Credit card interest is calculated daily using a daily periodic rate (your APR divided by 365). If you carry a balance, interest accrues on your average daily balance each day and compounds — meaning yesterday's interest starts earning interest today.
Your card's interest rate is expressed as an APR — the yearly price of borrowing. To calculate what you owe each day, issuers divide the APR by 365 to get the daily periodic rate (DPR). A 24% APR works out to a DPR of roughly 0.0658% per day.
Most issuers use the average daily balance method: they add your balance for each day in the cycle and divide by the number of days, then multiply by the DPR and the days in the cycle. Because interest compounds daily, unpaid interest is added to the balance and the next day's calculation starts higher.
Your statement may show multiple APRs — purchases, balance transfers, and cash advances each typically carry separate rates, with cash advances usually highest. When you pay more than the minimum, federal rules require the excess to be applied first to the highest-APR balance.
If your card has a grace period (most do for purchases), you avoid interest entirely by paying your statement balance in full by the due date each month. The grace period works only if you carried no balance from the prior cycle; once you carry a balance, new purchases typically begin accruing interest from the transaction date.