How Credit Card Interest Is Calculated, Step by Step
The interest charge on a credit card statement can look like it came out of nowhere. It did not. According to the Consumer Financial Protection Bureau, many card companies calculate the interest you owe daily, based on your average daily balance. Once you know the three pieces, the daily periodic rate, the average daily balance, and the number of days in the billing cycle, you can check the number yourself.
1. Piece 1: the daily periodic rate
Your card's APR is a yearly figure, but issuers usually charge interest by the day. The CFPB calls the rate charged each day the daily periodic rate. Regulation Z links the two: when periodic rates are used, the APR is the periodic rate multiplied by the number of periods in a year. Working backward, a daily rate is the APR divided by 365 (some issuers use 360; your cardholder agreement says which).
For a card with a 19.99% APR:
19.99% / 365 = about 0.0548% per day.
2. Piece 2: the average daily balance
The average daily balance is just what it sounds like. The issuer records your balance at the end of each day in the billing cycle, adds those daily balances together, and divides by the number of days.
Here is a 30-day billing cycle with one purchase and one payment, and a card with no grace period in effect because a balance was carried from the prior month:
| Days | Balance | Days x balance |
|---|---|---|
| Days 1 to 10 | $1,200 | $12,000 |
| Days 11 to 22 (after a $300 purchase) | $1,500 | $18,000 |
| Days 23 to 30 (after a $600 payment) | $900 | $7,200 |
| Total | $37,200 |
Average daily balance = $37,200 / 30 = $1,240.
3. Piece 3: putting it together
Interest for the cycle = average daily balance x daily rate x days in the cycle.
$1,240 x 0.0548% x 30 = about $20.37.
That is the number that would show up as the interest charge for this cycle, before any rounding conventions or fees. Some issuers compound daily, adding each day's interest to the balance used the next day, which nudges the figure slightly higher. Your agreement describes the exact method.
4. Why timing matters so much
Because interest accrues every day, the CFPB points out that if you do not have a grace period, the sooner you pay off all or some of your balance, the less interest you will pay. In the example, the $600 payment arrived on day 23. If the same payment had arrived on day 11, along with the purchase, the balance for days 11 to 30 would have been $900 instead of a mix of $1,500 and $900, and the average daily balance would have dropped to $1,000. Same payment, same month, lower interest: about $16.43 instead of $20.37.
Paying as early as you can, or splitting a payment into two during the month, lowers the average daily balance on a card that is already accruing interest.
5. The grace period changes everything
If your card has a grace period, the CFPB explains that you can avoid paying interest on purchases if you pay your balance in full by the due date each month. The grace period usually applies only to new purchases and only if you were not already carrying a balance. That is why the example above assumed a carried balance: once you carry one, new purchases often start accruing interest right away.
Cash advances and balance transfers often carry different rates than purchases, and usually do not get a grace period at all. Your statement must show each category with a different APR and the balance in each.
6. Where your payment goes
When you pay more than the minimum, the CFPB notes that the issuer must generally apply the amount above the minimum to the balance with the highest interest rate first, then to the others in descending order of rate. The issuer generally decides where the minimum payment portion goes. If you have a high-rate cash advance balance alongside purchases, paying more than the minimum is what directs money at the expensive balance.
7. Key takeaways
- Many issuers charge interest daily on your average daily balance, using a daily periodic rate equal to the APR divided by 365 or 360.
- Interest for a cycle is roughly average daily balance x daily rate x days in the cycle.
- In the example, a $1,240 average daily balance at 19.99% APR costs about $20.37 for a 30-day cycle.
- Paying earlier in the cycle lowers the average daily balance and the interest, even with the same payment amount.
- Paying in full by the due date keeps the grace period on purchases, which can make the interest charge zero.