Daily periodic rate: how card interest adds up

Turning an annual rate into a daily one

APR is expressed as a yearly rate, but interest can accrue every day a balance is carried. Card issuers convert the APR into a daily periodic rate using the divisor in the card agreement, generally 360 or 365, and use that daily rate in the account's interest calculation.

How the daily rate turns 24% APR into a monthly charge

Each day, the issuer takes that day's balance, multiplies it by the daily rate, and that's the interest charged for that one day. Those daily amounts accumulate across every day in the billing cycle to produce the total interest on the statement. If the balance changes mid-cycle, from a new purchase or a payment, each day still uses whatever the balance actually was on that day.

Simplified examples before rounding. Daily balances, posting dates, compounding, and issuer rules can change the statement charge.

Working out a daily periodic rate by hand

The calculation is straightforward once broken into its parts.

1. Find the divisor in the card agreement or statement, generally 360 or 365, then divide the card's APR by that number to get the daily periodic rate.
2. Multiply that daily rate by the balance for a given day to get that day's interest.
3. Repeat for every day in the billing cycle if the balance changes during the month.
4. Add up every day's interest for the cycle to get the total interest charge.
5. Compare that total to the number on the statement to check your math.

Why the same APR can produce a different interest charge month to month

The APR staying fixed doesn't mean the interest charge does. A higher balance carried for more days in the cycle raises the total, and a longer billing cycle, more days, does too. A purchase made early in the cycle accrues interest for more days than one made near the end, even at the identical rate.

Where this shows up on a real statement

Statements typically list both the APR and the corresponding periodic rate the issuer actually used, along with the balance the rate was applied to. Use an average daily balance calculator walkthrough to check how changing balances affect the cycle, then compare the result with your card's own numbers rather than assuming every issuer rounds or divides the same way. Daily card interest is different from capitalized interest, which adds unpaid interest to principal.

Common questions

Does every issuer divide by 365 exactly?

No. A daily periodic rate is generally APR divided by 360 or 365, depending on the issuer and card agreement. Check your card's agreement or statement for the divisor, periodic rate, and calculation method the issuer actually uses.

Why does my interest charge change even though my APR didn't?

Because the daily balance changed. Carrying a higher balance for more days in a given cycle, or having a cycle with more days than usual, both raise the total interest even without any change to the rate itself.

Does paying earlier in the billing cycle lower the interest charged?

Generally yes, since it lowers the balance the daily rate applies to for the remaining days in that cycle. The exact effect depends on when in the cycle the payment posts and how the issuer calculates the balance.