Credit card interest

Credit card interest is the price a borrower pays for carrying an unpaid balance on a credit card. It’s quoted as an annual percentage rate (APR) but applied much more frequently — typically daily — to the average daily balance, then billed as a monthly interest charge.

How it’s calculated

Most card issuers convert the APR to a daily rate by dividing by 365 (or 360), then apply that rate to the outstanding balance every day. The accumulated daily interest is then added to the balance once per billing cycle. For modelling purposes, this daily-compounded behaviour is well-approximated by applying one-twelfth of the APR to the balance each month.

A typical consumer credit card APR sits between 15% and 25%, with penalty rates and store cards often running higher. At 20% APR, a $5,000 balance accrues roughly $83 in interest in the first month alone.

Why credit card rates are so high

Credit cards are unsecured revolving credit — there’s no collateral, no fixed term, and the borrower can draw and repay flexibly. To compensate for the higher default risk and the operational cost of running a revolving line, issuers charge significantly higher rates than secured loans (mortgages, car loans). The rate also funds rewards programs, fraud reimbursement, and processing infrastructure.

The compounding trap

Credit card interest compounds against the borrower in two ways. First, interest accrues on a balance that includes previously charged interest, so unpaid interest itself starts earning interest the next month. Second, minimum payments fall as the balance falls, slowing the payoff and extending the period over which compounding works.

The combination is why credit card balances paid down at the minimum rate often take 25+ years to clear and cost two or three times the original balance in total interest.

How to limit the cost

• Pay the statement balance in full each cycle — when the balance is paid off, no interest accrues.
• Pay above the minimum — every dollar above the minimum reduces the balance disproportionately because it cuts both balance and time.
• Move balances to lower-rate products — balance-transfer cards or unsecured personal loans at lower rates often pay back the transfer fee within a few months.
• Avoid cash advances — these usually attract a higher rate than purchases and have no interest-free period.

Credit card interest is one of the most expensive forms of consumer debt available, but it’s also one of the easiest to avoid. The simplest discipline — never carry a balance month to month — eliminates the cost entirely.