How these numbers are put together

Where the data comes from

The Bureau of Transportation Statistics requires larger US airlines to file the scheduled and actual times of every domestic flight they operate — pushback, wheels up, wheels down, gate arrival — along with cancellations, diversions and, for late arrivals, a reason. The agency has published it monthly since 1987. It is a work of the US government and carries no copyright, which is why a site like this can exist.

Nothing here is scraped, modelled or filled in. Where a figure is missing from the filings, it is missing from this site too.

The window

Every page reads the trailing 36 months of filings — July 2023 through June 2026 — covering 21,076,383 flights. Three years is long enough for each calendar month to appear three times, so a single blizzard or a single meltdown cannot drag a month’s figure around on its own, and short enough that the numbers still describe how airlines and airports are running now rather than how they ran before the schedules changed.

The window moves. When a new month is published, the site is rebuilt and the oldest month drops off the back. No date on this site is typed by hand — the “data through” line in the footer comes from the data itself, and if it says June 2026, then June 2026 is what every page on the site is counting.

What “on time” means

Fifteen minutes. A flight counts as on time if it was less than 15 minutes behind its scheduled time — that is the government’s definition, it is the one airlines are held to publicly, and using anything else would make these pages incomparable with every other on-time figure you will read. Early counts as on time. Fourteen minutes late counts as on time. Fifteen does not.

Left on time

Pushed back from the gate less than 15 minutes behind the scheduled departure time.

Arrived on time

Reached the arrival gate less than 15 minutes behind the scheduled arrival time.

Cancelled

Share of scheduled flights that never flew, counted against everything scheduled.

Typical delay when late

The average delay among flights 15 minutes or more behind — the on-time flights are left out, because averaging them in produces a comforting number that describes nobody’s trip.

Departures on airport pages, arrivals on route and airline pages

An airport page asks a question you can act on before you book: which departure time out of this airport has the better history? That is a departure question, so every rate on an airport page — the hour grid, the monthly bars, the airline table — counts departures leaving that airport.

A route page and an airline page ask a different question: did the flight get there? Padding in the schedule means an airline can push back late and still park on time, and a traveler with a connection does not care which. So those pages count arrivals. Airline pages show both figures side by side, because the gap between them is worth seeing.

Every page says which basis it is using, in words, next to the number. If a figure here disagrees with one somewhere else, that is usually the reason.

When no number is printed at all

A percentage off a thin sample is worse than no percentage: it looks precise and it is not. Two rules apply everywhere on this site.

• Fewer than 100 flights, no percentage. Any on-time share, any cancellation rate, anything with a denominator under 100 is left blank and marked “not enough flights to say”. The flight count itself is still shown, so you can see how thin it was.
• Fewer than 30 late flights, no average delay. The “typical delay when late” figure needs at least 30 delayed flights behind it, or it is one bad Tuesday wearing a decimal point.

This is why a small airport’s grid has empty cells at 5am on a Sunday, and why some pages carry more blanks than numbers. The blanks are the honest part.

Cancellations and diversions

A cancelled flight is neither on time nor late — it leaves the on-time rate entirely and appears in the cancellation rate instead. That is why the two numbers have to be read together: an airline that cancels the flight which was going to be four hours late protects one figure at the expense of the other.

Diverted flights — roughly a quarter of one percent of all flights — are left out of the arrival figures, because a flight that landed at a different airport has no meaningful arrival delay at the one you were expecting. Most of them do eventually reach the destination on the ticket, hours late; they sit with the cancellations because neither one is an arrival you could have planned around. In the connection tool they are counted alongside cancellations, since from a connecting passenger’s seat the outcome is the same.

The airline in the data is not always the airline on the ticket

The government collects these records from the airline that operated the flight, not the one that sold the seat. Regional carriers — SkyWest, Envoy, Republic, PSA, Endeavor and others — fly short and medium routes under the majors’ flight numbers and file their own records under their own names. A ticket that says United, Delta or American may well have been flown by one of them.

This site follows the filings, so regionals appear as airlines in their own right and their numbers are not folded into the brand on the boarding pass. It is the honest way to present the data, and it also means an airline’s figure here covers only the flights it actually flew.

The five reasons

For every arrival 15 minutes late or more, the airline files what the delay is put down to, split across five categories. The percentages on this site are shares of delay minutes, not shares of flights.

On an airport page the mix is attributed to where the flight started: it covers flights that departed that airport and ran 15 minutes late or more, not flights that landed there. On an airline page it covers that airline’s own late arrivals, wherever they happened.

Late-arriving aircraft

The plane came in late from its previous leg, so it went out late again. Usually the largest slice, and the reason a morning problem is still a problem at dinner.

Airline operations

Crew, maintenance, fuelling, baggage, boarding — the parts the airline runs.

Air traffic and airport volume

Holding, ground stops, runway congestion, air traffic control restrictions. Weather severe enough to slow the whole system often lands in this bucket rather than the weather one, which is why the weather share looks smaller than travelers expect.

Severe weather

Conditions that stopped this specific flight — thunderstorms, blizzards, fog.

Security

Terminal evacuations, screening equipment failures, re-screening. Almost always a rounding error.

The lag, and how often this updates

The Bureau publishes each month’s filings about two months after the month ends. That is the state of the art in this data, not a shortcut on this site: nobody has a public on-time record for last week. When a new month appears, the site is rebuilt against the fresh window and every date on every page moves with it.

What this site will not tell you

Whether your flight will be late. Nothing here knows about tomorrow’s weather, this afternoon’s ground stop or the aircraft swap made an hour ago. What three years of records support is the shape of the thing — that early departures out of a big hub have a better history than evening ones, that July is not February, that two airlines flying the same route do not fly it equally well. Read every number on this site as a historical share, in the past tense, and it will not mislead you.

Smaller airports carry wider uncertainty than large ones even when a figure clears the minimum. A rate off a few hundred departures moves around far more than one off half a million, and no rounding convention makes that go away.