How Credit Card Interest Is Calculated (Average Daily Balance)

Most US credit cards charge interest with the average daily balance method. The issuer divides your APR by 365 to get a daily periodic rate, multiplies it by your average daily balance for the billing cycle and then by the number of days in the cycle. A $1,483.33 average daily balance at 22.99% APR over a 30-day cycle costs about $28.03 in interest. If you pay the full statement balance by the due date every month, the grace period means you pay no interest on purchases at all.

The formula

• APR: the annual rate on your statement. Cards often have separate APRs for purchases, cash advances and balance transfers, plus a penalty APR.
• Daily periodic rate (DPR): APR ÷ 365. A few issuers use 360, which makes the daily rate slightly higher.
• Average daily balance (ADB): the sum of each day’s ending balance divided by the number of days in the cycle.

Many issuers compound daily, adding each day’s interest to the balance so it earns interest the next day. That makes the true cost slightly higher than the simple formula. Your card agreement’s “How we calculate your balance” section says which method applies.

Worked example: one billing cycle

Your card has a 22.99% APR and a 30-day cycle. You started the cycle carrying $1,200 from last month (so there is no grace period this cycle).

1. Average daily balance: $44,500 ÷ 30 = $1,483.33
2. Daily periodic rate: 0.2299 ÷ 365 = 0.06299%
3. Interest: $1,483.33 × 0.0006299 × 30 = $28.03

With daily compounding the same cycle costs $28.27, a difference of about 24 cents. Notice that the timing matters: paying the $300 on day 11 instead of day 26 would have lowered the ADB and the interest. The credit card interest calculator runs this calculation for any cycle.

How the grace period works

A grace period is the time between the end of a billing cycle and the payment due date, at least 21 days by law when a card offers one. If you paid the previous statement in full, new purchases are not charged interest during that window. The catch: if you carry any balance, you usually lose the grace period, and new purchases start accruing interest from the day they post. You typically need to pay in full for one or two consecutive cycles to get it back.

Cash advances rarely have a grace period. They usually carry a higher APR plus an upfront fee, often 3% to 5% of the amount.

Why the APR understates the cost

An APR is a nominal rate. Because unpaid interest joins the balance, the effective annual rate is higher: 22.99% compounded daily works out to about 25.8% a year if the balance is never paid down. The guide on APR vs. APY explains the conversion.

Minimum payments: the slow road

Minimum payments are designed to keep the account current, not to pay it off. Many large issuers set the minimum at about 1% of the balance plus that month’s interest and fees, with a dollar floor. On $5,000 at 22.99%, the first month’s interest is about $95.79, so the first minimum is about $145.79, but only $50 of it reduces the balance.

Here is how long $5,000 at 22.99% takes to repay under different strategies, with interest estimated monthly at APR ÷ 12 and no new charges:

The minimum-payment path costs more in interest than the original balance. Locking in a fixed payment, even one only modestly above the first minimum, cuts years off. Under the CARD Act, your statement must show how long minimum payments would take and what payment would clear the balance in three years. Test your own numbers in the credit card payoff calculator.

How payments are applied across balances

If you have balances at different APRs (say a 0% promotional transfer and 24% purchases), the minimum payment can go to any of them, usually the lowest-rate balance. Anything above the minimum must be applied to the highest-APR balance first. That rule is one reason to pay more than the minimum when you have a promotional balance.

Ways to pay less interest

1. Pay the statement balance in full whenever you can, to keep the grace period.
2. Pay early or more than once a month. A mid-cycle payment lowers the average daily balance.
3. Target the highest APR first if you carry several balances; compare methods in debt snowball vs. avalanche.
4. Consider a balance transfer to a 0% promotional APR, after accounting for the transfer fee, typically 3% to 5%. The balance transfer calculator shows the break-even.
5. Ask for a lower APR. Issuers sometimes reduce rates for customers with good payment history.
6. Avoid cash advances and keep payments on time to avoid a penalty APR, which can apply after a payment is more than 60 days late.

Checking your statement

Your monthly statement lists the APR, balance subject to interest rate (the ADB) and interest charged for each balance type. Multiply the balance subject to interest by the daily rate and the days in the cycle; the result should match within a few cents, with any difference explained by daily compounding. If it does not, call the issuer, and if you cannot resolve a billing error, you can submit a complaint to the CFPB.

This guide is general information, not financial advice. Card terms vary by issuer and account; your cardholder agreement and monthly statement control how interest is charged.