How Runway Numbers Work: Headings, Letters and Reciprocals

A runway number tells a pilot the magnetic heading of that runway, divided by 10 and rounded to the nearest whole number. Runway 27 points to roughly 270° magnetic. Runway 18 points to 180°. Runway 04 points to 040°. The rule is simple: take the magnetic heading, drop the last digit, and round to the nearest whole number.

The International Civil Aviation Organization (ICAO) sets this standard, and the Federal Aviation Administration (FAA) applies it at every public airport in the United States. Every runway carries two numbers, one for each direction pilots can use it. The two numbers are always 18 apart, because a runway is a straight line and its two ends face opposite directions 180° apart, and 180 divided by 10 is 18.

Numbers run from 01 to 36 and always use two digits, so pilots read "zero-niner" or "two-seven" over the radio without confusion. A pilot lining up on Runway 09 knows without checking a chart that the nose points close to 090°. That number painted on the threshold saves a step during approach briefings, and it lets air traffic control assign a runway by heading rather than by a name that means nothing on its own.

The table below lists every runway number from 01 to 36, its magnetic heading, and the reciprocal runway found at the opposite end of the same strip. Runway heading equals the number multiplied by 10. Reciprocal runway equals the number plus 18, wrapped back into the 01 to 36 range once it passes 36.

Busy airports build more than one runway on the same heading to handle more traffic, and the plain number cannot tell those runways apart on its own. So airports add a letter: L for left, R for right, and C for center, based on how the runways sit when viewed from the approach end.

Chicago O'Hare's 27L and 27R are two separate strips pointing the same 270°, sitting side by side. Dallas Fort Worth goes further with 17L, 17C and 17R, three parallel runways read left to right by an arriving pilot. The letters flip depending on which end you approach from. A pilot landing from the opposite direction calls the very same pair of runways 09R and 09L instead, because left and right depend on which way the aircraft faces.

Air traffic control uses the full designator, letter included, in every clearance: "cleared to land runway two-seven left." Leaving off the letter at a busy parallel-runway airport risks lining up on the wrong strip, so pilots read back the full runway number and letter every time.

Runway numbers drift out of date because the number is tied to magnetic north, and magnetic north keeps moving. The magnetic pole shifts roughly 50 to 60 kilometers a year, so the magnetic heading of a runway that has not moved an inch can still change over time. Aviation authorities review magnetic variation on a set cycle and require a runway to be renumbered once its rounded heading shifts more than 5° from the number painted on the pavement.

Tampa International Airport is a documented example. On January 13, 2011, its runway 18R/36L became 1L/19R, because the runway's magnetic alignment had drifted from close to 180°/360° toward roughly 190°/010°. The concrete and the runway itself never moved. Only the number did, along with over a hundred signs.

This is also why some airports end up with a mix of runway numbers that look close together, like the 24 and 25 pairs at Los Angeles International, built and surveyed at different times. A runway number describes magnetic heading as of its last survey, not a permanent fact about the ground underneath it.

Think of the number painted on a runway threshold as a giant compass reading built into the pavement. Standing at the runway end and looking straight down the centerline, that number tells you exactly where the nose points on a magnetic compass, the same way a compass rose on an aeronautical chart shows every heading around a full circle.

Pilots use this every time they taxi into position. Lining up on Runway 09 means checking that the aircraft's own compass or heading indicator reads close to 090° before entering the runway, a quick cross-check that catches a wrong-runway lineup before takeoff.

This same runway heading is the exact number the crosswind calculator asks for at the top of this page. Enter a runway number, or the heading it represents, alongside wind direction and speed, and the calculator resolves the wind into crosswind and headwind components using the same magnetic reference the runway number already gives you. See how that plays out across every wind angle on the wind component chart.