A credit card balance grows faster than its stated annual rate suggests. The reason is that interest is calculated against a daily balance rather than once at the end of the year.
The daily periodic rate
Issuers take the annual rate and divide it by the number of days in the year. The result is a daily periodic rate, applied to whatever balance is outstanding on that particular day.
Each day's interest charge is added to the balance. The next day's calculation then runs against the slightly larger figure, and so on through the cycle.
By the time a statement is produced, the accumulated charge is a little higher than a single annual calculation would produce. The gap is small over one month and widens as a balance persists across many.
Why the grace period changes the picture
Most cards do not charge interest on new purchases if the previous statement was settled in full. That interest-free window exists between the purchase date and the payment due date.
The window is conditional rather than permanent. Once a balance is carried, many issuers begin charging interest on new purchases from the transaction date instead of the statement date.
This is why the transition from paying in full to carrying a balance feels abrupt. The mechanism changes, not just the amount owed.
Minimum payments and the order of application
A minimum payment is calculated as a small fraction of the balance plus accrued interest and fees. It is designed to keep an account current, not to reduce principal quickly.
When a payment arrives, issuers apply it according to rules that often direct the amount toward lower-rate balances first, above the minimum in some jurisdictions.
The practical effect is that a balance carrying several different rates unwinds unevenly, with the most expensive portion frequently the last to shrink.
Why cash advances behave differently
Cash advances usually carry a higher rate than purchases and typically have no grace period at all. Interest begins accruing from the moment the cash is drawn.
A separate transaction fee is generally applied at the same time, and that fee itself becomes part of the balance that interest is calculated against.
Two identical amounts drawn on the same card can therefore cost very different sums depending only on how they were taken.
The balance calculation method matters
Issuers differ in which balance they charge against. An average daily balance method smooths out the month, while other methods can include balances already paid down.
Two cards quoting the same annual rate can produce different charges purely through this choice of method.
The method is stated in the cardholder agreement rather than in headline marketing, which is why the quoted rate alone rarely explains a statement.