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A runway number, or designator, is the magnetic direction of the runway centreline rounded to the nearest 10°, with the last zero dropped. Runway 27 points roughly 270° magnetic, which is west. Each strip of concrete has two numbers, one for each direction, so the same runway is called 09 by aircraft heading east and 27 by aircraft heading west.
From heading to runway number
| Magnetic direction of centreline | Rounded | Runway number |
|---|---|---|
| 084° | 080° | 08 |
| 087° | 090° | 09 |
| 274° | 270° | 27 |
| 356° | 360° | 36 |
| 004° | 360° | 36 (never 00) |
A runway pointing north is always 36, never 00. ICAO uses two digits with a leading zero (09), while runways in the United States are usually painted with a single digit (9).
Reciprocal runways differ by 18
The two ends of a runway point in opposite directions, 180° apart. Dropping the last zero turns 180 into 18, so the two numbers always differ by 18: 09/27, 04/22, 12/30, 18/36. If you know one end, add or subtract 18 to get the other.
Parallel runways: L, C and R
Parallel runways share a number and add a letter as seen by the pilot approaching or taking off: L for left, R for right and C for centre when there are three.
- Two parallels: 12L and 12R. From the other end they become 30R and 30L. Left and right swap because you are looking the other way, so the reciprocal of 12L is 30R.
- Three parallels: 17L, 17C and 17R.
- Four or more parallels: airports usually split them into two neighbouring numbers. Los Angeles International, for example, uses 06L/06R and 07L/07R for four runways that all point roughly the same way.
Why airports renumber runways
Runway numbers follow magnetic north, and the Earth’s magnetic field slowly drifts. When the magnetic direction of a runway moves far enough to round to a different ten degrees, the airport changes the number. That means repainting the runway, replacing signs, and republishing charts and procedures. Tampa International in Florida renumbered its runways in 2011 for exactly this reason.
In parts of the far north, where magnetic variation is large and unreliable, some runways are numbered by true direction instead. In Canada these are marked with a T after the number.
Using the runway number with the wind
The runway number gives a quick estimate of the angle between the wind and the runway. Runway 27 with a wind of 300° at 15 kt has the wind 30° off the nose:
- Crosswind = 15 × sin 30° = 7.5 kt
- Headwind = 15 × cos 30° ≈ 13 kt
Use the exact published runway heading, not just the number, when the crosswind is close to a limit. Remember that wind from ATIS and the tower is magnetic, while wind in a METAR is true. The Crosswind & Headwind Calculator does the trigonometry, and Crosswind and Headwind Components Explained covers the method in detail.
Other markings near the number
- Threshold stripes mark the start of the usable landing surface. Under ICAO standards their number reflects the runway width: 4 stripes for 18 m, 6 for 23 m, 8 for 30 m, 12 for 45 m and 16 for 60 m.
- A displaced threshold is shown by white arrows leading to a threshold bar. You can use the arrowed section for take-off and roll-out, but not for touchdown.
- Aiming point markings are the two large solid bars a few hundred metres beyond the threshold, the visual target on a normal approach.
- Touchdown zone markings are pairs of bars that help judge distance along the runway.
Common mistakes
- Assuming the number is the exact heading. Runway 27 can be anywhere from about 265° to 274° magnetic.
- Mixing up left and right at the other end. 09L is 27R.
- Comparing METAR wind directly with the runway number where magnetic variation is large. Convert one of them first.
- Using old charts after a renumbering. Always use current publications.
Related tools and guides
- Crosswind & Headwind Calculator
- METAR & TAF Decoder for the current wind
- How to Read a METAR and TAF
Educational content only. Use current aerodrome charts and your operator’s procedures.
Sources and further reading
- AIM 2-3, Airport Marking Aids and Signs (runway markings: 2-3-3)
- FAA Advisory Circular 150/5340-1, Standards for Airport Markings
- ICAO Annex 14, Aerodromes, Volume I (runway designation markings)
Sources checked November 2026. Rules differ between countries and change over time: for real flights, use the current publications of the authority that applies to you, your aircraft flight manual and your operator’s procedures.