Consumer Reports: Making the most of your frequent flyer miles

NEW YORK (WABC) -- Robert Karp has been planning his family's vacations ever since he was 13 and figured out how to get the best bang for his parents' mileage.

"My family, we recently went to Hawaii last December and our business class airfare and nine nights in hotels were all covered on our miles and points," he said.

The airline you choose has a lot to do with whether you'll score free travel.

Consumer Reports analyzed the latest data from the U.S. Department of Transportation and found Southwest the most generous carrier, giving away 11.5 percent of its seats. JetBlue is the stingiest, giving away only 4-point-5 percent. United, American and Delta fall in between.

"Book several months in advance, when there are more awards seats available," Consumer Reports' Margot Gilman said. "Still, last minute searches can also work because people change their plans and seats become free."

If you're hoping to travel overseas and there are no awards seats available on your U.S carrier, check their foreign partners. You may be able to get one of their award seats. For instance, American's partners include British Airways. Delta has Air France and United has Lufthansa among others.

"Don't necessarily jump on any free seat," Gilman said. "You don't want to waste your miles on a cheap trip."

To figure out if it's worth it, divide the price of the ticket by the number of Frequent Flyer miles you're spending. If the result is less than a penny, you're better off paying cash.

And if you're short miles for a trip you want, don't buy them. They are not a bargain.

Keep in mind that if you don't have enough frequent flyer miles for a roundtrip ticket, many airlines now allow you to book one way with miles and pay for the return with cash.

From New York City, the most popular frequent flyer destinations are Miami, Los Angeles and Orlando.