Question: When one needs to make a connecting flight, say from Frankfurt, Germany, to Milan’s Malpensa in Italy, what is a reasonable layover time to ensure both passenger and bags get to the same place at the same time? — Melanie Gnad, Simi Valley, Calif.

Answer: There is what is an allowable connect time, and there is what is reasonable, and these often are not the same.

“Airlines frequently sell you on what they call ‘legal’ connecting times, which means if you’re under 25 and wearing track shoes, you just might make it,” Mark Anderson of Adventure Vacations said in an email.

Welcome to the complex and confusing world of minimum connecting times, or MCT. These are built into the ticket configurations you can create when using an online booking engine or are evident to a travel agent or other entity issuing tickets.

Consumers generally won’t have access to the complete listing of all MCTs, but you can Google your flight and your airports and “connect time” and get a better idea.

But that is a static formula. The airline takes into account the variables of its own operations and the airport you’re connecting in, but it can’t always account for the variables such as weather.

Here is what you need to consider:

>> Is there construction at the airport?

The answer is probably yes, at least domestically. About $32.5 billion in improvements is needed from 2017 to 2021, according to the Airport Consultants Council’s 2017 report. If there is construction, add time.

>> How is the airport configured?

Does it have more than one terminal or do you need to take intra-airport transportation?

Then, too, look at the sheer size. Denver, for instance, is about 53 square miles, according to the World Atlas. That makes Chicago’s O’Hare look shrimpy at about 10 square miles, but neither of these places is a walk in the park.

>> Are you taking the same airline on your connecting flight?

If yes, breathe a little bit easier. An airline’s gates tend to be clustered within one area. If you’re not on the same airline, breathe a lot harder because this could mean your new gate may not be close.

>> Do you have to go through customs?

If you’re coming into the U.S. from abroad, you will have to clear U.S. customs before you can go on to your next flight. Don’t count on what U.S. Customs and Border Protection calls “pre-clearance locations” to make that process any faster.

>> Here’s the best advice about connections: Don’t. Nonstop is almost always better. If you must, try to stay with the same airline, which will help you. If you cannot stay with the same airline (or choose not to, because fares that involve two airlines are often cheaper), allow yourself plenty of time.