Why Ships and Planes Use ‘Knots’ Instead of Miles per Hour

March 18, 2026 7 min read 6 Views

If you’ve ever glanced at a flight display or overheard a sailor talking shop, you might’ve caught a curious detail: speed isn’t measured in miles per hour. It’s measured in knots.

Not the kind you tie in a rope—though, interestingly enough, that’s not far from the truth.

At first, it feels like one of those odd traditions that just stuck around. Like why we still say “hang up” the phone. But knots aren’t just a quirky leftover from the past. They’re surprisingly practical, deeply tied to how we understand the Earth itself, and kind of elegant.

Let’s untangle it.

What exactly is a knot?

A knot is a unit of speed. One knot equals one nautical mile per hour. That’s the short version.

But then comes the obvious follow-up: what’s a nautical mile?

A nautical mile is based on the Earth’s geometry. Specifically, it’s equal to one minute of latitude.

If you imagine slicing the Earth into horizontal lines, each degree of latitude is divided into 60 minutes. One of those minutes equals one nautical mile, which is about 1.1508 regular (statute) miles, or 1.852 kilometers.

So when a plane cruises at 480 knots, it’s traveling 480 nautical miles per hour—directly tied to the Earth’s grid.

And that connection is where things get interesting.

It all started with rope, wood, and a bit of clever thinking

The term “knot” comes from a surprisingly hands-on method used by sailors centuries ago. Before GPS, radar, or even reliable clocks, mariners had to get creative.

They used a device called a chip log. Picture this: a piece of wood attached to a rope, with knots tied at regular intervals along the line. Sailors would toss the wood into the water and let the rope unspool behind the ship.

Then they’d measure how many knots slipped through their hands over a set period—often timed with a sandglass.

The number of knots counted was the ship’s speed.

It wasn’t perfect, of course. Wind, waves, and human reaction time all added some error. But for its time, it was brilliant. And the name stuck.

Why not just switch to miles per hour?

Fair question. After all, we use miles per hour on land, and kilometers per hour in most of the world. Why not standardize everything?

The answer has less to do with tradition and more to do with navigation.

Miles per hour are based on land measurements. They don’t map neatly onto the Earth’s coordinate system. Nautical miles do.

Because one nautical mile equals one minute of latitude, navigators can easily translate speed into position. If you’re traveling at 60 knots, you’re covering one degree of latitude every hour. That’s clean, intuitive math, especially when you’re plotting courses over long distances.

In other words, knots simplify navigation. They turn speed into something directly useful for figuring out where you are and where you’re going.

Knots respect that the Earth isn’t flat

Here’s a subtle but important point: the Earth is round (well, slightly squashed, but close enough). When you’re traveling across oceans or through the air, you’re moving along a curved surface.

Using nautical miles (and therefore knots) accounts for that curvature.

Latitude lines are evenly spaced, which makes them reliable for measurement. Longitude lines, on the other hand, converge at the poles, so they’re trickier. Nautical navigation leans heavily on latitude for this reason.

By tying speed to latitude, knots create a system that works smoothly across the globe. Whether you’re crossing the Atlantic or flying over the Pacific, the math stays consistent.

Pilots and sailors are basically speaking the same language

Another reason knots persist is standardization across industries.

Aviation and maritime navigation share a lot of overlap. Both deal with:

Long distances over featureless terrain (oceans, skies)
Global coordinate systems
Weather patterns that don’t care about borders

Using knots keeps communication consistent. A pilot, a ship captain, and an air traffic controller all understand speed in the same terms.

It’s a bit like how scientists use the metric system worldwide. It reduces confusion, especially when precision matters.

And in navigation, precision always matters.

There’s also a practical side in aviation

Aircraft performance is closely tied to airspeed, not ground speed. That distinction matters.

A plane flying into a headwind might have a ground speed of 400 knots but an airspeed of 480 knots. The engines, wings, and lift calculations all depend on that airspeed.

Knots integrate neatly with these calculations. Many aviation charts, instruments, and performance tables are built around nautical miles and knots. Switching to miles per hour would mean reworking a massive amount of infrastructure—not just physical instruments, but training, procedures, and safety protocols.

So yes, tradition plays a role, but it’s backed by a whole ecosystem of practical reasons.

How to translate knots to mph

If you’re more used to miles per hour, here’s a rough sense of how knots translate:

1 knot ≈ 1.15 mph
100 knots ≈ 115 mph
500 knots ≈ 575 mph

Commercial jets typically cruise between 450 and 500 knots. That’s about 520 to 575 mph, depending on conditions.

Ships, on the other hand, are slower. A large cargo ship might travel around 20 to 25 knots.

Why land travel went a different way

On land, we don’t use knots because we don’t need to.

Roads follow terrain, not latitude lines. Distances are measured between cities, landmarks, and intersections—not along a global grid. Miles and kilometers work perfectly well for that.

Also, land navigation doesn’t require the same level of global consistency. You’re rarely plotting a route across thousands of miles of empty space with no visual reference points.

So miles per hour stuck around for cars and trains. It fits the context.

Technology didn’t kill knots—it reinforced them

You might think modern technology would’ve replaced knots by now. GPS can calculate speed in any unit you want, after all.

But instead of fading away, knots became even more entrenched.

Why? Because all the underlying systems (navigation charts, aviation standards, maritime regulations, etc.) are built around them.

Organizations like the International Civil Aviation Organization (ICAO) and the International Maritime Organization (IMO) use knots as the standard unit. Changing that would be a massive, global effort with little payoff.

Plus, pilots and sailors train extensively using knots. It becomes second nature. Switching units could actually introduce confusion, which is the last thing you want in high-stakes environments.

So why do ships and planes use knots?

If we boil it down, it comes down to a few key reasons:

They align with the Earth’s coordinate system, making navigation simpler
They provide consistency across global industries, especially aviation and maritime
They integrate smoothly with existing tools and calculations
They’ve stood the test of time, proving both practical and reliable

Miles per hour might feel more familiar, but knots are better suited to the job.