Marriott Bonvoy members can certainly use their points for free stays at Marriott. But there’s another use for Marriott Bonvoy points: booking flights.

Marriott, like other hotel loyalty programs, allows members to convert points to airline miles. But unlike other hotel programs, Marriott partners with a whopping 40 airline transfer partners and offers a 3:1 transfer ratio for 38 of them.

Some airline partner transfers can be quite lucrative, while others aren’t great at all. Here are the best (and worst) Marriott transfer partners.

The value of Marriott Bonvoy points

But first before you transfer your Bonvoy points, you should know what a Bonvoy is actually worth. NerdWallet values Marriott Bonvoy points at 0.8 cent each. These points transfer to most airline partners at a rate of 3 Bonvoy points to 1 airline mile. When you’re trading 3 Bonvoy points (worth 2.4 cents according to our valuations), you’ll need to get at least 2.4 cents in value from every airline mile to break even.

This is a little easier if you take advantage of Marriott’s transfer bonus. For every 60,000 Bonvoy points that you transfer, Marriott will add 5,000 more miles. So with most airlines, instead of getting 20,000 miles for 60,000 Bonvoy points, you get 25,000 miles per 60,000 Bonvoy points. At that rate, you need to extract just 2 cents per airline mile to break even.

Note that this transfer bonus does not apply to American, Delta, Avianca or Korean Air, but that transfers to United come with an additional bonus (more on that below)

To figure out how much value you get from each airline mile, divide the cash price of your plane ticket by the number of miles you’ll have to redeem. For example, let's say a round-trip economy ticket from Los Angeles to London costs $750 in cash or 50,000 miles. Divide $750 by 50,000 and you'll find that you’re getting a value of 1.5 cents for each mile, which isn’t that good. But if you look at that same trip in a business class seat ($4,650 in cash or 117,500 miles), you'll get nearly 4 cents per mile — a very good value.