Runways are numbered according the magnetic compass direction they are oriented to. They consist of two numbers, one at each end of the runway, each being the reciprocal of the other on the magnetic compass (180° opposite from each other).
One number is formed by rounding the compass bearing of one end of the runway up or down to the nearest 10° and dropping the last digit; if this results in a single digit, add a zero to the left of it. The other number is the reciprocal of the first number (see the table of reciprocal runway numbers below). For example, if a runway is aligned north-south, then it is 18/36, not 00/18. The lower number is always listed first.
Runways have two ends. Each end is considered a different runway, even though it is the same actual runway surface.
If the compass heading of a runway is 122° you would round it down to 120 and drop the last digit, leaving you with 12. Thus it is called runway 12/30.
If the compass heading of a runway is 37°, you would round it up to 40 and drop the last digit, leaving you with 4. Since this is a single digit, you add a zero to the beginning, giving you 04. Thus it is called runway 04/22. Both Ellington and Hobby airports have a 04/22 runway.
|North/East end||South/West end|
For example, in the runway diagram below, this is considered runway 9/27. If you are looking to the east, you are on runway 9, if you are looking to the west, you are on runway 27.
On an airport diagram, you will see the exact magnetic heading of that runway printed along side the runway:
This is an excerpt from the KIAH airport diagram. It shows the layout and connecting taxiways for Runway 9/27. The actual magnetic bearing you would see on the compass when sitting on the runway facing east is 085°, and if you did the same on the other end, facing west, it would be 265°.
When an airport has multiple parallel runways, they will all be oriented the same direction. When this happens, they assign a letter, either L (left), C (center), or R (right) to the runway number.
Bush Intercontinental airport is a good example of this. They have two sets of parallel runways. There is 15L/33R – 15R/33L and 08L/26R – 08R/26L. See the examples below:
Runway 15L/33R and Runway 15R/33L
Runway 08L/26R and Runway 08R/26L
Sometimes when an airport has multiple parallel runways, the magnetic alignment of the runways may not be exactly the same. If the runways are far enough apart, the curvature of the earth comes into play, and the orientations are off slightly, but due to the rounding used to get the runway numbers, the actual assigned runway numbers remain unchanged.
The exact headings would only come into play if there is an instrument approach procedure for that runway. An example of this is at Dallas Fort Worth (DFW) airport, where Runways 13/31 are on opposite sides of the airport. Note the magnetic direction difference of about 4° on all 4 runways:
When airport designers first develop the plan for an airport, they look at historical weather data, mainly what direction the wind is blowing the majority of the year, and design the airport runway layouts to favor this.
For example, here in Texas, the wind is usually out of the south (approx. 160° to approx. 170°), so most airports in the Houston area will have their main runways running north / south. There will usually be other runways oriented in some east / west direction, again, as dictated by historical weather patterns or as dictated by local building restrictions, city planning, etc.