(Leif Parsons)

Translating easily between cash, frequent-flier miles, and loyalty-program points would require something like the travel version of the Rosetta stone—and, unfortunately, we haven’t found it just yet. But there are a few ways to judge whether you’re getting a good deal.First, let’s tackle the question of how much your miles are really worth. Earning 50,000 miles would take more than 11 round-trip flights from Atlanta to Seattle. You can also buy 50,000 Delta SkyMiles for $1,750 (a rate of 3.5 cents apiece), but you’re not likely to get a free ticket on a fare that high for fewer than 80,000 miles. The generally accepted street value per mile is more like 1.4 cents—that’s based on the average domestic airfare ($361*) divided by 25,000 (the minimum number of miles required for a domestic ticket). By that standard, 50,000 points are worth $700, which indicates that paying cash for this ticket is the better option.But calculating the dollar value of your miles is only one consideration. You also have to take into account how easy it is to actually use these 50,000 points. A recent survey by Idea Works Company, a Wisconsin-based transportation consultancy, tested the ease of redeeming miles for tickets on various carriers. The study found that 27 percent of Delta flights were available for reward bookings. So the “Use them when you can” argument definitely comes into play here. My advice? Save your miles for a more expensive fare—something in the $500 range, at least.Your other option is to turn to your Starwood Preferred Guest program for the miles, moving 40,000 Starpoints into your SkyMiles account (Starwood will give you a 10,000-mile bonus for transferring these points, getting you up to the 50,000-mile mark). But 40,000 Starpoints would also net you a full five nights in a Starwood Category 4 property—even at a low $150-a-night room rate, that’s worth at least $750 in free accommodations.The better choice: use Starwood’s new SPG Flights online service, which allows loyalty-club members to book reward tickets directly with airlines. There’s no need to transfer points (which can take up to eight weeks) and you won’t encounter the same availability problems as you do when booking rewards seats through airlines. It would take only 25,000 Starpoints to nab this flight to the Vineyard. Again, you’ll want to weigh whether you’d rather spend this amount on hotel stays: 25,000 Starpoints are worth roughly $400 in free room nights. (See “How to Maximize Your Loyalty Credits,” at right, for more on the value of hotel-loyalty programs.) But if you have points to burn, you might want to cash them in anyway.Assigning straight dollar values to any of these options can take you only so far. Ultimately, it comes down to the type of traveler you are and which account—bank, frequent-flier, loyalty-program—you feel most comfortable drawing on at any given moment. And if you still can’t decide, you can always skip the flight and drive instead.Have a travel dilemma? The trip doctor is in. Send your question to Amy at [email protected].

How to Maximize Your Loyalty Credits

Pay a service to manage your miles. App and website UsingMiles (free, but we like the $29.99-a-year service) keeps track of your accounts and helps determine if it’s better to use cash or miles on a given flight (caveat: frequent fliers on a few airlines, such as Southwest, can’t apply their miles to these services).Starwood, Marriott, IHG, and Wyndham have all introduced their own online flight-booking platforms for loyalty-club members, which offer cheaper, faster, and more flexible ways to cash in points for airline tickets. You don’t have to worry about seat availability with these programs. Even so, points go even further on free room nights.Use the 1.4-cents-per-mile rule of thumb when calculating the value of an award ticket. But factor in seat availability as well (see chart below). Airline miles are only worth something if you can actually spend them.Transferring hotel loyalty points to frequent-flier accounts is generally a bad idea. You’ll wait weeks before they post, and you’ll get better value using points for free rooms or upgrades.Highest Reward-Seat Availability Among Major U.S. Carriers*No. 1 Southwest Airlines% of Total Flights with Availability: 100No. 2 (tie) AirTran Airways% of Total Flights with Availability: 87.1No. 2 (tie) United Airlines% of Total Flights with Availability: 87.1No. 3 JetBlue% of Total Flights with Availability: 86.4No. 4 Alaska Airlines% of Total Flights with Availability: 59.3No. 5 American Airlines% of Total Flights with Availability: 45.7*From the annual Switchfly Reward Seat Availability Survey by the Idea Works Company; based on tests of reward-seat availability for June through October 2012.