How to Calculate Credit Card Interest, Step by Step

Quick Answer: To calculate credit card interest, divide the APR by 365 to get the daily periodic rate, work out your average daily balance for the billing cycle, then multiply: average daily balance × daily periodic rate × days in the cycle. With a 24.99% APR, a $1,233.33 average daily balance and a 30-day cycle, the interest charge is about $25.33.

Credit card interest is calculated from three numbers printed on or derived from your statement: the APR, the average daily balance, and the number of days in the billing cycle. Divide the APR by 365 for the daily rate, multiply it by the average daily balance, and multiply again by the days in the cycle. The result should land within cents of the interest charge your issuer shows.

How do card issuers calculate interest?

The Consumer Financial Protection Bureau's explanation, How does my credit card company calculate the amount of interest I owe? (read on 26 October 2026), puts it in two sentences: many card companies calculate the interest you owe daily, based on your average daily balance, and the rate charged each day is called the daily periodic rate.

That gives you the three ingredients:

1. APR (annual percentage rate). The yearly rate in your cardmember agreement and on the statement. Purchases, cash advances and balance transfers often have different APRs.
2. Daily periodic rate (DPR). The APR divided by the number of days the issuer uses for a year. Most US agreements use 365; your cardmember agreement names the divisor.
3. Average daily balance (ADB). The balance at the end of each day in the cycle, added up and divided by the number of days.

The formula: interest charge = ADB × DPR × days in the billing cycle.

You will notice the APR is never divided by 12. A "monthly rate" of APR ÷ 12 is a quick estimate, but issuers charge by the day, and billing cycles run anywhere from 28 to 31 days, so the daily method is what reconciles to your statement.

When is interest charged at all?

Before doing any arithmetic, check whether interest applies. The CFPB's page on credit card grace periods explains that a grace period is the time between the end of a billing cycle and the payment due date. If you pay the balance in full by the due date, you may not be charged interest on purchases. If you do not, interest is charged on the unpaid portion, and new purchases in the next cycle accrue interest from the date each one is made.

Two practical consequences:

• Paying in full each cycle usually means no purchase interest, so there is nothing to calculate.
• Cash advances typically have no grace period. The CFPB notes they generally accrue interest from the transaction date, which is why a cash advance shows interest even on a card you pay off every month.

A worked example with real statement numbers

Take a 30-day billing cycle on a card with a 24.99% purchase APR. You started the cycle carrying $1,200 from last month, made a $300 purchase on day 11, and paid $500 on day 21.

Step 1: daily periodic rate. 24.99% ÷ 365 = 0.068466% per day (0.00068466 as a decimal).

Step 2: daily balances. Group the days by balance:

Step 3: average daily balance. $37,000 ÷ 30 = $1,233.33.

Step 4: interest. $1,233.33 × 0.00068466 × 30 = $25.33.

Notice how timing changes the number. If you had made the $500 payment on day 1 instead of day 21, the balance would have been lower for twenty more days and the interest charge would have dropped. If nothing had happened all cycle and the $1,200 sat there for 30 days, the charge would be $1,200 × 0.00068466 × 30 = $24.65.

Why your number may be a few cents off

• Daily compounding. Many issuers add each day's interest to the balance used the next day. Run the same example with that method and the charge comes to about $25.59 instead of $25.33. The cardmember agreement says whether interest compounds.
• Posting date versus transaction date. Issuers use the dates in their own records. A purchase made on the 10th that posts on the 11th counts from the date the agreement specifies.
• Fees in the balance. Late fees and other fees can be included in the balance that accrues interest, depending on the agreement.
• Minimum interest charge. Some cards charge a small minimum whenever any interest is due. It appears on the statement as a separate line or as the interest charge itself.

How do you calculate interest with more than one APR?

Each balance type is calculated separately and then added. Suppose the same card also had a $200 cash advance for the last 15 days of the cycle at a 29.99% cash advance APR:

The cash advance's average daily balance is $100 because $200 sat on the account for 15 of the 30 days ($200 × 15 ÷ 30). Most US statements include an interest charge calculation section that lists each balance type, its APR and the balance subject to interest, which is where you find the inputs for this table. If you have never located that section, our guide to reading a credit card statement points it out, and finding your credit card interest rate covers where the APR itself is printed.

Rebuilding the calculation in Excel

Checking a single cycle by hand is fine. Checking a year of cycles, or several cards, is easier in a spreadsheet built from the statement's transaction list.

1. Get the transactions into rows. If your issuer offers a CSV download of activity, use it. If you only have the PDF statements, a converter such as QuickBankConvert turns each statement into rows with a date, description and signed amount (charges and payments carry opposite signs). See credit card statement to Excel for that step.
2. Add the opening balance as the first row, dated the first day of the cycle.
3. Sort by date and add a running balance column: in D3, =D2+C3 if column C holds amounts with charges positive (flip the sign if your export shows charges as negative).
4. Count the days each balance held. In E2, =A3-A2; for the last row, use the cycle's closing date plus one, minus the row's date.
5. Multiply balance by days. In F2, =D2*E2, filled down.
6. Average daily balance. =SUM(F:F)/30, replacing 30 with the actual days in the cycle (closing date minus opening date plus one).
7. Interest. =ADB*(APR/365)*days, with the APR entered as a decimal, such as 0.2499.

If several transactions fall on one day, only the end-of-day balance matters; the days-held column for the earlier rows on that date will be zero, which is correct. A spreadsheet like this is a check on the issuer's arithmetic, not a substitute for the statement: the interest charge on the statement is what you owe. If a QuickBankConvert file is your source, it is sent to our servers for parsing and a copy is kept until you ask us to delete it; plans and limits are on the pricing page.

How much interest will you pay over time?

The same daily-rate logic answers "how much interest am I paying." Add the interest charge lines from each statement for the period you care about; many issuers also print interest charged year to date on the statement. Projecting future interest depends on what you pay, when, and whether your APR is variable, so treat any projection as an estimate rather than a promise. The CFPB's credit card resources cover minimum payments, APR changes and your rights if you think a charge is wrong.

For the background on why interest builds this way, see how credit card interest works. For when a cycle starts and ends, which sets the "days" in the formula, see the guide to your credit card closing date.

Related guides

• How credit card interest works
• Find your credit card interest rate
• How to read a credit card statement
• Reconcile credit card statements in Excel