Everything You Need to Know About Nautical Miles & Knots

If you’ve ever thought about the speed a ship travels at and wondered why it’s measured in knots, or why sailors talk about nautical miles instead of regular miles, you’re not alone. These terms can feel a bit on the mysterious side if you’re used to driving cars, reading road maps, or figuring out distances on land. 

In reality, nautical miles and knots are logical, practical, and deeply tied to how humans have navigated the oceans for centuries. In this blog post, we break down what they mean, why they’re used at sea, and we’ll also throw in a few fun facts that make maritime navigation even more interesting. 

What is a Nautical Mile? 

The first thing to know is that a nautical mile is a unit of distance that’s used in both maritime and aviation navigation. 

One nautical mile is equal to: 

• 1.1508 statute miles (land miles) 

• 1.852 kilometers 

• 6,076 feet 

So yes, a nautical mile is slightly longer than the mile you see on road signs or on Google Maps. But the key difference isn’t just the length, it’s how the nautical mile is defined. 

Why Nautical Miles are Based on the Earth 

Bear with us now, because this is going to get mathematical!  

Unlike land miles, nautical miles are based on the geometry of the Earth and a nautical mile is defined as: 

One minute of latitude along the surface of the Earth. 

The Earth is divided into: 

• 360 degrees of latitude 

• Each degree divided into 60 minutes 

That gives us 21,600 nautical miles around the Earth from pole to pole and back again. 

And what this means is that nautical miles tie directly into latitude and longitude, which are the foundation of marine navigation. If you know your position on the globe, nautical miles let you calculate distance quickly and accurately without converting between systems. 

This is one of the main reasons nautical miles are used both at sea and in the air. 

Are you still with us?! If so, let’s move on to knots. 

How is a Knot Defined? 

The crucial thing to know is that a knot is a unit of speed, not distance. 

One knot equals: 

• One nautical mile per hour 

So, if a ship is traveling at 20 knots, it is moving at 20 nautical miles per hour. 

In land terms, that’s roughly 23 miles per hour and for most vessels, that’s a perfectly reasonable cruising speed. 

Where Did the Term “Knot” Come From? 

The word knot comes from an old and surprisingly clever method of measuring speed at sea. 

Before GPS and digital instruments became the norm, sailors used a device called a log line. This was a rope with evenly spaced knots tied along its length, attached to a piece of wood thrown over the stern. 

The process worked like this: 

• The wooden log was thrown into the water 

• A sandglass measured a fixed amount of time 

• Sailors counted how many knots passed through their hands during that time 

The number of knots counted equaled the ship’s speed in nautical miles per hour. 

Obviously ships today no longer measure speed this way but the term knot has remained part of maritime language. 

Why Maritime Terms are Different from Land Terms 

Shipping developed and evolved long before modern roads, cars, or standardized maps s sailors needed a system that worked on a curved planet with no fixed reference points. 

Nautical miles and knots make sense at sea because: 

• They align with latitude and longitude 

• They work consistently anywhere on Earth 

• They allow easier chart plotting 

• They support accurate dead reckoning and position fixing 

Using land miles at sea would require constant conversion and wouldn’t match chart scales. Therefore it’s fair to say that the maritime system isn’t old-fashioned – it’s efficient! 

Aviation uses the same system for similar reasons: aircraft navigate across the globe, not along roads. 

Nautical Miles vs Statute (Land) Miles 

Are you thoroughly confused?! Here’s a simple comparison: 

Measurement Type	Used For	Length
Nautical mile	Sea and air navigation	1.852 kilometers
Statute mile	Land travel	1.609 kilometers

Because nautical miles are longer, speeds measured in knots sound faster than the same number of miles per hour - this often causes confusion for people new to shipping. 

How Fast Do Ships Travel in Knots? 

Different types of vessels have very different speeds. 

Typical speeds include: 

• Large container ships: 18 to 22 knots 

• Cruise ships: 20 to 24 knots 

• Bulk carriers: 12 to 15 knots 

• Tankers: 12 to 16 knots 

• High-speed ferries: 30 to 45 knots 

• Naval vessels: 30 knots and higher 

Fuel efficiency, weather, and schedules all influence a ship’s speed. In recent years, many commercial ships have reduced speed to cut fuel costs and emissions, a practice known as slow steaming. 

Nautical Miles in Charts and Navigation 

Nautical charts are designed so that distance can be measured directly using latitude. 

If you measure the distance between two points on a chart using the latitude scale on the side, the result is in nautical miles. No conversion is required. 

This is one of the most elegant features of marine navigation and one reason the system has survived centuries of technological change. 

Useful Nautical Fast Facts 

• A ship traveling at 1 knot will cover one minute of latitude every hour. 

• The Earth’s circumference is about 21,600 nautical miles. 

• Wind speed at sea is also measured in knots. 

• Weather forecasts for mariners use knots, not miles per hour. 

• Even modern GPS systems still display speed in knots for maritime users. 

Once you start thinking in nautical miles and knots, the system becomes second nature. 

Why You Still See Nautical Terms Today 

Despite GPS, satellite tracking, and digital charts, nautical miles and knots remain the international standard. 

They are used by: 

• Commercial shipping lines 

• Navies 

• Fishing fleets 

• Yachts and sailboats 

• Aviation 

• Meteorological services 

This consistency is critical for safety and communication. For example, a ship’s master in Asia and a port authority in Europe must be speaking the same language when it comes to speed and distance. 

Handy hint: want to know the meaning behind more maritime phrases, words and acronyms? Take a look at Martide’s shipping glossary, which has you covered from A through Z!  

Common Misconceptions About Knots and Nautical Miles 

• A knot isn’t a distance, it’s a speed. 

• Knots per hour is incorrect: knots already mean per hour. 

• Nautical miles are not outdated – they’re purpose-built! 

Understanding these basics helps avoid some very common mistakes. 

Frequently Asked Questions 

Why do sailors not use kilometers or miles? 

Because nautical miles align directly with the Earth’s coordinate system, making navigation simpler and more accurate. 

Are knots used on rivers and lakes? 

Yes. Even inland waterways often use knots for consistency with maritime standards. 

Can I convert knots to miles per hour? 

Yes. Multiply knots by 1.1508 to get miles per hour. 

Do all countries use nautical miles at sea? 

Yes. Nautical miles and knots are international standards defined by global maritime and aviation organizations. 

Is GPS changing this system? 

No. GPS displays data, but the underlying units used at sea remain nautical miles and knots. 

Final Thoughts 

Nautical miles and knots aren’t quirky maritime traditions; they’re practical tools built around the shape of the Earth and the realities of navigating open water. 

Once you understand where they come from and why they’re used, they (hopefully!) start to feel logical rather than confusing. And the next time someone says a ship is making 20 knots, you’ll know exactly what that means and why it matters!