Key takeaways
- Issuers divide the APR by 365 (or 360, depending on the card's terms) to get a daily rate and apply it to each day's balance.
- Interest for a cycle is roughly average daily balance × daily rate × days; a hypothetical $1,000 cycle with one purchase and one payment at 24% APR costs about $21.04.
- Paying the full statement balance on time keeps purchases interest-free on a card with a grace period; cash advances accrue from the transaction date.
A cardholder carries $1,000 on a card with a 24% APR through a 30-day billing cycle, with no purchases and no payments. Twenty-four percent spread over 12 months is 2% a month, and 2% of $1,000 is $20. Depending on the card, the statement could instead show about $19.73, $20.00 or $19.92. The spread comes from how the issuer counts the days in a year and whether yesterday’s interest joins today’s balance. Card interest is not worked out once a month on a single balance. It is figured day by day, on a balance that moves with each purchase and payment, at a daily rate set by the card agreement.
Quick answer
Most US issuers convert the APR into a daily periodic rate (APR ÷ 365, or ÷ 360 on some cards), apply it to each day’s balance and total the results for the billing cycle. In practice the charge is average daily balance × daily rate × days in the cycle. On a card with a grace period, paying the full statement balance by the due date leaves new purchases with no interest, provided the previous balance was also paid in full.
From an annual rate to a daily one
The annual percentage rate, or APR, is the yearly cost of borrowing stated as a percentage. Issuers rarely apply it in one piece. The Consumer Financial Protection Bureau explains that many card companies calculate interest daily, based on the average daily balance, and that the rate charged each day is called the daily periodic rate.
To get that rate, the issuer divides the APR by the days in its chosen year. At 24% APR, dividing by 365 gives about 0.0657534% a day. Dividing by 360 gives about 0.0666667%. The second figure is higher because the same yearly rate is spread across fewer days.
Regulation Z, the federal rule that implements the Truth in Lending Act, does not fix that divisor. What section 1026.6 does require is that issuers disclose the balance computation method, meaning the way they work out the balance a finance charge applies to. The 365-or-360 choice lives in each card’s own terms. On a steady $1,000 balance over a 30-day cycle, the 365 version produces about $19.73 of interest and the 360 version $20.00, a gap of about $0.27.
One 30-day cycle, worked through
For example, suppose the same $1,000 balance sits on a 24% APR card at the start of a 30-day cycle. After ten days the cardholder makes a $400 purchase. Ten days after that, a $600 payment posts. The balance is therefore $1,000 for the first ten days, $1,400 for the next ten and $800 for the last ten.
The average daily balance method weights each balance by how many days it lasted.
| Stretch of the cycle | Daily balance | Days | Balance × days |
|---|---|---|---|
| First 10 days | $1,000 | 10 | $10,000 |
| Middle 10 days | $1,400 | 10 | $14,000 |
| Last 10 days | $800 | 10 | $8,000 |
| Total | — | 30 | $32,000 |
Hypothetical balances for illustration at a 24% APR. Average daily balance method as described by the CFPB (checked October 2026).
The rest takes three steps.
- Divide the total by the days in the cycle. $32,000 ÷ 30 is about $1,066.67, the average daily balance.
- Multiply by the daily rate. At about 0.0657534%, that balance costs roughly $0.70 a day.
- Multiply by the 30 days in the cycle. The interest charge comes to about $21.04, or about $21.33 on a card that divides by 360.
The charge is higher than on the steady $1,000 because the $400 purchase raised the balance for part of the cycle, not because of the daily method itself. The $600 payment helped even though it arrived two-thirds of the way through the cycle. Every day spent at a lower balance shrinks the last column. For someone carrying a balance, paying earlier in the cycle trims the charge slightly, while paying later lets the higher balance run longer.
To repeat the exercise on your own statement, look for the interest charge calculation section. It lists each APR and the balance it was applied to, which section 1026.7 of Regulation Z requires periodic statements to show. Multiply that balance by the APR, divide by 365 (or 360), and multiply by the days in your billing period. The result should land within a few cents of the printed charge.
