How Does Credit Card Interest Work?
Credit card interest is calculated daily, not monthly. Each day the issuer applies a daily periodic rate to your balance and adds that day’s interest into a running total. Understanding this is the key to understanding your statement.
The average daily balance method
Most U.S. issuers compute finance charges using an average daily balance. Every day of the billing cycle they record your ending balance, sum those daily balances, and divide by the number of days in the cycle. The result is multiplied by the daily rate and the number of days.
Daily compounding
Because interest accrues daily and gets added to the balance, next-day interest is charged on the previous day’s interest. Over a full year this compounding pushes the effective annual rate slightly above the stated APR.
Grace periods
If you paid your last statement balance in full and on time, most cards give a grace period on new purchases — no interest is charged if you again pay in full. Carrying any balance eliminates the grace period for the next cycle until the balance is repaid.
Practical example
On a $2,000 balance at 20% APR, the daily rate is about 0.0548%. Daily interest is roughly $1.10. Across a 30-day cycle that is about $33 in interest — which is why paying by the statement due date matters so much.
Frequently asked questions
- Does making a mid-cycle payment reduce interest?
- Yes. Because interest is calculated on the average daily balance, paying sooner in the cycle reduces the days on which interest accrues.
- Why does my interest change even if my balance did not?
- The average daily balance depends on when charges and payments post. Different cycle lengths and posting dates cause small differences month to month.