Wouldn't you love to find an investment that's guaranteed to double your money? While stocks and mutual funds carry risk of potential losses, investments like CDs and savings accounts--those with a guaranteed interest rate--are far less risky and can generally be expected, over time, to double your investment. Although, depending on the interest rate paid, it may not double in your lifetime. How Long To Double Your Money? There is an easy way to determine how long it will take for an interest-bearing investment to pay out enough compounded interest to double your principal. You can do this by using the rule of 72. The way it works is you divide 72 by your interest rate. The result is the number of years it will take your investment, growing at that rate, to double. For example, let's say you have funds in a savings account growing at 0.9 percent. If you divide 72 by 0.9, you'll learn that it will take 80 years to double your money. If, however, you find a fixed investment growing at 10 percent, you can expect your money to double in just 7.2 years. Not An Exact Science, But A Good Measure The rule of 72 isn't always exactly right and it's definitely not a guarantee of performance, but it gives a good, general time frame that can help you better evaluate investments for your portfolio. There are other things to consider along with the estimated number of years it gives you. First, the rule of 72 doesn't take into account the effects of inflation on your money, so it's not a good measure of future buying power, simply one to use for value growth. It also doesn't consider the toll that taxes will take from your earnings. Depending on the type of account you have, you may be expected to pay taxes each year on the interest you earn. If you aren't, then you may be required to pay taxes when you distribute the income out of the account. The rule of 72 is like having a relatively accurate crystal ball for your portfolio value. Using it can help you better estimate a reasonable retirement age and decide how aggressive your investing plan should be.