A compass painted on the ground
Why Are Runways Numbered?
The huge number at the end of a runway is not its name. It is the direction the runway points, rounded to the nearest ten degrees.
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Step 1 of 5
A compass circle has three hundred and sixty degrees.
North is 0, east is 90, south is 180, west is 270. Every direction on earth has a number, and a runway simply wears its own.
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Step 2 of 5
Drop the last digit and you have the runway.
A runway pointing at 270 degrees is Runway 27. Two digits are enough because a runway does not need to be placed more precisely than the nearest ten degrees.
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Step 3 of 5
Every runway has two numbers.
You can land from either end, and the two ends point opposite ways. The numbers always differ by eighteen, so one strip is Runway 9 at one end and Runway 27 at the other.
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Step 4 of 5
Aircraft take off into the wind.
A headwind gives lift for free, so the runway in use changes with the weather. That is why an airport with several runways still keeps them all busy.
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Step 5 of 5
The numbers change over the years.
The magnetic pole drifts, so headings slowly shift. Every few decades an airport has to repaint its runways and reprint its charts to keep up.
Where two parallel runways share a heading they get an L, C or R for left, centre and right, which is why big airports have a Runway 27L and a 27R.
The short version
A runway's number is its compass heading with the last digit dropped, and every strip carries two of them, eighteen apart.
Try it yourself
Find an airport on a map and read the runway numbers. Multiply by ten and the direction matches what you see.