So with summer coming, Hopper, an app that uses data to predict when airfare will be cheapest, analyzed its real-time flight search data to determine the most common and costly mistakes travelers make when booking.
Hopper found that, this summer alone, you could save up to $572 for domestic trips and $1,961 for international ones just by tweaking your booking strategy. Here are some of the biggest mistakes to try to avoid.
1. Booking too early or too late
Some of the most costly mistakes have to do with timing. "The worst thing travelers can do is book their flights too late," Liana Corwin, consumer travel expert at Hopper, tells CNBC Make It.
"On domestic trips it can cost you $139, and on international trips you're looking at being set back as much as $529," she says. Hopper calculated all costs from average fares for summer 2018 flights.
"Conversely, if you book too early, you can lose $38 on domestic trips and upwards of $474 on international ones," she says.
If you're traveling to a popular leisure destination, like Hawaii or Mexico, Corwin says the best fares pop up about one month before departure date. For popular business destinations, like New York City or Washington, D.C., you'll likely find the least expensive prices about three months ahead of time.
Hopper also found there are fewer deals offered on flights over the weekend, so booking then costs an average of $9 on domestic trips and $10 on international ones. And flying on Friday will cost you 20 percent more than flying on Tuesday.
2. Being inflexible about alternate airports or destinations
Flexibility is also key, according to the data: Not checking out alternate airports is costing travelers an average of $32 on domestic trips and $99 on international trips this summer, according to Hopper. Not being flexible about destinations costs an average of $131 for domestic trips and $397 for international jaunts.
3. Purchasing impulsively
Another easily avoidable but expensive mistake? Purchasing a plane ticket the same day you search for it. Hopper's study found that you can typically save 14 percent on your ticket if you wait a bit to book.
"A lot of travelers do this because they get sticker shock and are worried about ticket prices rising, but what we've found is that two out of three ticket prices will actually drop in the 24 hours after the first search," says Corwin.
4. Booking your flight before your hotel
Hopper also found that booking flights before hotels can cost an average of $34 per night. That's because you'll likely be better off booking your hotel further in advance (due to the way hotels do inventory and pricing). The sweet spot to book a hotel is around two-to-three months before your trip, according to Hopper.
5. Booking a trip for mid-summer
It helps to be strategic about the timing of when you actually take that summer vacation too.
According to 2017 flight data from Expedia, when it comes to summer vacations, August is the cheapest summer month for airfare, particularly the second half of the month. Expedia found that late June and early July is a bit more expensive and, last year, the average airfare ticket price in June was around $100 more than in August.
Don't miss: The best budget-friendly summer vacations
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