What is a flight delay index and how do I use it?

A flight delay index is a single number that captures how disrupted flights are at an airport, on a route, or across the system — combining multiple factors (delay frequency, delay length, cancellations, diversions) into one score that's easier to interpret than raw percentages.

Flight Check's delay data combines on-time rate, average delay when late, and cancellation rate to give each airport and route a reliability picture. Here's how to read the key numbers:

On-time rate: Percentage of flights arriving within 15 minutes of schedule. National average is about 78%. Below 75% is concerning; above 82% is good. Airports like SLC (85%+) are consistently reliable; SFO and EWR hover around 72-74%.

Average delay when late: Of the flights that ARE delayed, how long is the typical delay? The national average is about 25 minutes. Anything above 35 minutes means that when things go wrong at that airport, they go very wrong.

Cancellation rate: Percentage of flights cancelled outright. The national average is about 1.5%. Above 3% is a red flag (often weather-prone airports in winter).

How to use this practically: before booking, check the delay index for your airports and route. A flight through ORD in January has very different reliability than through PHX. If you have a tight connection, the delay index tells you whether you're playing with fire.