Only 39% of Americans with credit card balances know this key number

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More than half of balance-carrying credit cardholders don't know their annual percentage rates (APRs), a new survey finds. Of 1,681 adults with at least one credit card polled online, only 39 percent of those who carried a balance in the past six months said they definitely knew the interest rates on each of their cards, which are prone to change over time.

"An extrapolation based on the U.S. adult population reveals that 59 million people have little or no idea of how much interest they're paying on their card debt," the reports states.

"The problem is, if you don't know what you're paying for money, you're probably paying more than you think," said Ed Mierzwinski, senior director at the U.S. Public Interest Research Group.

APRs change in accordance with the prime rate, which is tied to the federal funds rate. The benchmark federal funds rate has been raised by a quarter percentage point six times since the end of the 2015, the report states, and it's projected to rise from its current median rate, which is between 1.50 and 1.75 percent, to 2.1 percent by the end of 2018 and 2.9 percent next year.

The average APR, meanwhile, is currently 16.71 percent. If the Federal Reserve hikes rates a full percentage point, your interest payments will increase $10 for every $1,000 you carry in your balance. So, if you are $5,644 in debt — like the average revolving American consumer — you'll be charged an extra $56.44 in interest per year.

The average household with revolving credit card debt already pays about $1,000 in interest every year. That's why many experts recommend paying off your balance as soon as you can and billionaire Mark Cuban says getting rid of debt is the "best investment you can make."

If you don't currently have the funds to get out of the red, you should at least be aware of what kind of interest you're paying, as well as how that number might change.

"You can save a lot of money if you look at your statements for one minute a month," Mierzwinski said. "The credit card companies take advantage of the fact that everyone is too busy."

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Why Mark Cuban and other famous people don't use credit cards
Why Mark Cuban and other famous people don't use credit cards