Find your card’s APR
Divide the APR by 365 to get the daily periodic rate
Divide the APR by 12 to estimate the monthly periodic rate
Determine your average daily balance
Multiply the average daily balance by the daily periodic rate
Multiply the result by the number of days in the billing cycle
Add any fees or new purchases if your issuer includes them in the balance
Subtract any grace period benefit if you paid the full statement balance
Repeat for each balance type if your card has different APRs
Check your statement for the exact method your issuer uses