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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:
The Rule of 72 is a quick and simple technique for estimating one of two things: 1  the time it takes for a single amount of money to double with a known interest rate, or. 2  the rate of interest you need to earn for an amount to double within a known time period.he rule states that an investment or a cost will double when: When interest is compounded annually, a single amount will double in each of the following situations: The Rule of 72 indicates than an investment earning 9% per year compounded annually will double in 8 years.