- 1). Divide the annual interest rate by 100 to convert to a decimal from a percentage. For example, if your annual interest rate equals 3.42 percent, divide 3.42 by 100 to get 0.0342.
- 2). Divide the annual interest rate by 12 to convert to a monthly interest rate. In this example, divide 0.0342 by 12 to get a monthly rate of 0.00285.
- 3). Multiply the monthly rate by the balance of your CD to find the amount of monthly interest earned. In this example, if you had $3,510 in the CD, multiply $3,510 by 0.00285 -- your monthly interest is $10.
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