Here’s how to reinstate those lost miles.

Buying Airline Miles
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Few things are more frustrating than losing your hard-earned miles. For many airline loyalty programs, miles vanish if you don’t use them by a certain date or keep your account active. But did you know you can buy them back?

Miles expire after 18 months on American, United, and Hawaiian Airlines. Alaska Airlines and Southwest have a more generous 24-month policy. And miles never expire on Delta and JetBlue.

When to Buy Back Your Miles

“I think most people that are letting their miles expire are better off keeping the cash,” says Gary Leff, founder Book Your Award, a mileage-redemption service. “But for someone who has a lot of miles, and is ready to use them, it can be worth it.”

If you are going to use them for premium cabin international travel, for example, Leff says you should reactive those miles. That’s because you are going to get substantially more value than the cost of the points.

Most airlines charge about a penny per mile to buy back expired miles. Even at the best promotional prices, most airlines do not sell them for less than 1.7 cents. That makes it a “good deal” by comparison.

“However, there are a few catches,” says Ben Schlappig, founder of PointsPros and the One Mile at a Time blog. He advises you to reactivate miles only if you know what you want to use them for and won't let them expire again.

“There's a reason airlines let people reactive miles at a fairly reasonable cost,” he says. “They know these are also the people who may forget the expiration date yet again.”

“Be sure to check paid fares before you pull the trigger,” adds Zach Honig, ‎editor-in-chief at The Points Guy. “Since in some cases you could purchase a ticket for less than you’ll spend to restore your frequent flyer balance.”

How Much You Should Pay

Schlappig says some airlines, including American and United, have better deals for those reactivating a lot of miles. “Both airlines have tiered pricing,” he says. “For example, if you have 500,001 to 750,000 United miles expiring, you can buy them back for just $700, which is roughly 0.1 cents per point — that's less than a tenth of what I value them at.”

That means for $700 you’d have enough miles for six business-class tickets between the U.S. and Europe, which Schlappig points out “would otherwise each cost thousands of dollars.”

American charges quite a bit more for miles. For example, buying back 250,001 to 500,000 miles costs $2,000. “But I still consider that a good deal, as it's less than a penny per point,” says Schlappig.

Know When They Expire

Having trouble keeping track of the expiration dates? Leff uses to keep track of frequent flier miles, hotel awards and credit card points. “They send me an email several months before awards are going to expire,” he says. “Usually I can buy something online I was going to purchase anyway, to reset my account.”

Leff says expiration policies can vary wildly, especially, for non-U.S. airlines. For example, Singapore Airlines miles expire after three years, even if your account has recent activity. But for a fee you can extend the miles for six months — or 12 months for elite travelers.

Alaska Airlines closes mileage accounts and deletes balances after 24 months — but will reinstate miles for a $75 fee for up to one year after the date of deletion.

“At the end of the day, cash should always trump miles,” says Schlappig. “Cash is a very real currency that you own, while the miles in your account can be devalued without any sort of advance notice.”