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Contact your lender or consult your account documents to determine the account APR. Use the APR rather than the APY, which stands for annual percentage yield.

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Divide the APR by 12 to calculate the monthly interest rate expressed as a percentage. For example, if the APR equals 9 percent, you would divide 9 by 12 to get 0.75 percent for the monthly rate expressed as a percentage.

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Divide the monthly interest rate expressed as a percentage by 100 to calculate the monthly interest rate expressed as a decimal. Finishing this example, you would divide 0.75 percent by 100 to find the monthly rate expressed as a decimal to be 0.0075.