How Airports Are Sized and Coded, Explained: Large vs Medium, IATA vs ICAO

You book a flight to LHR, your bag gets a tag that says LHR, and the departure board says LHR. But the pilot's flight plan calls the same airport EGLL, and a map of world airports labels it large while the airfield down the coast is medium. None of this is arbitrary — there are two naming systems and one size scale at work, each built for a different job. Once you know what they mean, the airports map reads like a chart instead of a soup of codes.

Size classes: how big a deal is this airport

The colour of every dot on this map is its size class — a classification from OurAirports, the open registry the map is built on. It isn't a single measurement like runway length or passenger count; it's an overall judgement of an airport's scale and role.

• Large airports are the major hubs — the ones with long runways, heavy traffic, and usually international and intercontinental flights. Think of the airport you'd change planes at on a long trip. On the map these are amber.
• Medium airports are the regional workhorses. Most countries run several: proper terminals, scheduled airlines, but serving a city or region rather than the entire world. These are charge-blue.
• Small airports are everything smaller that still runs — minor regional fields and airstrips. On this map they appear only when they carry scheduled passenger service, in radar-green.

The size class is deliberately a shorthand rather than a precise threshold, because "how important is this airport" depends on more than any one figure. A short-runway airport packed with flights can matter more than a long-runway one that barely sees traffic. Colouring by size class lets the map say, at a glance, where the world's big gateways are and where the smaller regional network fills in around them.

Two codes, two audiences

Nearly every well-known airport has two codes, and they exist because two organisations name airports for two completely different groups of people.

The IATA code is the three-letter one you already know — LHR, JFK, LAX, SYD. It's run by the International Air Transport Association, and it's built for passengers and commerce: short, memorable, printed on tickets and luggage tags. It's the travel-facing name.

The ICAO code is the four-letter one — EGLL, KJFK, KLAX, YSSY. It's run by the International Civil Aviation Organisation, and it's built for pilots and air-traffic control: globally unique, structured, and used in flight plans and operations. ICAO codes aren't random — the leading letters encode a region. Anything starting with K is the contiguous United States; EG is the United Kingdom; Y is Australia. That regional structure is why ICAO codes are longer: they have to be unambiguous across the entire planet, with no two airports ever sharing one.

So when you see an airport's detail on the map showing both IATA LHR and ICAO EGLL, you're seeing the same place named twice — once for the person catching the flight, once for the person flying the plane. Big airports carry both. Smaller fields might have only an ICAO code, or only a local one, which is why some airports on the map show a single code rather than a pair.

What "scheduled service" filters in

The full OurAirports dataset is enormous — more than 80,000 entries — because it logs every kind of aviation facility: heliports, seaplane bases, balloon ports, tiny private strips, and airports long since closed. That's valuable for aviation, but it's not what most people mean by "an airport."

So this map applies one honest filter: it keeps every large and medium airport, plus small airports flagged as running scheduled service — regular, timetabled passenger flights you could actually book. Everything else is left out. The result shows far fewer than 80,000 points, but with a clear promise behind each one: every dot is a place an ordinary traveller could board a flight. A patch of tarmac that only sees private jets, crop dusters or helicopters doesn't make the cut, because it wouldn't help you answer the question the map exists for — where can I fly from?

Reading the map

Put the three ideas together and the map tells a real story about a country. The spread of colours shows the shape of its air network — a few amber hubs feeding a layer of blue regional airports, with green scheduled-service fields reaching into the places airlines still serve but only lightly. The count reflects scope: a country threaded with remote communities served by air can show a surprisingly high number of small scheduled-service airports, because flying is simply how people get around there. And the codes in each airport's detail tell you whether you're looking at a globally significant gateway with a full IATA/ICAO pair, or a smaller field known mainly to the pilots who use it.

It's a dated snapshot, not a live flight feed — it shows where airports are, not which planes are in the air right now. For that, the live flights overlay on the Earth canvas tracks aircraft in real time. But as a map of the fixed places the world's air travel runs through, every label on it now means something specific.