I have missed three connections. Two were entirely my own fault for booking a tight one to save money, and the third was not, and the difference in how they were resolved is the whole point of this.
The minimum is a minimum
Airports publish a minimum connection time, and booking systems will sell you an itinerary at that figure.
It assumes the inbound flight is on time, that you disembark promptly, that immigration and security have no queue, and that the gates are not at opposite ends of a large building.
It is achievable and it is achievable in the best case, which is not the case you should plan for. Any delay at all consumes the entire buffer.
What actually eats the time
Worth listing because the driving factors are not the ones people worry about.
Disembarking. Between landing and being off the aircraft is frequently twenty minutes and can be considerably more if a stand is not available and you are bussed.
Immigration, if the connection involves entering a country. This is the largest variable and can be an hour or more at busy times, and it applies in more situations than people expect.
Terminal changes. Some large airports require a train, a bus, or a long walk between terminals, and this can be forty minutes on its own.
Security, again, if you have re-entered a landside area.
And baggage recollection, if the bag is not checked through, which happens on separate tickets and on some itineraries that look continuous.
The single ticket question
The most important distinction and one many people do not know exists.
On a single ticket, the airline is responsible for getting you to the final destination. Miss the connection because the first flight was late, and they rebook you, generally at no cost, and provide assistance depending on the jurisdiction.
On separate tickets, you have simply failed to arrive for a flight you bought. The second airline has no obligation. You buy a new ticket at whatever the price is on the day, which near departure is the worst price available.
Separate tickets are frequently cheaper and the saving is a bet against disruption. I have made that bet and lost it, and the loss was considerably larger than the saving.
How much buffer
What I use now, having got this wrong.
Within the same terminal, no immigration, single ticket: ninety minutes.
Terminal change or immigration involved: two and a half hours.
Separate tickets, anything: four hours minimum, and preferably an overnight stop, because there is no protection and the downside is unbounded.
Last flight of the day as the connection: add more, because a missed connection there means a night somewhere rather than a two hour wait.
The direction of the day matters
Something that took me a while to notice.
Delays accumulate through the day. An aircraft that starts late stays late, and the effect compounds across its rotations.
Which means a morning connection is considerably safer than an evening one, on the same route with the same buffer.
The first flight of the day is the most reliable departure available and it is worth the early start for anything with a connection behind it.
What to do when it goes wrong
From having done it three times.
Start the rebooking process before landing if you can see you will miss it. Many airlines will have already rebooked you and the information may be available on the app or by message.
Go to the transfer desk rather than out through arrivals, if you are on a single ticket. Leaving the secure area complicates everything.
Join the queue and call the airline simultaneously. Whichever answers first is the one you use, and phone queues are frequently shorter than physical ones.
Ask specifically about other airlines and other airports, because rebooking systems default to the same carrier and the alternatives are often better.
And know what you are entitled to, which depends on where you departed from and which airline. Some jurisdictions require accommodation, meals and compensation, and the airline does not always volunteer this.
The one that was not my fault
Single ticket, inbound delayed by weather, connection missed by twenty minutes.
I was rebooked before landing, put on a flight four hours later, given vouchers I did not ask for, and arrived the same day.
Total cost to me: an afternoon in an airport.
The separate-ticket version, on a different trip, cost me a new one-way fare bought two hours before departure and a night in an airport hotel. The original saving on that itinerary had been modest, and I have not booked separate tickets since.
Where the bag is going
One detail worth checking at the first check-in desk, because it determines what happens if things go wrong.
Ask whether the bag is checked through to the final destination or only to the connecting point. On separate tickets it is nearly always only to the connecting point, which means collecting it, clearing customs, and checking in again — all of which takes time that the connection may not contain.
Even on a single ticket it is worth confirming, because some routings require recollection for customs reasons regardless.
The answer changes how much buffer you need by an hour or more, and it takes one question to get it.