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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:
Compound Interest. Simple Interest. The Doubling Time formula is used in Finance to calculate the length of time required to double an investment or money in an interest bearing account. It is important to note that r in the doubling time formula is the rate per period.f one wishes to calculate the amount of time to double their money in a money market account that is compounded monthly, then r needs to express the monthly rate and not the annual rate. The monthly rate can be found by dividing the annual rate by 12.