Learn how runway numbers are determined from magnetic direction, why opposite ends differ by 18, and how parallel labels and renumbering work.
In Aviation & Real-World Flying, runway numbers are determined from the runway centreline’s magnetic direction as seen on approach, rounded to the nearest 10 degrees and shortened to two digits: 090° becomes 09. The opposite end is 180° away, so it receives 27; physical alignment is chosen for wind, terrain and airport constraints.
How is a runway number calculated?
The calculation converts the runway’s magnetic azimuth into a two-digit designator between 01 and 36.
- Take the centreline direction. Measure its magnetic azimuth clockwise from magnetic north, looking in the direction an arriving aircraft travels.
- Round to the nearest 10 degrees. A direction of 087° rounds to 090°, while 274° rounds to 270°.
- Remove the final zero. The rounded directions become Runway 09 and Runway 27.
- Add a leading zero when needed. Runways 1 through 9 are written as 01 through 09 and spoken digit by digit, such as “zero nine”.
- Number the reciprocal end. Add or subtract 18 from the designator because the opposite direction is 180° away. Runway 09 therefore becomes 27 at the other end.
| Magnetic centreline direction | Runway number | Reciprocal end |
|---|---|---|
| 087° | 09 | 27 at approximately 267° |
| 274° | 27 | 09 at approximately 094° |
| 002° | 36 | 18 at approximately 182° |
A direction near 360° or 000° is designated Runway 36, never Runway 00. Most countries use magnetic north for these designations. In high-latitude areas where magnetic references are unreliable, an aviation authority may use another published reference, so the applicable airport chart remains definitive.
Why is Runway 27 not always aligned at exactly 270°?
A runway number is a rounded identifier rather than an exact heading, so Runway 27 can have a centreline direction such as 274°.
The difference is ordinarily about five degrees or less when the number is assigned. Magnetic variation can then change while the pavement remains fixed, and renumbering is a coordinated airport change rather than an immediate response to every small shift.
An ILS course, flight-management database or chart may consequently show a more precise value than the painted number. Our guide to checking published ILS courses and runway data explains why those figures need not match exactly.
What do L, C and R mean after a runway number?
The letters identify parallel runways with approximately the same magnetic direction: L is left, C is centre and R is right when viewed by an aircraft approaching that end.
The perspective reverses at the opposite end. Runway 09L therefore normally becomes 27R, 09C becomes 27C, and 09R becomes 27L. Airports with more than three parallel runways may divide them between adjacent numbers, even though the pavements have almost the same physical alignment.
Why do runway numbers change?
Runway numbers change when long-term movement of magnetic north makes the old rounded designation unsuitable.
The runway itself has not rotated. The airport must coordinate new pavement markings, signs, charts, instrument procedures and navigation data. This can produce temporary disagreement between documentation and older flight-simulator scenery, where the navigation database may use the new number while the ground texture still displays the old one.
Use the applicable publication rather than guessing from magnetic heading alone; our explanation of how to verify runway designations on aviation charts covers the practical chart-reading process.
How are the physical and active runway directions chosen?
The pavement’s alignment and the direction used for a particular operation are separate decisions.
Physical runway alignment
Airport planners try to align a runway with the prevailing winds because aircraft normally take off and land into wind. The final direction must also account for:
- terrain and obstacles beneath approach and departure paths;
- available land, drainage and surrounding development;
- controlled airspace and conflicts with nearby airports;
- noise-sensitive areas and operating restrictions;
- taxiway layout, terminal access and required airport capacity.
The runway number is assigned after that physical alignment has been established; the number does not dictate where the runway must be built.
Direction used for take-off or landing
Either end of the same pavement can usually be used, with the active direction selected according to wind, traffic flow, runway length, approach availability, surface condition and local restrictions.
For example, using Runway 09 means approaching from the west and travelling roughly east; using Runway 27 means approaching from the east and travelling roughly west. Some airports restrict one direction because of terrain, noise or procedure limitations. We cover the operational decision in our guide to selecting a runway for landing from wind and airport conditions.
Do not compare a runway number blindly with every reported wind direction. METAR wind is referenced to true north, while controller and locally broadcast wind is normally magnetic, subject to national procedures. Our explanation of the direction reference used for ATIS winds clarifies that distinction.
Common runway-number mistakes
- Treating the number as an exact course. Runway 27 means a rounded direction near 270°, not necessarily precisely 270°.
- Using 00 for north. The valid north-facing designation is 36.
- Keeping L and R on the reciprocal end. Left and right reverse with the pilot’s viewpoint, so 09L usually pairs with 27R.
- Expecting the aircraft’s nose to match the runway number. The designator describes centreline direction; a crosswind crab can make aircraft heading differ from its track along the approach.
- Assuming a simulator mismatch is pilot error. Painted scenery, ATC data and navigation databases may represent different dates, especially after a real runway has been renumbered.