Q: What kinds of investments could make the value of my 401(k) double in five years?

A:   Doubling in five years. That’s the widely held rule of thumb and a target held by many relatively aggressive investors.

Why is doubling in five years such a lofty goal? Because reaching that goal implies you’re going to beat the market, which for most people is difficult to do. Simply stated, to double your 401(k) in five years, you’ll need to get a 14.9% average annual return on your investment in each of the five years. This calculation, for now, ignores the contributions you make to your 401(k) by taking money from your paychecks. We’ll get to that next.

But first understand that while a 14.9% annual average return might not sound like that much, in reality, it is. Yes, I understand the tech-heavy Nasdaq composite index is up about 15% this year, and the year isn’t over yet. Even the broader Standard & Poor’s 500 is up 10% this year. Once you add the expected 2% dividend yield, that means the S&P 500 has posted a total return of about 12% this year.

But historically, stocks don’t return anywhere near 14.9% a year. Going back to 1928, the S&P’s 500 index has generated an average annual compound rate of return of 9.3%, says IFA.com. And that return includes dividends. That’s a far cry from the 14.9% return you’ll need to double your 401(k) in five years.

What kind of investments would you need in your 401(k) to get a decent shot at a 14.9% average annual return? You’d need a pretty risky portfolio. The most risky diversified portfolio at IFA.com is called Portfolio 100. This portfolio is designed for investors with the biggest appetite for risk. And even this portfolio has returned just 11.1% a year on average since 1928. Even if you put all your money into one of the most risky asset classes, emerging markets stocks, those returns wouldn’t get you to 14.95%. The IFA Emerging Markets index has returned just 13.9% a year on average since 1928.

But wait. Don’t get frustrated. You can still have a shot at doubling the value of your 401(k) in five years. How? Your contributions. One of the key traits of a 401(k) is the fact that you, the employee, are continually adding to it. You contribute to your 401(k) through automatic pre-tax contributions. These contributions are designed to be made before the money lands into your bank account, making the saving less painful.

These contributions are the secret sauce that could make doubling your 401(k) possible. To demonstrate how powerful these contributions are, we’ll answer your question using a somewhat typical 401(k) situation:

Let’s say your 401(k) is currently worth $20,000 and you earn $50,000 a year. And let’s say you plan to contribute 7% a year of your gross pay to your 401(k) through monthly contributions of $292. We’ll also assume you get no raises in five years. Using all these assumptions, what kind of returns are needed to double your 401(k) in five years?

Thanks to your 7% a year contributions, made monthly, you only need your portfolio to return 1.7% a year for the value of your 401(k) to double in five years. And that’s very doable. If you’re early in your career, you could put 30% of your portfolio into bonds and the rest in stocks. Five-year government bonds have returned 5.0% a year on average, says IFA. Stocks have returned 9.3% on average. Combining the two, you’re looking at a expected return of about 8% a year.

So here’s the deal. If you want to double the value of your 401(k) in five years, you can try to shoot for the moon and chase after the riskiest possible individual stocks in a hope of getting a 15% average annual return. That’s quite a huge gamble, though, to play with your nest egg. A much better alternative is to save at least 7% of your gross pay. That way doubling the value of your 401(k) is quite within your reach while taking dramatically less risk.

Matt Krantz is a financial markets reporter at USA TODAY and author of Investing Online for Dummies and Fundamental Analysis for Dummies. He answers a different reader question every weekday in his Ask Matt column at money.usatoday.com. To submit a question, e-mail Matt at mkrantz@usatoday.com. Follow Matt on Twitter at: twitter.com/mattkrantz