Nautical mile and knots: what is it and how to quickly calculate it?

Nautical miles (nm) and knots (kn or kt) are the most important units of measurement in marine and air navigation. The origins of both terms lie in geographical calculations, and the units of measurement themselves have been actively used by sailors around the world for hundreds of years. But why is one nautical mile equal to 1852 meters, and a knot to 1.85 km/h? Let's look at it step by step.

This article is an adaptation of the material "Nautical mile and speed in knots: where do these maritime measurements come from?" by Yves Melchior for the website BoatNews.com

Nautical mile: where did it come from and how to use it?

Every (conscientious) sailor knows that a nautical mile is equivalent to a distance of 1852 meters. Why such a value? Why not adopt a more precise unit of measurement? The short answer: the value of the nautical mile is related to the curvature of the Earth's surface.

Nautical Miles and Planet Earth

A nautical mile is a unit of distance used in marine and air navigation. Unlike the kilometre, which is part of the International System of Units (SI), the nautical mile was defined to facilitate the measurement of distances at sea. 

The nautical mile was originally defined as one minute of arc of the earth's meridian, or 1/60 of a degree. This is a convenient unit of measurement because coordinates on maps are given in degrees of latitude and longitude. The nautical mile makes it easy to calculate distances on such maps.

To put it in slightly more mathematical terms:

• The Earth is a sphere, so we can draw a continuous arc between the equator and the pole.
• This arc represents 90 degrees of latitude. Each degree is divided into 60 minutes, or 5400 minutes between the two poles.
• The distance from the equator to the pole is approximately 10,000 km, so dividing by 5400 minutes gives 1852 meters per arc minute.

Thus, one arc minute is equal to 1852 meters.

Historical features associated with the nautical mile

Early nautical mile values varied between countries. For example, the British nautical mile was 1853.2 meters, while the French mile was 1852.3 meters. This led to confusion for a long time due to the lack of standardization, but this all changed in the early 20th century.

In 1929, the International Hydrographic Conference decided to standardize the nautical mile at 1852 meters. Today, this value is the international standard.

Another interesting point that for some reason not everyone knows: the nautical mile is different in length from the land mile. The land (or regular) mile is shorter, its length is 1609.34 meters. The difference is due to the fact that the regular mile is based on old Roman units of measurement, and the nautical mile, as we found out, is associated with the geographical features of the planet.

Knots: The Right Speed for Navigation

At sea, the speed of any vessel is expressed not in the familiar kilometers per hour, but in a more exotic unit of measurement - knots. One knot (aka knot, kt, kn, nd) corresponds to the actual speed of 1 nautical mile per hour or 1.852 km/h.

The name of the unit of measurement originates in the 16th-17th centuries, when sailors measured the speed of a ship using the prototype of a modern log - the so-called "side log". This "speedometer" was a reel with a long rope, on which knots were tied at equal intervals. The principle of application was as follows:

• one end of the rope was tied to a piece of wood (lag), which was thrown overboard;
• the ship moved in the opposite direction from the log, gradually unwinding the rope;
• The sailors would time themselves (usually 30 seconds on an hourglass) and count how many knots were unwound.

The number of these knots was equal to the ship's speed in knots. Although this method of calculating speed is no longer used, the unit "knot" retains its name as a unit of ship and wind speed.

One of the advantages of measuring speed in knots is that knots are identical to miles per hour. That is, a speed of 10 knots corresponds to 10 nautical miles. This makes it easy to convert speed to distance traveled.

Convert Nautical Miles and Knots to SI

While nautical miles and knots are indeed convenient for sailors, there are often times when you need to quickly convert them to standard international units. Here are some tricks to help speed up the process.

Conversion of knots to km/h

Value: 1 knot ≈ 1.85 km/h.

Rule of thumb: multiply the nodes by 2 and subtract 10% from the result.

Example: 10 nodes are given. 

1. 10 * 2 = 20
2. 20 - 10% (i.e. 2) = 18 km/h

Convert km/h to knots

Value: 1 km/h ≈ 0.54 knots.

A rule of thumb is to divide the km/h value by 2 and add 10%.

Example: given 20 km/h.

1. 20 / 2 = 10
2. 10 + 10% (i.e. 1) = 11 nodes

Convert Nautical Miles to Kilometers

Meaning: 1 nautical mile ≈ 1.85 km.

Rule of thumb: multiply the nodes by 2 and subtract 10% from the result.

Example: given 5 miles.

1. 5 * 2 = 10
2. 10 - 10% (that is 1) = 9 km.

Convert kilometers to nautical miles

Value: 1 km ≈ 0.54 nautical miles.

A rule of thumb is to divide the km/h value by 2 and add 10%.

Example: 10 km are given.

1. 10 / 2 = 5
2. 5 + 10% (i.e. 0.5) = 5.5 miles.

For a faster but less accurate calculation, you can simply multiply or divide the actual values by 2. This result will be rounded, but for those cases when there is no time to carefully calculate, it will do. Here is a small comparison table of translation results:

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13.03.2025