United MileagePlus has introduced a way for members to convert the value of their unused gift cards to miles. The MileagePlus Gift Card Exchange allows members to convert retail gift cards with 60 major retailers into miles. Participating retailers include Barnes & Noble, Bed Bath & Beyond, Office Depot, Sears, Target and Sam’s Club, to name a few.

To check out the exchange rates, visit http://www.ualmiles.com/mpgiftcardexchange. Select a retailer from the MileagePlus Gift Card Exchange and enter the gift card information. You must have a gift card with a minimum balance of $25 and you can’t select how much of the remaining balance you want to have converted into miles–the entire balance of the card will be exchanged into miles. You don’t need to mail in your gift card and you will receive your miles within five business days. According to the FAQ, “Rates are determined by market pricing” so different gift cards will yield different mileage ratios. After entering your gift card information, you’ll receive a mileage offer. If you decide to accept the offer and convert your miles, you won’t be able to reverse your decision. All gift card exchanges are final.

We entered a $25 Barnes & Noble gift card and received an offer of 670 miles, a value of $.037 per mile, which is almost identical to the cost of buying miles directly from the airline.

Bottom line: Although the value per mile for this latest award offer from United will most likely not be the best, it offers members another option to spend miles and a use for a gift card that might otherwise go unused. It is estimated that the unused amount of gift card value in the U.S. for 2010 reached $3.6 billion, and in 2011, estimates have been as high as $2 billion. Since 2005, the value of unused cards is estimated at $41 billion.