more modern unit of measurement and therefore is based on the Earth rather than a walking concept from thousands of years ago.

A standard mile works well for measuring distance on land, but the curvature of the Earth means that this unit of measurement would be inaccurate over a large distance. This is also the reason that kilometers aren't typically used at sea or in the air, although they may be used for short distances where the Earth's curvature doesn't have a massive impact.

We don't use nautical miles to measure distance on land because it can't be accurately measured in paces. Nautical miles were also founded much later than miles, which is the unit for distance used in the imperial system.

Why is it called a nautical mile?

The word 'nautical' is often used to describe ships, sailors, and maritime navigation. It is derived from the Latin word 'nauticus', meaning sailor, and the Greek word 'nautikos', meaning naval. Traditionally, nautical miles were used to measure distances at sea, although they are now also used by air and space navigation.

Is there a 'nautical kilometer'?

No, there's no such thing as a nautical kilometer in the metric system. The nautical mile is used internationally in countries whether they use the imperial system or not. As the nautical mile has no real connection to a standard mile, it isn't directly connected to the metric or imperial system.

How fast is a knot?

Speeds at sea are measured by knots. A single knot is equal to one nautical mile per hour, 1.15 statute miles per hour, or 1.852 km per hour.

The table below shows the speed of knots in comparison to the average speed of people and other vehicles.

KnotsSpeed in mphSpeed in kmphContext
Three3.445.55Average walking speed
Seven8.0012.87Average running speed
Ten11.5118.55Average cycling speed

A nautical mile is used at sea and in the air to measure distance than land units of measurement would more accurately. It was officially recognized as a unit by the International Hydrographic Organization in 1929, although it wasn't adopted by the US and the UK until several decades later.

Vessels at sea measure their speed in knots — one knot is equal to one nautical mile per hour. To put this in context, three knots is the average speed that someone can walk.

The official conversion rate between nautical miles and kilometers is 1.852km in one nautical mile. There are approximately 1.15078 miles in one nautical mile.