When's the best time to book a flight?

The cost of filling up a 747 aircraft is at a multi-year low and yet airfares are on the rise.

According to FareCompare.com, major carriers are increasing fares by $10 per round trip, making it the third increase this year.

The fare hikes are being driven by increasing demand, fewer empty seat, as a time when business travel is not slowing down.

According to a new study by CheapAir.com, the best time to book a domestic flight is 54 days, which is between 7 to 8 weeks before a trip.

The reason, according to Airline Weekly's Seth Kaplan, is because passengers who have guaranteed trips are likely to purchase way in advance, while last minute passengers are more willing to pay a premium.

The study looked 1.3 billion airfares from 2015 across 3 million different trips on each day of the year.

If you're looking for a flight with prices on the decline, Puerto Rico might be your best bet. According to Reuters report, the lowest fares to San Juan, Puerto Rico have fallen 22 percent on average since last year.

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