How this works
We take the past year's arrival delays for your first flight (or, if it doesn't fly often enough, the same airline on the route, or the whole route) and count how often it landed too late to leave you time to reach the next gate. Cancelled and diverted flights are reported separately: on one booking the airline rebooks you. We scale that by how your travel month compares with the rest of the year on the route, then check whether the onward flight is the last of the day.
An estimate, not a guarantee. It assumes the onward flight leaves on time; sometimes it's late too, which helps. On one booking the airline must rebook you if you misconnect. On separate tickets it doesn't have to. Layover guide by hub · Method.