Difference Between Nautical Miles and Knots?

1. What is Nautical Miles?

The nautical mile is the unit of measurement of distance at sea. An arbitrary value of 6080 ft or 1852 meters is accepted as the length of one nautical mile. These valves are set by IHO (International Hygroscopic Organization). A nautical mile is a standard quantity that is fixed all over the earth.

The nautical mile has also been defined concerning the earth’s surface. A Nautical Mile is the distance you cover when you travel along a longitude, within a slice of latitude that’s one-sixtieth of a degree (1/60).

Diagram of nautical mile 

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2. Why Nautical Mile?

While we commonly use meters and kilometers to measure distances on land, these units become impractical when navigating on water. In the maritime industry, the nautical mile becomes the preferred measure for measuring the distance covered or remaining. Unlike land-based units, the nautical mile is specifically designed to address the challenges and scale of sea travel, making it a more suitable and accurate metric for distances over open water.

2.1 Symbols for Nautical Miles

The symbol used for the nautical mile is not internationally agreed. Hence, there are various symbols set by different organizations. 

  • M    – International Hygroscopic Organization 
  • NM – International Civil Aviation Organization
  • nmi – Institute of Electrical and Electronic Engineers
  • nm  – Used in maritime applications and text, US Govt Coast Pilots

One-tenth of a nautical mile is called a Cable.

2.2 Sea Mile

The sea mile is the distance of one minute of arc measured along the meridian at the latitude of measurement(Great Circle). This distance is not fixed as the earth is flattened at the poles and is not the true sphere.

The distance is approximately 1843 meters at the equator and 1862 at the poles. 

2.3 Cable

Cable is defined as one-tenth of the nautical miles i.e. 184.3m to 186.2m or 100 fathoms according to latitude. These are used for the short-range navigational purposes. Cable is the unit named after the length of the anchor cable on a sailing vessel.

The cable is approximately 0.1’ or 0.1nm.

2.4 Fathoms

Fathom is a traditionally used nautical linear measure that is equal to 6 feet. It is used to measure the length of the ropes, hawsers (thick ropes), etc. Fathom is also used as a unit while taking soundings and depth of tanks and holds.

It is superseded by the meters i.e.,

1 fathom = 1.8288 meters 

1 meter = 0.5468 fathoms 


Lead Line Fathoms

The length of one outstretched arm is equal to one fathom or 6 feet

3. What is Knots?

Knots are used as a unit of speed used in marine navigation. It indicates nautical miles coveted per hour i.e.,1.852 km per hour.

In the 17th century, sailors used a “common log” to measure ship speed. This log had a rope with knots at intervals, connected to a floating piece of wood shaped like a slice of a pie. Mariners used to lower the wood piece and allow it to float freely behind the ship for a specific period. When the time was up, by counting the knots passed in a set time, sailors estimated their ship’s speed. This practice gave rise to the term “knot” for measuring nautical speed

howing knots 

3.1 Symbols For Knots

A knot is used in meteorology, maritime, and navigation, and the symbol representing a knot is:-

  • Kn – ISO Standard/IEEE
  • Kt – ICAO 

4. Frequently Asked Questions

Q1 One Nautical Mile is equal to how many kilometers?

Ans. One nautical mile is equal to 1.852 Kilometers.

Q2 Why the distance at the poles is not equal to the distance at the equator?

Ans. Since the earth is not a perfect sphere and is flattened at the poles therefore distance at the poles is greater as compared to the distance at the equator.

Q3 Why do sailors use knots?

Ans. In simpler terms, knots are a way to measure distance on the water and they are easier to use than land miles. This is because knots are based on the Earth’s lines of latitude. One nautical mile (knot) is equal to one minute of latitude. 

Q4 Are nautical miles the same as air miles?

Ans. In simpler words, an air mile is the same as a nautical mile. It is a standard measure used worldwide and is equal to 6,076 feet or 1,852 meters.

Q5 What speed is 1 knot?

Ans. One knot is equal to one nautical mile per hour.

Q6 Why knots instead of miles per hour?

Ans. Knots are easier and more accurate to navigate with nautical charts all around the world.

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