How Flying Works
What Do Runway Numbers Actually Mean?
The large painted numbers at each end of a runway look arbitrary until you know what they encode: a compass heading. Once that clicks, every runway designation on every airport chart in the world becomes instantly readable — including why the same physical strip of pavement carries two entirely different numbers.
The number is a rounded compass heading
A runway’s number is simply its magnetic compass heading, rounded to the nearest 10 degrees, with the trailing zero dropped for a clean two-digit marking. A runway that points due west — a magnetic heading of 270 degrees — is painted “27.” One pointing northeast at roughly 40 degrees becomes “04.” A runway aligned almost exactly with magnetic north is designated “36” (360 degrees, rounded and shortened) rather than “0,” keeping every runway number a clean two-digit code.
Why the same runway has two numbers
Any single strip of pavement can be used in either direction, and those two directions of travel are always exactly 180 degrees apart — the reciprocal heading. A runway numbered 27 on the western approach is, by definition, numbered 9 on the eastern approach (270 minus 180 is 90, which rounds to “09”). Which number is in use at a given moment depends entirely on which direction aircraft are taking off and landing that day, which itself is usually set by wind direction — aircraft generally take off and land into the wind, so the “active” runway number can genuinely change over the course of a day as conditions shift.
Why the numbers sometimes have to change
Magnetic north isn’t fixed — it drifts slowly over time, a real, tracked and documented phenomenon rather than a rounding curiosity. Most runways sit comfortably in the middle of their 10-degree bracket and never need attention, but for a runway whose true heading sits close to a bracket boundary, that slow drift can eventually push its actual magnetic heading into the next bracket. When that happens, the airport has to repaint the runway numbers and update every relevant chart and database to match — not a frequent event, but a genuinely recurring maintenance task some airports have had to perform more than once.
Sources & Further Reading
Common Questions
Frequently Asked Questions
Why is a runway numbered "27" and not something like "270"?
The number is the magnetic compass heading the runway points toward, rounded to the nearest 10 degrees with the final zero dropped — a runway heading 270 degrees (due west) becomes simply "27." It keeps the markings short and immediately readable rather than displaying the full three-digit bearing.
Why do the two ends of the same runway have different numbers?
Because a single strip of pavement points in two exactly opposite directions depending on which way an aircraft is using it, and those two directions are always 180 degrees apart. A runway numbered 27 on one end is numbered 9 on the other (270 minus 180 equals 90, rounded to 9) — same physical runway, different number depending on landing direction.
Does a runway number ever actually change?
Yes — magnetic north itself drifts slowly over time (a documented, tracked phenomenon), and if that drift eventually pushes a runway's true magnetic heading past the rounding threshold into the next 10-degree bracket, the airport has to repaint the numbers and update its charts to match. It doesn't happen often, but it's a real, recurring task for some airports.
Why magnetic north and not true north?
Because aircraft compasses are magnetic instruments, so runway numbers are set to match what a pilot's compass actually reads, not an abstract geographic reference. Using magnetic heading keeps the number directly useful in the cockpit rather than requiring a mental conversion during approach or takeoff.