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Your annual percentage rate (APR) is the yearly cost of borrowing money on your credit card, expressed as a percentage. But the way it's calculated—and what you actually pay—involves several moving pieces that aren't always obvious from the rate alone.
APR reflects the interest rate you're charged on a carried balance, annualized. If your card has a 20% APR and you carry a $1,000 balance for a full year without making payments, you'd owe roughly $200 in interest (before accounting for how interest compounds).
The catch: most people don't carry a balance for a full year all at once. The way interest actually accumulates depends on how your card issuer calculates it and when they apply charges.
Your APR is determined by your creditworthiness. Issuers assign different rates to different borrowers based on factors like credit score, income, and payment history. You may be offered one rate at approval and see it change over time if your agreement allows it.
Your balance is what interest is applied to. This sounds straightforward, but issuers use different methods to determine which balance:
Your grace period affects whether you pay interest at all. Most cards offer a grace period (typically 21–25 days) during which no interest accrues if you pay your full balance by the due date. Once you carry a balance past this period, interest starts accruing immediately on new purchases.
Daily compounding is standard. Issuers typically calculate interest daily, which means interest is charged on interest. This compounds the effect over time, especially if you're making only minimum payments.
Here's the basic formula issuers use:
Daily interest rate = APR ÷ 365 daysDaily interest charge = Balance × Daily interest rate
For a 20% APR with a $1,000 balance:
That charge repeats every day until the balance is paid off or the billing cycle closes.
Because the method of calculation differs, two cards with identical 20% APRs can charge different amounts of interest on the same balance.
| Factor | Impact |
|---|---|
| Balance calculation method | Average daily balance typically results in higher charges than adjusted balance |
| Grace period length | Longer grace periods lower interest if you pay in full |
| When interest starts accruing on purchases | Some cards charge immediately; others wait until after the grace period |
| Fees (annual, late payment, cash advance) | Add to total cost of borrowing, separate from APR |
Your APR doesn't include:
These add to your cost of borrowing without changing the APR figure.
Check your card's Schumer Box—the standardized disclosure table on every credit card offer or statement. It shows:
Your monthly statement also discloses interest charges and explains how they were calculated. If you're unsure whether your card uses average daily balance or another method, this information is on your statement or the card issuer's website.
The best APR for you depends on whether you expect to carry a balance. If you pay in full every month, APR is irrelevant—you'll pay no interest regardless of the rate. If you do carry a balance, comparing APRs is useful, but the method of calculation and any fees matter just as much as the percentage itself.
