Credit card interest: why it rarely matches a simple estimate

Credit card interest is calculated daily. The issuer divides your APR by 365 (some use 360) to get a daily periodic rate, applies it to your balance each day of the billing cycle, and adds the results up, usually through a method called the average daily balance. None of that applies to purchases you pay off in full by the due date, because of the grace period. Carry part of a balance past the due date and interest runs on what you left unpaid and on new purchases from the day you make them.

From APR to a daily rate

The APR on your statement is a yearly rate, and for a credit card it is the interest rate itself; annual fees are not part of it. To use it, the issuer converts it to a daily rate. Take an illustrative APR of 24%: the daily rate is 0.24 ÷ 365, about 0.0658% a day.

A common shortcut is to divide the APR by 12 and call that the monthly rate. It is close but not exact. Over a 30-day cycle the daily rate adds up to 1.97%, slightly less than the 2% the shortcut gives; over a 31-day cycle it is 2.04%, slightly more. When your actual charge differs from a quick estimate, the bigger reason is usually the balance the estimate used, not the rate. The difference between an APR and a plain interest rate on other loans is covered in APR vs. interest rate.

The average daily balance, worked out

The issuer does not charge interest on the balance printed on your statement. It adds up your balance at the end of each day in the cycle and divides by the number of days.

Say your cycle is 30 days. You start with $2,000, and on day 11 you pay $500, so the balance is $1,500 for the remaining 20 days:

• 10 days × $2,000 = $20,000
• 20 days × $1,500 = $30,000
• Total of $50,000 ÷ 30 days = an average daily balance of $1,666.67

Interest for the cycle is the average daily balance × the daily rate × the days in the cycle: $1,666.67 × (0.24 ÷ 365) × 30 = $32.88. Estimating from the ending balance ($1,500 × 2% = $30) would have been too low, and estimating from the starting balance ($2,000 × 2% = $40) too high. The timing of your payment within the cycle moves the charge, which is why paying early in the cycle, rather than on the due date, lowers interest when you carry a balance.

What daily compounding adds

Many issuers compound daily: each day’s interest is added to the balance that the next day’s interest is calculated on. Your card agreement says which method it uses. Over one cycle the effect is small. On $2,000 for 30 days at 24%, simple daily interest is $39.45, and daily compounding makes it $39.83, a difference of 38 cents.

It grows if interest is never paid off. An APR of 24% compounded daily works out to about 27.1% over a year on a balance that is left untouched, interest included. That is the real cost of letting a balance sit while paying less than the interest each month.

For reference, the Federal Reserve’s average APR on card accounts that were charged interest is:

22.36%Credit card APR, accounts paying interestAugust 2026 · FRED

The grace period is what makes purchases free

A grace period is the time between the end of a billing cycle and the payment due date. If you pay the full statement balance by the due date, you pay no interest on the purchases from that cycle. No law requires an issuer to offer one, but most cards do on purchases, and Regulation Z requires card issuers to deliver your statement at least 21 days before the due date (12 CFR 1026.5(b)(2)(ii)).

How long the free ride lasts depends on when you buy. Suppose your cycle closes March 25 and payment is due April 20, and you always pay on April 19:

• A purchase on March 5, early in the cycle, goes 45 days without interest.
• A purchase on March 24, the day before the cycle closes, gets 26 days.

The CFPB’s explanation of grace periods notes that they typically do not apply to cash advances or checks from your issuer, which accrue interest from the date of the transaction. That is part of why cash advances cost so much.

Losing the grace period, and the residual interest after

Pay less than the full statement balance and two things happen. You are charged interest on the unpaid part, and new purchases in the next cycle accrue interest from the day you make them. The CARD Act limits how far back that goes. If you had been paying in full, the issuer cannot charge interest on days in earlier cycles or on the part you did pay by the due date (12 CFR 1026.54).

An example at 24%: your statement shows $1,000 of purchases and you pay $900 by the due date. In the new 30-day cycle you are charged interest on the $100 you left, about $1.97, plus interest on a $300 purchase made on day 10 for the 20 days that remain, about $3.95. Small amounts, but they continue every cycle until you pay in full again.

Getting the grace period back typically takes paying the full statement balance for one or two cycles, depending on the issuer. Expect one more charge along the way: residual interest. If you were carrying $1,000 and pay it all 25 days after the statement closes, the interest that accrued in those 25 days, about $16.44, appears on the next statement. Pay that too and you are clear.

Balance transfers create a related surprise. A card that carries a transferred balance typically gives no grace period on new purchases, even when the transfer is at 0%, as the CFPB’s 2023 credit card market report described. Use a different card for spending while you pay down a transfer; our balance transfer math has the rest.

When the rate itself changes

Most card APRs are variable: the prime rate plus a fixed margin, so they rise and fall with the index without any notice beyond what your agreement already disclosed. Other increases are limited by the CARD Act. An issuer can raise the rate on your existing balance as a penalty only after a payment is more than 60 days late, and if it does, it must restore the old rate after six consecutive on-time minimum payments. A penalty rate on new purchases requires 45 days’ notice and cannot start in the account’s first year. A 0% or other promotional rate must last at least six months and can only be ended early under that same 60-day rule, not for a payment a few days late. After any penalty increase, the issuer must review the account at least every six months.

Payments help in a defined order. Anything above the minimum goes to the balance with the highest APR first (12 CFR 1026.53). Paying only the minimum, by contrast, mostly covers interest, as the minimum payment trap shows.

To see how long your own balance would take to clear at your card’s APR and a payment you choose, use the payoff calculator: