If you’ve flown lately, you know there aren’t many empty seats for frequent fliers. In fact, airlines don’t actually have enough seats to redeem all the frequent flier miles they’ve awarded.

So, they have to convince us to use them for other things; like school supplies, hotels, or magazines. 

“The cost of giving away a free seat is higher than ever. And so airlines would love to get us to use our miles in some other way,” says Seth Kaplan, an analyst at Airline Weekly.

But that may not be the best use of your miles.

Think about it: “Would you rather have a $70 calculator or fly first class to the Bahamas?” asks Brian Kelly, founder and editor-in-chief of thepointsguy.com. “There’s an opportunity cost to every redemption.”

Kelly says it may be easier to just redeem your points for that calculator rather than trying to negotiate a free flight. But he says the best use for miles is, well, on planes. That’s where you get the most value per mile.

Still Kelly says, if your miles are about to expire, yeah, go ahead and get that calculator.