Why yesterday’s interest can earn interest today
Some agreements build each day’s balance from the previous day’s balance plus new charges, minus payments and credits. If that previous balance includes the interest added the day before, interest compounds daily. Whether a particular card works this way is part of the balance computation method that Regulation Z § 1026.6 requires issuers to disclose, so the card’s own terms are the place to check.
For example, a $1,000 balance at 24% APR ÷ 365 picks up about $0.66 of interest on day one. On day two the balance is about $1,000.66, so the next charge is figured on that slightly larger amount. Over 30 days with no purchases or payments, daily compounding produces about $19.92 instead of $19.73, roughly 19 cents more.
Within a single month the effect is small. Over many months of carried balances it adds up, because each month’s charge feeds the next month’s average.
The grace period, and what ends it
Regulation Z defines a grace period as a period within which credit may be repaid without incurring a finance charge due to a periodic interest rate. In plain terms, it is the window between the statement and the due date. The CFPB says that if a card has one, paying the balance in full by the due date each month avoids interest on purchases. That is the payment that makes the charge zero: the full statement balance, on time, every cycle.
The conditions matter. According to the CFPB, the grace period usually applies only to new purchases, and only if the cardholder was not already carrying a balance. Paying the minimum, or anything short of the full statement balance, usually switches it off. New purchases then start accruing interest at once, and the issuer runs the daily calculation shown above.
A cardholder who has been carrying a balance and then pays the statement in full may still see a small interest charge on the next one. Some issuers charge interest for the days between the statement closing date and the day the payment arrives, and the card’s terms say when the grace period returns.
Cash advances and balance transfers follow a different clock. Sample disclosure wording in Regulation Z’s model clauses says interest on these transactions begins accruing on the transaction date, and cash advances generally have no grace period. A cash withdrawal from an ATM on a credit card therefore starts the daily calculation immediately, even for someone who pays in full every month. The card’s terms list any separate APR or fee for these transactions.
What the Minimum Payment Warning box shows
Every periodic statement must carry a box headed “Minimum Payment Warning,” a requirement set out in Regulation Z § 1026.7(b)(12). It opens with fixed wording: “If you make only the minimum payment each period, you will pay more in interest and it will take you longer to pay off your balance.” Unless repayment at the minimum would take 3 years or less, the box adds a 36-month comparison: the monthly payment that would clear the balance in that time, the total cost on that schedule and the savings.
The box is the daily-interest arithmetic run forward month after month. When a payment barely exceeds the month’s interest charge, the balance falls slowly, the average daily balance stays high, and the next charge stays high with it. The 36-month line shows the trade in dollars: a bigger fixed payment, a smaller total.
Where your statement may differ from the math
Several things can pull a real charge away from the textbook figure. A card can hold separate balances, such as purchases, cash advances and promotional transfers, each with its own APR and its own average daily balance. The statement adds the results. Billing cycles vary by a few days, so the day count changes from month to month. A variable APR can move between cycles, and the statement shows the rate actually applied. Fees, such as late fees or cash-advance fees, are listed separately from interest.
The 24% rate in these examples is a round figure chosen for the arithmetic. The Federal Reserve’s G.19 consumer credit release of September 8, 2026, put the annualized rate on credit card accounts assessed interest at 22.15% for the second quarter of 2026, up from 21.52% in the first quarter. Measured across all accounts, including those charged no interest, the second-quarter rate was 20.94%. The first series is the better benchmark for anyone carrying a balance, because accounts that pay in full pull the second one down. Both series are quarterly and can be revised in later releases.
The day-count divisor and the compounding rules are set by each issuer, not by federal regulation, so two cards with the same APR can produce slightly different charges. These mechanics follow US Regulation Z; the UK, Canada and Australia set their own rules for card interest and disclosure.
To check a charge, multiply the balance subject to interest by APR ÷ 365 and the days in the cycle; a bigger gap than a few cents points to a different method.