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The worst month to fly, measured

Everyone has a theory about summer storms and Christmas queues. Here is what the flights themselves did, month by month.

Computed from 10,423,751 flights · January 2025 – August 2026 · recalculated whenever new data lands

Pooled across every flight in the record, June is the worst month to fly and September is the best — 72.4% on time against 83.4%, a swing of 11 points.

Delay and cancellation do not peak together. The month with the most cancellations is January at 4.18%, which is worth knowing separately: a delayed flight costs an afternoon and a cancelled one costs a day.

One caution about reading this as advice. These months are pooled across four countries and both hemispheres, so a northern winter and a southern summer are averaged together. The seasonal view on an individual route page is the more useful one for an actual trip.

Month On time Cancelled Flights
January 77.5% 4.18% 699,539
February 80.6% 1.97% 667,284
March 77.1% 2.78% 785,978
April 80.4% 0.87% 781,861
May 78% 0.9% 807,958
June 72.4% 1.46% 1,001,285
July 72.7% 2.4% 749,518
August 77.4% 1.08% 602,391
September 83.4% 0.51% 562,439
October 79.7% 0.53% 605,844
November 79.4% 2.49% 570,550
December 73.2% 1.55% 582,304

Every month in the record, pooled across all routes.

More analysis

Why some airlines run late and others do not Two carriers fly the same routes into the same weather. One arrives on time far more often. The record shows where the difference actually comes from. Why your return flight runs later than your outbound The same route, flown the other way, is measurably worse. We checked every route in the record to see how often that holds. A cancellation costs you more than a delay — and the airlines differ wildly Cancellation is the outcome that loses you a day and the one that triggers a refund. Some carriers do it five times as often as others.