I'm not a professional pilot, just an enthusiast, so this might seem obvious to others; but I noticed that airports don't use sequential numbering for runways, starting from 1.

Liverpool (for example) has runway 9/27; so how are these numbers assigned? Also, what do Left and Right signify?

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    $\begingroup$ See also: en.wikipedia.org/wiki/Runway $\endgroup$ – yankeekilo Dec 18 '13 at 10:57
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    $\begingroup$ @abelenky I guess it's just one things I've never looked up before now! It's planes that interest me, not airports. $\endgroup$ – Danny Beckett Dec 18 '13 at 15:52
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    $\begingroup$ Someone needs to send this to Hollywood so they can stop talking about "Runway 47" or other such garbage occasionally $\endgroup$ – SSumner May 5 '14 at 15:42
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    $\begingroup$ @SSumner well if you add another runway after having 63 parallel runways all going north, 47 is next in the bag... $\endgroup$ – Nick T Jun 1 '14 at 18:23
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    $\begingroup$ @jwzumwalt there was a debate around that question, yes, but I agree with you that it should have stayed, exactly like this one. unfortunately you decided to delete the question and not bring the discussion to meta. also, please do not look at the fake internet points when deciding if one is an enthisiast or not. $\endgroup$ – Federico Jun 3 '18 at 6:26

Runways are usually numbered according to their direction, more precisely called runway magnetic bearing or QFU (see Q codes).

Consider a plane flying toward the runway on final approach in a day without any wind. Divide its magnetic heading by 10, round it to the nearest whole number and you'll usually get the runway number. For example, if the magnetic heading is 345° then 345/10=34,5, so the runway number will be 35 which will be a runway used for landings (and takeoffs) to the north.

Opposite ends of the same runway have different numbers, 18 (which represents 180 degrees) apart. A runway with 35 for landings to the north will have runway 17 for landings to the south. Even though these are the same strip of concrete, they are treated as separate runways by pilots and controllers.

If there are two airports near one another with runways at the same angle, sometimes one of the airports will add or subtract one from the runway number to help planes differentiate between the airports.

Occasionally a runway number will change when the magnetic declination angle changes in such amount, making the runway magnetic bearing divided by 10 and rounded to the nearest whole number increase or decrease.

Some runways in areas of large magnetic declination use true instead of magnetic headings for the runway numbers. This is not unusual in northern Canada and Greenland.

When there is more than one parallel runway at an airport, L, R, or C may be appended to the runway number for Left, Right, or Center. These are based on the approach direction, so, for example, the runway 35L would be called 17R from the opposite direction.

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    $\begingroup$ Yes, the Runway numbers at PAO changed from 12/30 to 13/31 around ten years ago or so. $\endgroup$ – Edward Falk Dec 18 '13 at 19:26
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    $\begingroup$ Just a remark, I believe that the 4 runways of CDG are all parallel, just numbered one-off. So your 3rd paragraph can apply to a singe airport, too. $\endgroup$ – yo' Feb 4 '14 at 17:29
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    $\begingroup$ @tohecz Not quite: they appear to be parallel but, according to Wikipedia, they're numbered 08L/26R, 08R/26L, 09L/27R and 09R/27L. The six runways at Denver International are 16L/34R, 16R/34L, 17L/35R, 17R/35L (all parallel) and 7/25 and 8/26 (parallel to each other, at right-angles to the other four but presumably not labeled as an L/R pair because they're at opposite sides of the airport). Meanwhile Chicago O'Hare's runways include the parallel 9L/28R, 9R/27L, 10L/28R and 10C/28C but no 10R/28L -- I guess it used to exist but was closed? $\endgroup$ – David Richerby Mar 11 '14 at 20:11
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    $\begingroup$ I would add that some airports far north have runway with suffix T (e.g. 17T/35T in Resolute Bay) meaning the number is derived from true heading rather than the usual magnetic heading. $\endgroup$ – Jan Hudec Aug 7 '14 at 4:43
  • $\begingroup$ KATL has 9R/9L and 10R/10L, which are all parallel. Presumably after 3 parallel runways they drop the "Center" designator? $\endgroup$ – zymhan Nov 3 '20 at 16:24

Runway numbers are determined by their heading, e.g. for Liverpool, runway 9/27 is facing approx. magnetic heading 90° in one direction, and 270° in the opposite direction.

