"Double your money, fast!" Do those words sound like the tagline of a get-rich-quick scam? Whether you want to evaluate offers like these or establish investment goals for your portfolio, there's a quick-and-dirty method that will show you how long it really will take you to double your money. It's called the Rule of 72, and it can be applied to any type of investment.

How the Rule Works

To use the Rule of 72, divide the number 72 by an investment's expected annual return. The result is the number of years it will take, roughly, to double your money. For example, if the expected annual return of a bank Certificate of Deposit (CD) is 2.35% and you have $1,000 to invest, it will take 72/2.35 or 30.64 years for you to double your original investment to $2,000.

Depressing, right? CDs are great for safety and liquidity, but let's look at a more uplifting example: stocks. It's impossible to know in advance what will happen to stock prices. We know that past performance does not guarantee future returns. But by examining historical data, we can make an educated guess. According to Standard and Poor's, the average annualized return of the S&P index, which later became the S&P 500, from 1926 to 2018 was 10%. At 10%, you could double your initial investment every seven years (72 divided by 10). In a less-risky investment such as bonds, which have averaged a return of about 5% to 6% over the same time period, you could expect to double your money in about 12 years (72 divided by 6).

Keep in mind that we're talking about annualized returns or long-term averages. In any given year, stocks might return 25% or lose 30%. It's over a long period of time that the returns will average out to 10%. The Rule of 72 doesn't mean that you'll definitely be able to take your money out of the stock market in 10 years. You might have actually doubled your money by then, but the market could be down and you might have to leave your money in for several more years until things turn around. If you must achieve a certain goal or be able to withdraw your money by a certain time, the Rule of 72 isn't enough. You'll have to plan carefully, choose your investments wisely, and keep an eye on your portfolio.

Achieving Your Investment Goals

A professional financial advisor may be your best bet for achieving specific investing goals, but the Rule of 72 can help you get started. If you know that you need to have a certain amount of money by a certain date, for example, for retirement or to pay for your newborn child's college tuition, the Rule of 72 can give you a general idea of which asset classes you'll need to invest in to achieve your goal.

First, you can use the Rule of 72 to determine how much college might cost in 18 years if tuition increases by an average of 4% per year. Divide 72 by 4% and you know that college costs are going to double every 18 years.

Right now you have $1,000 to invest and with an 18-year time horizon, you want to put it all in stocks. We saw in the previous section that investing in the S&P 500 has historically allowed investors to double their money about every six or seven years. Your initial $1,000 investment will grow to $2,000 by year 7, $4,000 by year 14, and $6,000 by year 18. Suddenly 18 years isn't as long a time horizon as you thought, perhaps leading you to rethink your investment strategy.

The Bottom Line

While the Rule of 72 is a good investment guideline, it only provides a framework. If you're looking for a more precise outcome, you'll need to better understand an asset's future value formula. The Rule of 72 also does not take into account the effect of investment fees, such as management fees and trading commissions, can have on your returns. Nor does it account for the losses you'll incur from any taxes you have to pay on your investment gains.