SFO Airport Code Origin — Why San Francisco Uses Three Letters

Why is the SFO airport code San Francisco? Short answer: it isn’t hiding anything. S-F-O pulls straight from the city name, which makes it one of the more logical codes in the American airport system. But having flown in and out of the Bay Area dozens of times for work trips, family visits, and one deeply regrettable red-eye back from Tokyo, I can tell you the code is actually the least surprising thing about this airport. Where SFO gets interesting is the geography, the history, and the fact that it sits inside a county most passengers have never heard of.

SFO — San Francisco International

The three-letter IATA code system was formalized in the 1930s, when the aviation industry needed a standardized way to identify airports across an expanding global network. Before that, airports sometimes borrowed two-letter National Weather Service station identifiers. When the International Air Transport Association stepped in and pushed for three-letter codes, many airports simply expanded what they already had.

San Francisco’s airport did exactly that. The city abbreviation SF plus an O — standing for “Oakland” or just used as a disambiguating letter depending on who you ask — produced SFO. Some sources say the O was added simply to make the code three characters, with no deeper meaning. Others suggest it helped distinguish San Francisco from other SF-prefixed identifiers already in use at the time. Either way, the result is one of the most intuitive codes in the system.

Compare that to some of its neighbors. Oakland International uses OAK, which is clean and obvious. But then you get to Los Angeles — LAX — where the X is essentially a placeholder that came from an older two-letter code, LA, when airports were required to go to three letters. Chicago O’Hare is ORD because it was once called Orchard Field. Midway is MDW. None of that history shows on the boarding pass. SFO, by contrast, basically just says what it is.

Opened in 1927 as Mills Field Municipal Airport on a stretch of mudflats south of San Francisco, the airport has been through several name changes and expansions. It became San Francisco International Airport officially in 1955. The terminal complex that most travelers walk through today took its current shape after a $2.4 billion renovation program that stretched across the late 1990s and early 2000s, including the International Terminal — still one of the largest airport terminal buildings in the United States at roughly 2.7 million square feet of total floor area.

The Bay Area Airport Trio

Probably should have opened with this section, honestly, because for a lot of travelers the real question isn’t just what SFO stands for — it’s which Bay Area airport to actually use.

Three major commercial airports serve the greater San Francisco Bay Area.

• SFO — San Francisco International Airport — The largest of the three. Served by most major domestic carriers and a wide range of international airlines including United, which uses SFO as a hub. Lufthansa, Singapore Airlines, Japan Airlines, and Air Canada all operate flights here. If you’re flying internationally, SFO is almost always your only realistic option among the three.
• OAK — Oakland International Airport — Sits on the east side of the Bay. Southwest Airlines dominates here, which means you’ll find aggressively priced fares if you’re flexible. Lyft or BART can get you into San Francisco from OAK in roughly 30 to 45 minutes depending on traffic and which route you take. Parking at OAK tends to run $18 to $22 per day in the economy lots, noticeably cheaper than SFO’s economy rate of around $25 to $28.
• SJC — Norman Y. Mineta San Jose International Airport — Located in San Jose, which puts it closer to the South Bay and Silicon Valley than to San Francisco proper. Alaska Airlines and Southwest both have strong presences here. If your destination is Cupertino, Sunnyvale, or Palo Alto, SJC is genuinely worth considering over SFO — you skip a lot of Bay Area traffic.

I made the mistake once of booking into SFO for a meeting in Mountain View and then sitting in 101 traffic for an hour and forty minutes in a rental car. Should have flown into SJC. Learned that lesson the expensive, sweaty way.

For travelers coming from Europe or Asia, SFO is the gateway. For budget domestic flights, OAK is underrated. For anyone doing business in the valley itself, SJC saves time in ways that don’t show up until you’re actually on the ground.

Why SFO Is Not Actually in San Francisco

Here’s the part that trips people up. Despite carrying the city’s name and code, San Francisco International Airport is not located in San Francisco. It sits in unincorporated San Mateo County, roughly 14 miles south of downtown SF along the Bay shoreline.

Unincorporated means it doesn’t technically belong to any city. The surrounding communities — Millbrae, San Bruno, Burlingame — frame the airport on its land-side edges, but the airport property itself falls under county jurisdiction. The City and County of San Francisco owns and operates the airport through its Airport Commission, which is what creates the ongoing political and administrative connection to the city, even though the physical location is somewhere else entirely.

Surprised by this — as I genuinely was the first time someone pointed it out — most people instinctively assume the airport sits inside the city limits. It doesn’t. San Francisco proper ends well north of the airport. The BART extension to SFO, which opened in 2003, actually crosses county lines during the trip, passing through Daly City and South San Francisco before reaching the airport station.

The geography made sense historically. San Francisco is a dense, hilly peninsula with very little flat land. Building a major airport inside the city was never realistic. The Bay mudflats to the south offered space, even if it required extensive land reclamation work. The airport sits on fill and engineered ground — a fact that becomes relevant every time engineers discuss seismic risk assessments for the facilities.

So the code SFO represents the institution and the operator rather than the literal address. The City of San Francisco built it, named it, and runs it. The county just happens to be where the runways ended up. It’s a common arrangement in American aviation — think of Dallas/Fort Worth International, which sits in Irving, Texas — but SFO is one of the more dramatic examples of a city putting its name on an airport that exists somewhere else entirely.