If an airport has parallel runways, these would then be marked Left, Center and Right, e.g. 9L/9C/9R.

Heathrow has two parallel runways, the 'right' runway has 09 R on the east and 27 L on the west end (and the other is 09L/27R).

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    $\begingroup$ An interesting follow-up to this is that sometimes runway numbers change! The surface of the earth is not stationary, it moves small amounts over time and over larger periods of time, it can move enough that a runway's direction can shift to where an increase or decrease in its number is warranted. $\endgroup$ – mah Dec 18 '13 at 11:28
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    $\begingroup$ A minor point, but I think the headings are magnetic rather than true. $\endgroup$ – user1994911 Dec 18 '13 at 11:47
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    $\begingroup$ @mah The reason for changing designations is more or less all about magnetic pole movement rather than continental drift. $\endgroup$ – yankeekilo Dec 18 '13 at 12:30
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    $\begingroup$ It's not "more or less", it's a fact. Runway numbers can be changed if the magnetic variation changes (so if the location of the magnetic poles change position). So if runway 26/8 has a magnetic heading of 264.9˚/84.9˚ and it changes to 265.2˚/85.2˚, it will probably be renamed to 27/9 $\endgroup$ – Philippe Leybaert Dec 18 '13 at 14:18
  • $\begingroup$ @PhilippeLeybaert Ah, that cleared a confusion I had. Since the magnetic poles can change, I always thought what happens when they change significantly. $\endgroup$ – Farhan Dec 18 '13 at 16:10

The information from the US AIM may be useful here:

b. Runway Designators. Runway numbers and letters are determined from the approach direction. The runway number is the whole number nearest one‐tenth the magnetic azimuth of the centerline of the runway, measured clockwise from the magnetic north. The letters, differentiate between left (L), right (R), or center (C), parallel runways, as applicable:

1. For two parallel runways “L” “R.”

2. For three parallel runways “L” “C” “R.”

As to why the runway is actually constructed on heading 1, 9 or something else, that is determined by the local prevailing wind conditions among other considerations (terrain etc.). Aircraft always take off or land into the wind (as far as possible), so that is a key factor in how runways are positioned. NASA has a very good article on airport layouts that goes into more detail.

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    $\begingroup$ Some times, like KMIA, you face 3 parallel runways, but these are designated as 08R, 08L, and 09. Just because the runways 08R and 08L are 'grouped'. $\endgroup$ – Ygor Montenegro Dec 18 '13 at 13:02
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    $\begingroup$ @YvesKlett Indeed, the ILS/LOC approaches for 8L, 8R and 9 at KMIA all show 092 as the final approach course. I have no real idea why, but my guess would be that since there is a large distance between 9 and 8L/8R, using the same number for them all would be confusing, especially in limited visibility. There are also procedural differences based on the distance between parallel runways; perhaps the numbering helps to manage/implement that too. $\endgroup$ – Pondlife Dec 18 '13 at 13:40

Runways are numbered based on compass heading. (Note that within the US, we use MAGNETIC heading; this is not necessarily the case in other countries! (e.g: Canada, which is closer to the north pole, where they use true north to compensate for compass north being off)

(Meaning each physical runway is two numbers!)

The heading of the runway is rounded and truncated to the first two digits; For example, a runway with a heading of 093 degrees becomes runway 9. (Runway 27 from the opposite end).

Now you may ask, what if there are 2 parallel runways?

One becomes runway 9L (left) and the other becomes runway 9R. A third runway would cause one to become 9C (center).

But what if there are MORE runways on the same heading? (Such as the 4 at Harstfield-Jackson International Airport in Atlanta?)

In this case, the airports will typically change the number of the runways by one, even though the actual heading of the runway may be no different! (For example, you would now have runways 8L, 8R, 9L, and 9R that are all parallel.

Wouldn't this cause problems with pilots not knowing the actual heading of the runway though? (Remember that the headings are rounded, and based on magnetic (less accurate) headings!)

