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Among the nearly half of U.S. households with credit card balances outstanding, the average debt is $15,564. Total U.S. credit card debt was $905 billion.

All U.S. household debt totals $12.96 trillion, including $8.74 trillion in mortgage debt, $1.21 trillion in auto loans and $1.36 trillion in student loans, in addition to credit card debt.

The data were reported today by NerdWallet and are based on an online survey conducted in early November of 2,089 U.S. adults ages 18 and over that included 1,201 people who have been in credit card debt.

The average American household with credit card debt pays $904 annually in interest charges. According to the NerdWallet survey, consumers accumulate debt for a number of different reasons, including spending above their means, becoming unemployed and paying for essentials that their income doesn’t cover.

The study also noted that household income has risen by 20% over the past decade while the cost of living has risen by 18% in the same period. But significant expenses have risen faster than income: medical expenses are up 34%, “other” expenses are up 30% and food and beverage expenses are up 22%. Housing expenses have risen 20% as well.

Health care accounted for $12 billion in credit card debt. As many as 27 million Americans pay for medical expenses with a credit card. This costs them an average of $471 a year in interest for out-of-pocket medical spending.

NerdWallet’s calculated total of $905 billion does not include overdraft lines of that are commonly cited as part of the total but which the researchers exclude because the charges do not necessarily belong to credit card users.

Americans appear to know why they have piled up so much debt. Some 41% say that spending more than they could afford on unnecessary purchases was a contributing factor to their debt load and 33% said they were forced to pay with credit for necessities not covered by their income. Non-medical emergencies was cited by 33% as a driver of credit card debt, and 23% said they ran up debt after becoming unemployed.

The NerdWallet study and charts and detail are available at the website.