See how long it takes to get rid of your credit card debt, and how much you actually pay in interest, with a fixed monthly payment.
The calculator simulates the payoff month by month: each month, interest is deducted from the remaining balance, and the rest of your payment goes toward reducing the debt. Because credit card interest is normally very high (often 15–30% annually), a large share of a low monthly payment goes toward interest at first, not the debt itself.
Because credit card interest rates are very high compared to other loans. With a 20% annual rate on $3,000, you pay over $30 in interest in the very first month alone — only what's left after interest actually reduces the debt.
Then the debt grows instead of shrinking, no matter how much you "pay" each month — the calculator warns you about this, since it means the debt will never be paid off at that amount.
As a rule of thumb, it's rarely worthwhile to save at a lower return than the credit card interest rate you're paying — getting rid of high-interest debt often gives a better "return" than most forms of saving.