You can try the Omni Calculator tool finance charge or do as follows:
Get the information about your annual interest rate. Let's say 20% on a $1000 credit card balance.
Divide the interest rate by 100 and then by 365. Multiply the result by the credit card balance: ((20 / 100) / 365) × 1000.
We obtain the result $0.547. That's your daily finance charge. Multiply it by 30.
Your monthly finance charge is $16.40.
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