Credit card interest is usually calculated daily. Your issuer divides the APR by 365 to get a daily periodic rate, multiplies it by your average daily balance, then multiplies by the number of days in the billing cycle. If you pay the full statement balance by the due date, a grace period usually means no interest on purchases.
Run your numbers in the credit card interest calculator. It shows a month of interest for any balance and APR, and how the balance moves under your payment.
The formula, step by step
The CFPB explains that many card companies calculate the interest you owe daily, based on your average daily balance. Most card agreements spell out the same three steps. Here they are with symbols:
- Daily periodic rate (DPR). DPR = APR ÷ 365. A PNC Bank agreement filed with the CFPB, for example, states that its daily rate is the APR divided by 365.
- Average daily balance (ADB). Write down the balance at the end of each day in the cycle, add them up, and divide by the number of days in the cycle.
- Interest for the cycle. Interest = DPR × ADB × D, where D is the number of days in the billing cycle.
Put together: Interest = ADB × (APR ÷ 365) × D.
The APR on your statement comes from the same rate, run in reverse. Regulation Z has the issuer multiply the periodic rate by the number of periods in a year to get the APR it discloses (§ 1026.14(b)). So a daily rate times 365 is the APR, and the APR divided by 365 is the daily rate.
A monthly version you can check quickly
If a cycle lasted exactly 365 ÷ 12 days (30.41667), the formula above reduces to ADB × APR ÷ 12. That’s the shortcut most calculators, including ours, use.
On a $5,000 balance at 24% APR, one month of interest at APR ÷ 12 is $100.00. At the average rate for card accounts that pay interest, 22.15% as of Q2 2026, here is one month of interest at a few balances:
| Balance | One month of interest at the average APR |
|---|---|
| $1,000 | $18.46 |
| $5,000 | $92.29 |
| $10,000 | $184.58 |
| $20,000 | $369.17 |
For the exact daily figure on your own card, put your balance, APR and cycle length into the credit card interest calculator instead of working it by hand.
How the average daily balance moves
The average daily balance is why timing matters. A purchase on the second day of a 30-day cycle sits in 29 of the daily balances. The same purchase on day 29 sits in two. A payment works the other way: the earlier it lands, the more days it lowers.
The CFPB puts it plainly: since interest accrues daily, the sooner you pay off all or part of your balance (when you don’t have a grace period), the less interest you pay. Paying half your planned amount mid-cycle and the rest on the due date lowers the average a little compared with one payment on the due date.
Agreements differ on what counts in the daily balance. Many use an “average daily balance (including new purchases)” method, which adds purchases on the day they post. Some separate balances by type, so purchases, cash advances and balance transfers each get their own average and their own APR. The CFPB notes your statement must show each category with a different APR and the balance in each.
The grace period
A grace period is the time between the end of a billing cycle and the payment due date. The CFPB’s definition: you may not be charged interest during that time if you pay your balance in full by the due date. Card issuers aren’t required to offer one. If they do, Regulation Z requires statements to be mailed or delivered at least 21 days before the due date (§ 1026.5(b)(2)(ii)).
Three things commonly end or skip the grace period:
- Carrying a balance. Typical agreements, such as the PNC one above, give new purchases a grace period only when the previous statement balance was paid in full by its due date. Carry a balance, and new purchases start accruing interest the day they post.
- Cash advances. These usually have no grace period, so interest starts on the transaction date. They can carry a higher APR too.
- Balance transfers. Agreements like PNC’s give these no grace period either, which matters once a promo rate ends.
Federal rules limit how far back an issuer can reach when you lose the grace period. It can’t charge interest on balances from days in earlier billing cycles, or on the part of a balance you repaid within the grace period (§ 1026.54). That rule ended the practice called double-cycle billing.
Does credit card interest compound?
Often, yes, and daily. The CFPB’s key terms page describes the daily periodic rate this way: the day’s interest is added to the previous day’s balance, which means interest compounds daily. Tomorrow’s interest is then charged on today’s interest.
