But as this summer travel season approaches, anyone hoping to use miles to upgrade to business class to Europe, say, should first read the fine print — and then take out the calculator.

Here’s why: Getting an upgrade can involve just cashing in frequent-flier miles or cashing in miles and paying an additional fee — and the amounts vary by airline. Then, you have to consider the type of seat you buy, because airlines differ on which seats can be upgraded. So while all the airlines allow upgrades from economy-class seats, some set a higher hurdle for the kinds of economy tickets you must buy to be upgraded.

Some programs, like those of American Airlines and Continental Airlines, appear to offer more flexibility than Delta Air Lines, for example. And benefit programs, even among airlines that are more flexible, may vary.

Randy Petersen, who oversees the Web site of WebFlyer, says that travelers may want to believe that all frequent-flier plans are created equal, but they are not. “The two most important parts are miles and the money to upgrade.”

Tim Winship, publisher of FrequentFlier.com, which tracks the frequent-flier business, agreed. “The whole question of which fares are upgradeable is a dirty little secret. The airlines will tell you that if you do your research, it will be disclosed that only such and such a fare base is upgradeable,” he said. “There is an incentive to mislead people because it makes the miles look more valuable if you don’t get into much detail before you can upgrade.”

There are other factors at work, too. One, for example, is how many seats are available for upgrades, although the airlines keep much of that information private. And much of the choice of frequent-flier programs is dictated by where you live and which airlines dominate your hometown market. For instance, neither United, which has a sliding scale policy related to the price of an economy ticket, nor US Airways, which charges $300 and 30,000 miles each way to upgrade regardless of ticket price, now fly directly to Europe out of New York’s airports.

Passengers who choose Delta Airlines instead of American or Continental are generally required to buy a more expensive economy-class ticket if they want to become eligible to upgrade to business class for overseas trips.

Asked about the comparisons, Paul Skrbec, a Delta spokesman, said: “The description is accurate. We are very clear on what people need to do within our program rules.”
Continental and American let passengers upgrade from an economy-class ticket — even the least expensive ones. And even though those airlines charge a fee for the upgrades as well as take miles out of your account, a recent check of upgrade costs on Delta for trips to London from New York and to Rome from Chicago, showed that the out-of-pocket costs on Delta were higher.

An upgradeable ticket on a 10-day trip from New York to London starting on June 1 would cost $2,356 round trip on Delta. If an upgrade were available, it would require 25,000 miles each way, but there would be no additional charge. A simple economy-class ticket would cost $953.60, but would not allow a passenger to apply for upgrades.

The same trip on American turned up the same price, $953.60, for the ticket. To upgrade — if the upgrade were available, and that is an “if” on any airline, would cost $350 and 25,000 miles each way. If a traveler is cleared for upgrade in both directions, the fare would be $1,653.60. For an upgrade in just one direction, the fare would be $1,303.60.

Also noteworthy, an American Airlines passenger can request an upgrade for just one leg of the trip. With Delta, on the other hand, a passenger who wants to upgrade in only one direction would still have to pay for the higher round-trip ticket to qualify for the one-way upgrade (though he or she would use only 25,000 miles).

Continental has its own version of the upgrade. On a route from New York to Amsterdam, for instance, where both Delta and Continental fly direct, an upgradeable Delta ticket was $2,710.40 and would require 50,000 miles to upgrade both ways. Continental has a slightly more complex, but less costly formula. For its lowest fare, for the first week in June, a ticket would cost $1,145 round trip along with $500 to upgrade each way. But Continental takes only 20,000 miles for each upgrade. So a total round-trip upgrade would cost $2,145 and 40,000 miles.

Continental also has a sliding scale for the fees. For the traveler who buys a higher-class economy ticket for $1,776.41, the charge for upgrade drops to $250 each way (and 20,000 miles each way). Continental also offers a full-fare economy ticket, for $2,417 round trip, that allows an upgrade for 40,000 miles, without extra fees.

Airlines vary as well on the numbers of miles required for each leg of an upgrade. United Airlines, like Continental, charges 20,000 miles for each leg of most upgrades overseas, while both American and Delta charge 25,000.

Of course, these fares change frequently; what was true in the summer may not be true in the fall, and pricing gaps between airlines on the same route could shrink or widen.
For a round-trip ticket from New York to London leaving on Oct. 1, a recent check showed the price of a Delta upgradeable ticket at $2,376.50, nearly the same as in June.
American’s price, meanwhile, jumped to $1,099, from $953.60 in June.

Travelers should expect the cost of upgrading to continue to rise.
“Between the rebounding of the economy and the cutback in inventory, the airlines have more pricing power, and I expect that to continue for the foreseeable future,” Mr. Winship said. “Everything points in the direction of increased ticket prices.”