Daily periodic rate
The card's APR divided by 365 — the actual rate applied each day to your balance to calculate interest.
Credit card APR is quoted as a yearly figure, but nothing about card interest is yearly. The issuer converts it into a daily periodic rate by dividing by 365. A 24.99% APR becomes a daily rate of about 0.0685%. That small number is applied every day, to whatever the balance is that day.
The issuer then computes an average daily balance for the billing cycle: it records what you owed each day, averages those numbers, and multiplies by the daily rate and the number of days in the cycle. Owe $2,000 for all 30 days at 24.99%, and interest for the cycle is roughly $41.
Two consequences follow. First, when a payment lands within the cycle matters — an earlier payment lowers more daily balances than a later one. Second, interest quietly runs every day the balance is above zero, which is why one small unpaid amount can prevent the whole grace period from applying.
Plumb is financial education, not financial, investment, tax, or legal advice.