APR stands for annual percentage rate, which equals the periodic rate times the number of periods per year. The APR does not take into consideration the effects of interest compounding so you can easily calculate the monthly rate. You may want to calculate the monthly rate if you are leaving your money in an account for a short period of time or if your account compounds interest on a monthly basis.

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Contact your lender or consult your account documents to determine the account APR. Make sure that you find 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.