Targeting millionaire status by your 65th birthday? Depending on how old you are today, the S&P 500 may be all you need to get there.

The S&P 500 is a basket of the U.S.'s largest public companies. Those 500-plus organizations collectively account for about 80% of the stock market's total value. As such, they drive much of the performance for the market as a whole. That's why when someone says, "The market was up 5% today," it often means specifically that the S&P 500 went up 5%.

Now let's get back to that millionaire goal of yours. The S&P 500 has three characteristics that make it a suitable anchor for your retirement portfolio. To start with, it's diversified. Holding hundreds of different stocks spreads out your risk so that you're not dependent on any one of them.

Green highway sign that reads Millionaire next exit

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Financial strength, proven business models

Second, the companies in the S&P 500 have financial strength, proven business models, experienced leaders, and loyal customers. If you use products or services from Apple, Amazon, Visa, or American Express regularly, you are one of those loyal customers.

These are the qualities you want in your retirement portfolio. That account could be with you for 60 years or more, so you need companies you can safely hold for long periods of time.

Strong long-term growth

The third characteristic, though, is the kicker. The long-term average annual growth rate of the S&P 500 is about 7% after inflation. At that rate, you can reach millionaire status in 30 years by investing about $880 a month, including your employer match. If you're earning the median U.S. salary of $53,508 for workers over 24, you could cover that monthly contribution with a 15% paycheck deferral and 5% employer match.

That might sound like a stretch right now, but here's some perspective. To reach the million-dollar mark in a high-yield cash savings account earning 0.55%, you'd have to contribute $2,500 monthly for 30 years. That's really only an option for someone who makes six figures and also has low living expenses.

You can also still ride the S&P 500 to millionaire status even if your retirement timeline is shorter than 30 years. It will take higher monthly contributions, but you can use a few tricks to make it easier. First, streamline your household spending and use the savings to increase your contributions. Second, invest in a tax-advantaged account, such as a 401(k) or HSA with employer match. Third, make sure you are reinvesting any dividends. And finally, increase your contributions every time you get a raise or cash windfall.

Managing the volatility

Before you invest your last dollar in the S&P 500, know that the index can be volatile in shorter periods of time. Volatility doesn't actually cost you anything, unless you have to sell your stocks when the share prices are down. You can mitigate that risk in two ways. One, keep an emergency account with enough cash in it to cover at least three months of your living expenses. And two, don't invest money you might need in the next five years.

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Image source: Getty Images.

How to invest in the S&P 500

You can invest directly in the S&P 500 by individually purchasing all 500-plus index constituents. But the easier route is to buy a low-cost S&P 500 ETF instead. You will incur fund expenses this way, but it's vastly more efficient to manage one position versus hundreds. Plus, if you choose the right fund, those expenses will be less than 0.05% of your invested capital.

The table below shows key metrics for three popular S&P 500 index funds.

Fund Name

Expense Ratio

10-Year Average Return

Assets Under Management

Vanguard S&P 500 ETF (NYSEMKT:VOO)

0.03%

13.39%

$658 billion 

iShares Core S&P 500 ETF (NYSEMKT:IVV)

0.03%

13.37%

$256 billion

SPDR Portfolio S&P 500 ETF (NYSEMKT:SPLG)

0.03%

13.29%

$9 billion 

Table data source: Vanguard, iShares, SSGA

You may be wondering what gives with the average returns in excess of 13%. Over the past 10 years, the S&P 500 has actually grown by 13.43% annually, which is well above its long-term average. While that growth trend is fantastic, it's not realistic to assume it will continue indefinitely. You're better off planning more conservatively, and then celebrating if your retirement account outperforms the plan.

Ride the market

Investing in the S&P 500 can make you a millionaire retiree. That holds true even if you're stretching today to contribute a few hundred dollars a month. What's important now is to get moving in the millionaire direction. You can always find ways to increase your contribution over time.

And who knows? The market might continue its unusually strong performance. If it does, you'll want to be there to reap the benefits.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.