Why planes use knots: airspeed and aeronautical navigation

Pick up a flight manual, listen to air traffic control, or look at an aviation chart, and you’ll hear and read one unit for speed: knots. A Boeing 737 cruises at 450 knots, not 450 mph or 450 km/h. A Cessna 172 might cruise at 110 knots. This is not because the US aviation industry rebelled against metrication — it’s because aviation, like maritime navigation, is built on latitude and longitude, and the knot is the unit that makes that system work. This guide explains the aeronautical logic behind the knot and how to convert any airspeed into miles per hour.

The knot in aviation: the conversion

In aviation, as on water, one knot equals approximately 1.15078 miles per hour. To convert any indicated airspeed to mph, multiply by this factor. Here are the speeds you’ll encounter in general aviation and commercial flight:

A commercial jetliner at 450 knots is moving at 518 miles per hour through the air — fast enough to cover a coast-to-coast flight in about 5 hours with prevailing winds and routing.

Why aviation adopted the knot: the latitude connection

To understand why pilots use knots, you need to understand how air navigation works. A pilot flying from New York to Los Angeles navigates using aeronautical charts. These charts are covered in a grid of latitude and longitude lines. Latitude is measured in degrees and minutes of arc. The equator is 0° latitude; the North Pole is 90° North; each degree is divided into 60 minutes. This coordinate system is the same one that maritime navigation relies on, which is why nautical distances work differently from land miles.

One minute of latitude, measured along any meridian, is a fixed distance: exactly one nautical mile (1,852 meters). It doesn’t matter where on Earth you measure it — one minute of latitude is always one nautical mile.

So one knot — one nautical mile per hour — means a plane travels exactly one minute of latitude per hour. A pilot heading north and traveling at 100 knots will cover 100 minutes of latitude in 1 hour (about 1.67 degrees). Look at your aeronautical chart with its latitude scale, and you can instantly see how far north or south you’ve gone. Groundspeed, wind correction, fuel burn — all of it ties back to that direct relationship between speed in knots and distance in minutes of latitude.

Miles per hour has no such tie-in. A statute mile is a land measurement (1,609.344 meters) with no connection to latitude. If pilots switched to mph, every time they calculated distance to a waypoint, they’d have to convert between their chart (marked in minutes of latitude) and their airspeed (now in mph). That’s an extra mental step on every flight, in every calculation, for no gain.

Aviation inherited knots from maritime tradition — and never had reason to switch

When aviation began in the early 1900s, the first pilots borrowed heavily from maritime navigation. They used the same charts (plotting courses by latitude and longitude), the same speed unit (knots), and the same basic principles. Aircraft were slower than ships then, but the navigational system was the same. And because all international aviation operates on a shared standard, there was never a moment when a single country could or would switch unilaterally.

The International Civil Aviation Organization (ICAO), formed in 1944, mandates knots for all aviation speed reporting. Japan uses km/h on roads but knots in the cockpit. China does the same. France, Germany, and every other nation follows the ICAO standard. A pilot traveling internationally must understand knots; even a plane flying entirely within a country that uses km/h on roads will report airspeed to air traffic control in knots.

This is why airspeed in knots is not an American metrication holdout — it’s a global aeronautical standard, rooted in the navigational geometry of latitude and longitude.

Airspeed vs. groundspeed: why pilots care about both

A concept critical to aviation is the distinction between airspeed (speed through the air) and groundspeed(speed over the ground). Airspeed is what the aircraft’s instruments measure — how fast the plane is moving relative to the air mass around it. Groundspeed is what the flight plan needs — how fast you’re actually moving across the Earth.

Wind changes everything. Suppose a plane is flying from west to east and maintains an airspeed of 450 knots. If there’s a 50-knot jetstream wind pushing from the west, the groundspeed is 450 + 50 = 500 knots. If flying into a headwind, it’s 450 − 50 = 400 knots. The pilot’s altitude, fuel planning, and arrival time all depend on groundspeed, not airspeed. But controlling the aircraft depends on airspeed. This is why aviation charts list both: true airspeed for flying, and estimated groundspeed for navigation.

Both are reported in knots, because both are measured against navigation grids based on latitude and longitude.

A typical flight example

Imagine you’re planning a flight in a general aviation aircraft — say, a Piper Cherokee with a cruise speed of 110 knots. Your destination is 350 nautical miles away. At 110 knots, that flight will take 350 ÷ 110 ≈ 3.2 hours of flying time (not counting climb and descent). If you convert to mph, 110 knots ≈ 126.6 mph, and 350 nautical miles is 403 statute miles, so 403 ÷ 126.6 ≈ 3.2 hours. Same answer.

But the pilot works in the first set of numbers because the chart is marked in nautical miles (to match latitude/longitude), and the airspeed indicator reads in knots. The conversion to mph is for telling friends on the ground what speed you’re traveling, or for understanding an aircraft’s top speed in land terms.

The knot is not outdated — it’s the right tool for the job

Every pilot in the world, from bush planes in Alaska to commercial jets over the Atlantic, uses knots. Not from stubbornness, not from an American veto of metrication, but because the knot is the only unit that directly connects speed to the latitude-and-longitude navigation system. A pilot who knows their airspeed in knots can instantly translate that into progress on a chart marked in degrees and minutes of latitude.

It’s not a US eccentricity. It’s international aeronautical standard, rooted in navigational geometry. The next time you hear a pilot report 450 knots or see an aircraft spec in knots, remember that the unit is there for a reason: to connect flying to the Earth’s coordinate system.