Southwest Airlines and AirTran Airways are letting customers move frequent-flier credits from one airline’s program to the other, opening up new possibilities for free flights.

The conversion program allows customers to consolidate frequent-flier points and credits from the two carriers, and to access more destinations. AirTran, now a Southwest subsidiary that once served Charleston, offers some destinations outside the U.S., while Southwest offers a more extensive domestic network.

The ability to turn Southwest points into AirTran credits also could allow travelers to score better flights for fewer points.

For example, let’s say you wanted to fly from Atlanta to San Francisco, May 19-26. Southwest was offering the best deal in dollar terms for that week, with a web-only fare of $464 and no baggage charges. But if you wanted to fly using Southwest Rapid Reward points, AirTran would be the way to go.

On Southwest, you would need 25,200 points for the least costly flight, and you would have to make a stop between Atlanta and San Francisco, for a flight time of at least seven hours each way.

But for 19,200 points, swapped for AirTran A+ reward credits, you could get a direct flight on AirTran, cutting 90 minutes off your flight time and saving 6,000 reward points.

Here’s how that works.Southwest customers are able to convert Rapid Reward points into AirTran A+ Reward credits at the rate of 1,200 points per credit. That works out to 19,200 points for 16 credits, which is the amount needed for a round-trip ticket anywhere than AirTran flies.

Conversions are made on Southwest’s website, and once made, they cannot be reversed. A free frequent-flier account with each airline is needed.

People living in the Charleston area likely will be most interested in new ways to use Southwest points. AirTran credits expire after 12 months, and the airline hasn’t served Charleston International Airport since 2009, so local residents probably don’t have many unused AirTran credits.

Southwest has been serving Charleston since March 2011, but you don’t have to fly to get frequent-flier points.

People who took advantage of Southwest’s recent credit card promotion, where new applicants received 50,000 Rapid Rewards points in exchange for a $65 annual fee, have enough points for two round-trip AirTran tickets plus a one-way ticket, with points to spare.

AirTran may no longer serve South Carolina, but the company’s Atlanta hub is close enough that thrifty people might drive there for a good frequent-flier deal on a long flight.

Converting Southwest points to AirTran credits is pretty straightforward at 1,200 points per credit. Putting AirTran credits into a Southwest account is more complicated because Southwest switched from a system of credits to a point system last year.

The airline won’t convert AirTran credits to Southwest points because credits expire and points do not, but it will allow points and credits to be combined within a Southwest account for an award.

In Southwest’s program, reward points are used like money. More expensive flights and fares with fewer restrictions require more points.

In AirTran’s program, 16 reward credits equal a free flight anywhere, so expensive or long-distance flights are a frequent-flier bargain. AirTran charges baggage fees, unlike Southwest, and some taxes and fees apply to “free” tickets on any airline.

With Southwest, 19,200 reward points are worth a maximum of $320 in flight costs. Those same points converted to 16 A+ credits will take you anywhere AirTran flies, whether it’s Charlotte or Seattle.

And unlike most airlines, AirTran’s basic round-trip award can be used for destinations outside the continental U.S. It might be worth the drive to Atlanta now that you can turn 19,200 Southwest points into a round-trip direct flight to Cancun, San Juan or Aruba.

Reach David Slade at 937-5552 or Twitter @DSladeNews.