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Suppose you invest $100 at a compound interest rate of 10%. The rule of 72 tells you that your money will double every seven years, approximately: If you graph these points, you start to see the familiar compound interest curve:
You can also run it backwards: if you want to double your money in six years, just divide 6 into 72 to find that it will require an interest rate of about 12 percent. Years to double your investment Required Interest Rate Exact Answer: % Rule of 72 Estimate: %.ou can also run it backwards: if you want to double your money in six years, just divide 6 into 72 to find that it will require an interest rate of about 12 percent. Years to double your investment Required Interest Rate Exact Answer: % Rule of 72 Estimate: %.