Those giant numbers painted on US runways are not random: They tell pilots the direction they’re facing, with Runway 18 pointing roughly south and 9 east

If you’ve ever looked out a plane window at a US airport, you’ve probably seen them. Big white numbers painted directly on the runway. They might look like a decoration, but they are not. Every number is a short compass reading that tells a pilot which way the runway points, according to the Federal Aviation Administration’s Aeronautical Information Manual. When you know the trick, you can read a runway like a pilot, and a plain strip of pavement suddenly starts to make sense.

How a runway gets its number

Start with a compass, which is just a circle divided into 360 degrees. North is 0, which is also written as 360. East is 90, south 180, and west 270. The runway number is based on the runway direction, as measured on a magnetic compass, rounded to the nearest 10 degrees. The examples help. Runway 18 is a runway that runs 183 degrees. Runway 9 is a runway that is aligned at 87 degrees. In other words, you round the direction and drop the zero.


Runway 18 points roughly south, and Runway 9 points roughly east. By the same logic, Runway 36 is roughly north, and Runway 27 is roughly west. This is the direction a compass needle points, which is not exactly true north. There's one little quirk, too. If the direction ends in a 5, say, 185, the runway could be called either 18 or 19.

Why one strip of pavement has two names

A runway works like a two-way street because a plane can land from either end. Each end gets a number. The two numbers differ by 18. This is because the ends face in opposite directions, and opposite directions are 180 degrees apart. The number is also determined from the approach direction, and the angle is measured clockwise from magnetic north. Put simply, the number you see is the direction you are going when you come in to land. A single runway may be Runway 9 to a pilot approaching from the west and Runway 27 to a pilot approaching from the east.

At larger airports, there are often multiple runways in the same direction. Every number gets a letter to keep them apart. This is shown left to right as seen from the direction of approach. Two parallel runways get L and R; three get L, C, and R. C is for center. So a pilot told to land on Runway 28L, for example, knows to aim for the left-hand strip. At airports with more than three parallel runways, one set may be numbered 10 degrees away from the others (for example, 17 and 18) so that every runway still has a unique name.

Why the wind decides which way planes land

The wind direction given by the tower is also magnetic, and wind speed is given in knots, so a pilot can compare the wind with the runway number at a glance. Let's say the wind is blowing from 180 degrees, which is from the south. Runway 18 points south, so a plane landing there will fly head-on into that wind. The normal aim is to land into the wind, and FAA pilot handbooks include a chart for calculating headwind, crosswind, and tailwind.

When the wind is 5 knots or greater, controllers use the runway that most closely matches the wind. When the wind is less than 5 knots, they use what is called the calm wind runway. Another runway may work better for the airport's operations, or a noise-reduction plan may be in effect. Still, the manual says that tailwind and crosswind issues trump concerns about delays and noise plans.

Why the paint sometimes changes

You may think a runway number is permanent. The National Centers for Environmental Information, part of NOAA, explains that Earth's magnetic field is constantly changing and over time, this can even require airports to rename runways. For instance, Fairbanks International Airport in Alaska renamed its runway from 1L-19R to 2L-20R in 2009 because magnetic north had shifted enough to require it. NCEI expects the airport will need to update the name again around 2033. The runway itself never moved. Magnetic north did. The numbers are following magnetic direction, so the paint has to follow the numbers.

So the next time you see a runway number from your window, give it a try. Multiply by 10, and you have a rough idea of the heading. Runway 9 is approximately east, and runway 18 is approximately south. It is a small detail, but it makes the runway easier to read.