Two ways to ask the question
The calculator works in either direction. With a monthly payment, it counts the months to a zero balance and adds up the interest along the way. With a number of months, it solves for the payment that clears the card right on time.
Both start from the same three facts: the balance, the APR, and either the payment or the timeline. The results also show total paid, which is the starting balance plus all the interest, and the month the card reaches zero.
What happens to a card balance each month
Card interest is commonly charged monthly at roughly the APR divided by 12. A 24% APR works out to about 2% a month. On a $1,000 balance, that is about $20 of interest for the month.
If the payment is $50, about $20 covers that interest and $30 reduces the balance. The next month starts at $970. Interest on $970 is about $19.40, so $30.60 goes to the balance. The split shifts a little every month.
The same cycle repeats until the balance reaches zero. The final payment is usually smaller than the rest, since it only has to cover what is left plus that month's interest.
Why the first payments barely seem to move it
Early on, interest takes its biggest bite because the balance is at its highest. In the example above, 40% of the first payment goes to interest. As the balance drops, interest shrinks and more of each payment goes to principal, but the change is gradual.
The closer a payment sits to the monthly interest, the slower this goes. A payment just a few dollars above interest can take many years. If a payment is less than one month of interest, the balance would never shrink, so the calculator raises it to interest plus $1 and says so in the results.
The credit card interest calculator breaks the rate down by day and month, and the minimum payment calculator shows what happens when the payment follows the card's shrinking minimum.
Picking a timeline instead of a payment
A shorter timeline needs a larger payment, but the total interest is lower because the balance spends less time sitting at the APR. A longer timeline lowers the payment and raises the interest. Some people start from what fits the budget each month; others start from a month they want the card gone by.
Total paid makes the trade-off visible in dollars. Trying a few different timelines side by side shows how the monthly payment and the total interest move in opposite directions.
For more than one card at once, the multiple credit card payoff calculator handles the order and the rollover between cards.
What the estimate assumes
The math assumes no new purchases, a fixed APR, and one interest charge per month. Statements usually figure interest on the average daily balance, so payment timing and the number of days in a billing cycle can move the real totals slightly. The result is a close estimate, not a statement.