Since the dawn of travel loyalty programs 25 years ago, mileage collectors have found themselves bedeviled by overabundance: too many miles earned in too many different programs.

Mileage overabundance is the result of indiscriminate earning behavior, racking up miles in the program of whatever airline happens to fit the bill for any given trip. With no sustained loyalty or overarching plan to focus on a single airline's program, travelers accumulate a few miles in this program, a few more in that program, and so on.

Then comes the day of reckoning. While the total number of accrued miles may be significant, they don't add up to a single free ticket because they were earned in different programs and reside in multiple uncombinable accounts. Inevitably, the question arises: How can I consolidate my miles in a single account?

If frequent-flier miles were truly like the currencies to which they're often compared, it would be a simple matter to convert miles from several programs into miles in a single program, just as yen and euros and pounds can all be exchanged readily for dollars. In fact, mileage exchange is often impossible, rarely easy, and almost never worthwhile.

Before addressing the available options for consolidating miles, it's worth considering the standpoint of the airlines and hotels that operate the schemes. Rewards programs foster loyalty by doling out awards and perks to consumers who purchase more or less exclusively from the airline or hotel that hosts the program, plus a network affiliated with that program. But if American, for example, were to allow members of the program of its archrival, United, to transfer their miles into American's program, the loyalty effect of that program would be diluted. Why bother to fly American exclusively if miles for United flights can be transferred into an American account?

EXCHANGE MART

Points.com is the only company in the business of exchanging miles and points among different programs.

Points.com members can convert miles and points among retail, hotel and airline programs, including those of Air Canada, Alaska, American, Delta, Frontier, Hawaiian, Midwest, Southwest, and US Airways.

While the Points.com conversion procedure is quick and easy, a significant number of the original miles will be lost in the process. Exchanging 10,000 American miles, for instance, would net only 1,581 Frontier Early Returns miles, an 84 percent loss, or 948 Alaska Mileage Plan miles, a 91 percent loss. And some airlines, like Delta and US Airways, permit miles to be transferred in but not out.

Other systems for exchanging miles rely on a "middleman" strategy: converting miles from one airline program into points in an intermediary program, and then redeeming those points for miles in a second airline program.

DINERS CLUB REWARDS

The Diners Club Rewards program, for instance, allows program members to exchange 10,000 American AAdvantage miles for 5,000 Club Rewards points. The Club Rewards points can then be redeemed for miles in the programs of Air Canada, Alaska, American, Delta, Frontier, Hawaiian, Midwest, Southwest and 13 other airlines. Since points convert 1 to 1 to miles in most participating airline programs, 50 percent of the original miles will be lost when converting from American to another program.

Until recently, United miles also could be converted into Club Rewards points. But now only American miles may be converted, severely limiting the Diners program's utility as a conversion service.

HILTON HHONORS

Another vehicle for converting miles is Hilton's HHonors program. Through the program's Reward Exchange feature, members may exchange miles from the programs of American, Amtrak, Hawaiian, LAN, Mexicana, Midwest, South African, and Virgin Atlantic for HHonors points. And HHonors points may be redeemed for miles in the programs of 29 programs, including Air Canada, Alaska, American, Continental, Delta, Hawaiian, Midwest, Northwest, and Southwest.

For most airlines, 5,000 miles convert to 10,000 HHonors points. (There's also a $25 processing fee when exchanging American miles to points.) And when exchanging HHonors points for miles, the conversion rate is 10,000 points for either 1,000 or 1,500 miles in most of the participating airline programs. So at the end of the conversion process, 70 percent to 80 percent of the initial miles will have disappeared.

PRIORITY CLUB REWARDS

Another hotel program through which miles can be converted is InterContinental's Priority Club Rewards.

Similar to Diners Club conversions, only miles from American's AAdvantage program can be exchanged for Priority Club points, at a rate of 1,000 miles for 800 points. Points are then redeemable for miles in 36 programs, including those of all major U.S. carriers, generally yielding 2,000 miles for every 10,000 miles. So a typical conversion from American to another program would result in a loss of 80 percent of the original miles.

AMTRAK GUEST REWARDS

Somewhat improbably, the latest option for mile conversion is courtesy of Amtrak's Guest Rewards program.

As with the Diners, Hilton and InterContinental programs, mileage conversion is a two-step process. First, miles from the programs of Continental or Midwest are exchanged for points in the Amtrak program. And second, those points are redeemed for miles, again in the programs of either Continental or Midwest.

Here the original miles' value is preserved. — Thus 10,000 Continental miles can be exchanged for 10,000 Midwest miles, and vice versa. But with only two airline programs participating, its value to would-be converters is limited.

Reach Tim Winship at questions@frequentflier.com