Not every issuer does it. A Navy Federal Credit Union agreement in the CFPB’s database subtracts unpaid interest charges when it builds each daily balance, so interest doesn’t earn interest inside the cycle. Your agreement’s “How we calculate interest” section says which method applies.
Daily compounding makes the true yearly cost slightly higher than the APR on a balance you carry all year. The difference is small next to the APR itself. The bigger driver is how long the balance stays.
Why your statement won’t match an APR ÷ 12 calculator exactly
Our calculators take the monthly rate as APR ÷ 12. (APR ÷ 365 × 30.41667 is the same number.) The planner does not model daily-balance compounding; card issuers usually use a daily periodic rate on the average daily balance, so real statements differ by a few dollars. The gaps come from:
- Cycle length. Billing cycles vary by a few days. A cycle longer than 30.4 days charges a bit more than a month at APR ÷ 12, and a shorter one a bit less.
- Timing. Purchases and payments in the middle of the cycle change the average daily balance. A monthly model treats the balance as fixed for the month.
- Daily compounding. Interest on interest within the cycle adds a little.
- Several APRs. A cash advance or expired promo balance at a different rate changes the blend.
- Variable rates. Many card APRs are variable and track the prime rate, so the rate itself can change between statements.
Regulation Z accepts this kind of gap for the payoff estimate printed on your statement. Appendix M1 lets issuers assume either a monthly or a daily periodic rate, treat all months as the same length, and still count the estimate as accurate if it’s within 2 months of the actual figure.
How to pay less interest
- Pay the statement balance in full when you can. That keeps the grace period and brings purchase interest to zero.
- Pay earlier in the cycle if you carry a balance. Each day the balance is lower, the average falls.
- Pay more than the minimum. When you do, federal rules make the issuer apply the excess to your highest-APR balance first (§ 1026.53). See how minimum payments work for what the minimum alone costs.
- Lower the APR. Ask your issuer, or compare a balance transfer or a loan. What is a good APR for a credit card covers where rates sit today.
- Skip cash advances. No grace period and a higher rate make them the most expensive way to use a card.
To see how fast a carried balance disappears at your payment, try the credit card payoff calculator. For the terms themselves, see what APR means.
FAQ
How is credit card interest calculated?
Most issuers divide the APR by 365 to get a daily periodic rate. They multiply that rate by your average daily balance and by the number of days in the billing cycle. If you paid the previous statement in full by the due date, a grace period usually means purchases cost no interest at all.
How do I calculate my credit card interest per month?
For a close estimate, multiply your balance by the APR and divide by 12. The exact statement figure uses your average daily balance, the daily rate and the cycle’s actual length, so it can differ by a few dollars. The credit card interest calculator does either version from your numbers.
Do you pay interest if you pay the full balance every month?
Usually not on purchases. If your card has a grace period and you pay the full statement balance by the due date, purchases don’t accrue interest. Cash advances and most balance transfers are different: they typically have no grace period and start accruing interest on the day they post.
Why was I charged interest after I paid my balance in full?
If you carried a balance the month before, you had lost the grace period. New purchases then accrued interest from the day they posted until your full payment arrived, and that interest shows up on the next statement. Under agreements like PNC’s, the grace period comes back once you pay a statement balance in full.
Does credit card interest compound daily?
On many cards, yes. The CFPB describes the daily periodic rate method as adding each day’s interest to the previous day’s balance, which compounds daily. Some issuers leave unpaid interest out of the daily balance instead. Your card agreement’s section on calculating interest says which method your issuer uses.
Is credit card interest charged monthly or daily?
It accrues daily and is billed monthly. Each day’s balance is multiplied by the daily periodic rate, and the total for the cycle appears as one interest charge on your statement. That’s why paying earlier in the cycle, not only by the due date, lowers the interest you pay.