Index fund investing is popular for a reason, but can it really deliver for investors with lofty retirement goals? The answer isn't the same for everyone, but there are some people who can absolutely reach millionaire status just by investing in the S&P 500. Unfortunately, there are a few other important steps you'll have to take to get there.

Average returns

Obviously, the S&P 500's ability to make millionaires depends on its future performance. We can't know for sure where the market index is going, but we can lean on history to make an informed guess.

The S&P 500 has delivered a 10% average rate of return over the long-term. When we factor in fees, taxes, trading costs, and other expenses, that nets out to 7-8% for most people. When most people think about average market returns, those are the commonly cited numbers that they expect to achieve. At an 8% rate of return, $100,000 would turn into $1,000,000 after 30 years. If you have 30 years and $100,000, then there's a good chance that the S&P 500 can make you a millionaire retiree.

Affluent smiling man in a sweater standing on a dock in front of his yacht.

Image source: Getty Images

It gets a bit tougher if you have a shorter time frame. You'd have to start with $250,000 to reach $1 million within 20 years. That number swells to $475,000 if you only have a decade until retirement.

Most people aren't starting with a lump sum of cash to invest. Instead, financial planning usually involves monthly contributions to an account that grows over time. Consider a hypothetical family starting with $0 in a retirement account that is able to hit 8% returns year after year.

That family would only need to contribute $750 per month for 30 years to retire as millionaires. If they only have 20 years, that number swells to $1,900 per month. With 10 years left, it's a shocking $6,000 per month. This illustrates just how important it is to start saving early and often.

Real life examples

As we know, the stock market doesn't move in neat, straight lines. Things aren't always smooth and predictable, and life will throw financial curveballs at you. The S&P 500 might average an 8% rate of return, but most years are actually a lot better or worse.

To be a millionaire today, you would have needed a $350,000 investment in the S&P 500 in 2012. That's even after the huge drop over the first half of this year. One of the best decades in stock market history has created lots of millionaires this decade.

If we go back 20 years, the outlook isn't quite so rosy. People were right in the midst of the Dot Com Bubble's collapse, and the stock market was also dealing with a rate hike-induced recession. Even though we're adding a whole 10 more years of growth, it still would have taken $300,000 invested in the S&P 500 to surpass $1 million today. Other comparable real life figures are $120,000 invested 30 years ago and $600,000 five years ago.

Some of these matters are out of your control as an investor, and we have to understand that timing plays a major role. Still, all of these scenarios show us that the S&P 500 is clearly good enough to make you a millionaire retiree, even if the details can vary greatly from case to case.

Keys to success

There are definitely more active investment strategies that can outpace the market. However, most people who attempt to deploy these strategies fail, and they come with additional risk.

If you're comfortable with passive investing and index funds, then there are a few truths to keep in mind. Retiring a millionaire with the S&P 500 requires persistence, discipline, and an early start. Most families need to start putting money away early on. The $800 per month required to hit $1 million isn't a trivial amount. Most households require excellent savings habits to save that amount of cash.

It's also important to consider other index funds. You can have a completely passive strategy while combining other investments with the S&P 500, such as the Nasdaq and other growth indexes.