Capital One cardholders can get a 20% bonus when they transfer miles to Air France-KLM's frequent flyer program - here's what they need to know

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• Capital One Venture and Spark Miles users can transfer their miles to Air France/KLM's frequent flyer program - "Flying Blue" starting June 1.
• For the month of June, Capital One will offer a 20% bonus on top of the usual 2:1.5 transfer ratio, meaning that for every 1,000 Capital One miles you transfer, you'll get 900 Flying Blue miles.
• If you're looking to earn more transferable Capital One miles, the Capital One Venture Rewards Credit Card currently offers 50,000 bonus miles after spending $3,000 in the first three months.
• The Capital One Spark Miles for Business offers 50,000 miles when you spent $4,500 in the first three months.

The most valuable kind of credit card reward is transferable points- points that can be transferred to hotel and airline partners.

At the very end of last year, Capital One brought its travel rewards program in line with long-time top contenders like Chase and AmEx by introducing transfer partners to the Venture and Spark Miles cards. Every mile you earned with the cards - which earn 2x miles on every dollar spent - could still be redeemed to "erase" travel purchases, but can now also be transferred to any of 15 airline frequent flyer partners.

Miles typically transfer to airline partners at a 2:1 or 2:1.5 ratio - an example of the latter would be converting 1,000 Capital One miles to 750 Air Canada Aeroplan miles.

Transferable credit card rewards programs sometimes offer transfer bonuses to specific partners, and since upgrading its program, Capital One has been aggressive in offering bonuses. A week-long promotion in March offered a bonus on miles transferred to Emirates' Skywards frequent flyer program, while a month-long bonus on transfers to JetBlue ends today, May 31.

Friday, Capital One announced its latest promotion: a 20% bonus on transfers to Air France/KLM's Flying Blue program. The bonus is available for the month of June, ending at 11:59 p.m. ET on June 30.

In effect, the transfer ratio will be 2:1.8, meaning 1,000 Capital One miles will convert to 900 Flying Blue points.

Flying Blue can be a useful program for booking flights to Europe on either Air France or KLM, or flights on any SkyTeam Alliance member, including Delta, Korean Air, and many more.

Flying Blue also offers monthly discount promotions, which can be paired with the transfer bonus to get a significantly cheaper flight than normal.

According to Capital One, Flying Blue's June discount promotions will feature a number of routes from North America:

For the month of June, FlyingBlue promos include a 50% discount on standard economy reward bookings for routes between Atlanta (ATL) and Europe and Houston (IAH) and Europe. There will also be a 25% discount on standard business class reward bookings for routes between Chicago (ORD) and Europe, Montreal (YUL) and Europe, Edmonton (YEG) and Europe, and Calgary (YYC) and Europe. These FlyingBlue promo flights must be booked in the month of June and travel must be completed in the month of August or September.

Even if you don't live in one of those cities, it could be worth finding a cheap positioning flight to get you there in order to take advantage of a great deal (should one be available that works with your schedule).

Also worth noting: American Express, which along with Chase is also a Flying Blue transfer partner, periodically offers bonuses on transfers to the program, meaning there is often a chance to get better rates than normal.

Keep in mind that miles transfers only work one way, so you should generally have a specific redemption in mind before moving miles from Capital One to Flying Blue - otherwise, you might end up stuck with Air France/KLM miles you don't plan to use. Be sure to check availability for the flights you want, too, to know for sure that you can book them with miles. You can find availability on line, but it's typically a good idea to call the Flying Blue reservations desk to confirm it before you make the transfer.

The Capital One Venture is a formidable rewards card, offering 10x miles per dollar spent at Hotels.com (when the reservation is made through Hotels.com/Venture), and 2x miles per dollar on everything else.

Plus, you can earn a sign-up bonus of 50,000 Capital One miles when you spend $3,000 in your first three months with the card.

If you own a small business - or even if you have a solo side gig - the Capital One Spark Miles for Business offers 50,000 miles when you spent $4,500 in the first three months.

Click here to learn more about the Capital One Venture from Insider Picks' partner: The Points Guy.

Click here to learn more about the Capital One Spark Miles from Insider Picks' partner: The Points Guy.

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