Cashing in your frequent-flyer miles for airplane seats can seem like smashing open a piñata: you’ll sometimes find something you like inside, but you can never be sure in advance what you’ll actually get.

Virgin Australia airbus. Image by Aero Icarus / CC BY-SA 2.0

Virgin Australia airbus. Image by Aero Icarus / CC BY-SA 2.0 Image by Aero Icarus / CC BY-SA 2.0

Airlines give away miles much more easily than they offer free flights in exchange for miles earned. For travelers, that makes it worthwhile to know which airlines have the most seats available for redeeming miles.

Airberlin and Southwest Airlines are the top two carriers globally when it comes to having the most number of reward seats up for grabs — at least one for every flight, on average. That’s according to travel consultancy IdeaWorksCompany, which ran a survey this spring for Switchfly, a company that sells e-commerce software.

Not all airlines are as generous. This spring, availability of award tickets on Virgin Australia was twice as good, for example, as on LAN, at the basic mileage level. Virgin Australia had reward seats on 96.4% of the flights searched, up from only 80% of flights in 2013. LAN was the stingiest of the 25 airlines surveyed, with availability only 44% of the time.

Ticketing glitch sees Singapore Airlines flights reduced to up to half price.

SIngapore Airlines ranked high for reward seats. Image by Aero Icarus / CC BY-SA 2.0

Other top-ranked airlines included Air Canada (90.7%), Singapore (90%), JetBlue Airways (87.1%), and Lufthansa/SWISS/Austrian (87.1%). All of those percentages are above the industry average of 74% this year.

The survey is based on 7,640 booking queries for economy-class or “saver-style” seats on popular routes made by testers at IdeaWorksCompany on the websites of the 25 largest frequent flyer programs during March 2015 for round-trip tickets through October 2015.

Some critics claimed the survey shouldn’t have focused on economy-class tickets, claiming that, as a rule, the best value for frequent flyers can be obtained by redeeming miles for premium-cabin seats instead. Consider the consensus of the editors of FlyerTalk, one of the leading advice forums for mileage fanatics, which is that the best value for mileage redemption on traditional carriers with extensive international networks is — in terms of monetary value per mile redeemed —for business and first-class tickets rather than economy-class ones.

But if you don’t have hundreds of thousands of miles saved up, an economy-class ticket is the most relevant option. In that case, the survey is worth a close read.