One of the pieces of information about airports that is published on charts and such is the actual heading of the runway.

Grass runways are suffixed with a G (E.g. 12G), although since a pilot usually isn't going to have any trouble identifying a grass runway the G is not usually marked. I can't speak to the numbering of water runways; I have limited experience with amphibious aircraft, and I find it highly implausible that they paint the runway numbers! ;)

  • $\begingroup$ I wasn't sure whether or not "water runways" was a joke, so I looked up whether or not there really is such a thing. Turns out there is; the David Wayne Hooks Memorial Airport (KDWH) in Spring, Texas has one. $\endgroup$ – Terran Swett Apr 11 '17 at 23:10
  • $\begingroup$ @TannerSwett Water runways used to be much more common, when international travel was typically by flying-boat rather than a heavy airliner (for which suitable runways were rare). They can be found on historical coastal charts. $\endgroup$ – Chromatix May 17 '18 at 15:42
  • $\begingroup$ The suffix indicates type of runway not surface type. S = STOL runway, G = glider runway, W = water sealane or waterway, and U = ultralight runway. $\endgroup$ – PlaneGuy May 18 '18 at 22:42
  • $\begingroup$ @TannerSwett Notably, KDWH has 17R/35L, 17L/35R, and 17W/35W, with the latter not being parallel to the former two despite the numbering. $\endgroup$ – StephenS Feb 28 '19 at 21:47

Runway designations are defined by ICAO Annex 14. More than 3 parallel runways on the same bearing would have the runway identifier adjusted up or down for the ones in excess of 3, to ensure uniqueness. In other words, if there were 9 parallel runways all true north, they would be designated 35L, 35C, 35R, 36L, 36C, 36R, 01L, 01C, and 01R. The runway number is always two digits. The runway designator is always unique at a given airport. The use of the 'T' runway suffix to designate true, while possibly still existent on the physical marking of the runway, is no longer used in the industry. These rules limit the number of runways at a given airport to 108.

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    $\begingroup$ I would be most interested to see an airport where the maximum number of runways numberable was becoming a factor! $\endgroup$ – FreeMan Dec 1 '16 at 18:21
  • $\begingroup$ A can think of a few dry lakes... $\endgroup$ – jwzumwalt Jun 3 '18 at 6:34
  • $\begingroup$ "These rules limit the number of runways at a given airport to 108." Only if you count the reciprocal as its own runway. If instead you count slabs of ground, which makes more sense for figuring out aircraft movements (hello Tenerife), then the maximum number is 54, because you can have 18 (for example 01 through 18 inclusive, with the reciprocal being 19 through 36 inclusive) sets of L/C/R, for 18x3 = 54. $\endgroup$ – user Jun 3 '18 at 13:12

Runway numbers are given as per their direction on compass. Only difference in compass reading and airport runway numbers is that compass reading may be in 3 figures like 120 or 230 etc, but runway numbers are in 2 digits for universal understanding and ease in understanding without confusion.

It is simple: when an aircraft is taking off or landing on a runway that has been assigned, say 09; it will mean that when the aircraft would be ready for take off or coming in for landing, its compass reading should indicate 090 (Last digit is not advised as it is understood as zero)

Similarly if an aircraft coming in to land on, say, runway 25, on lining up with the runway while in the air, the aircraft compass should read 250 degrees. Some airports show single digit as runway number, (like 3 or 7). In such cases a zero is to be added before and after the number. Hence 3 will mean 030 and 7 would mean 070.

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    $\begingroup$ Nice first answer - one thing worth noting is that the compass reading might be off by a few degrees, i.e. indicate a heading of 092 for runway 9. $\endgroup$ – Danny Beckett Aug 7 '14 at 21:18
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    $\begingroup$ This is incorrect. The last digit of the aircraft's heading is not assumed to be zero: the heading of an aircraft taking off from runway 09 could be anything between 085 and 095. In fact, it could be even farther from that because airports quite often have a runway that, for example, "should" be labelled 08/26 but is labelled 09/27 to avoid confusion with some other runway 08/26. (E.g., at Denver, runways 07/25 and 08/26 are actually parallel.) $\endgroup$ – David Richerby Aug 9 '14 at 11:24
