It’s a tricky question, with several variables affecting the answer. Nonstop is best, but here’s what to consider if connections can’t be avoided.

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When one needs to make a connecting flight, what is a reasonable layover time to ensure both passenger and bags get to the same place at the same time?

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,” says Mark Anderson, of Adventure Vacations.

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 conditions at the airport or weather.

Here is what you need to know to gauge whether you’ll have enough time:

• Is there construction at the airport?

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

How can this foul you up? Let us count the ways, which include late flights because of, say, runway closures.

Self-defense: Look at the airport website. If there’s construction, add time.

• Does the airport have other issues?

“Other issues” may involve the configuration of the airport. Does it have more than one terminal or do you need to take intra-airport transportation?

Then, too, look at the sheer size.

Denver’s airport , 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.

Self-defense: Look at gate configurations and where they are in relation to one another. If gates are scattered or in other terminals, add time.

• 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.

Self-defense: Look at the airport’s map with proximity in mind.

If you’re not on the same airline, breathe a lot harder because this could mean: Your new gate may not be nearby.

If your bag is not checked all the way through, you’ll have to claim it and recheck it.

If you have to change terminals, you may have to go through security all over again.

Self-defense: Any of those three? Add time. All three? Triple the time.

• 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.

Those locations — there are more than a dozen of them — allow you to clear U.S. customs in a foreign airport.

Self-defense: Figure customs into your equation. Check out where there is pre-clearance and how that could factor into your itinerary.

• 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 the “hacker” fares that involve two airlines are often cheaper), allow yourself plenty of time.

And maybe find a travel agent who can book your ticket, which they’ll find easier to manipulate than you will booking online.

Usually, you can stay up to four hours at your connecting airport without paying more, said Tom Spagnola, a senior vice president of supplier relations for CheapOair.com.

“Better to be relaxing in the airport lounge with a good book or a celebratory lunch,” agent Anderson said, “than be in a hellbent-for-Alaska panic trying to make that too-tight connection.”