Building a million-dollar portfolio may seem like an impossible goal that's reserved for only elite investors. But it's more attainable than you might think.

You don't need to be wealthy to make a lot of money in the stock market, and you also don't need to know all the ins and outs of investing. What you will need, though, is the right investment.

Fortunately, there's one ETF that's perfect for beginners and advanced investors alike, and with a few hundred dollars per month, it could turn you into a millionaire. Here's how.

A powerful investment

If you want a safe investment that's going to consistently earn positive returns over the long run, you can't go wrong with an S&P 500 ETF.

An S&P 500 ETF -- such as the Vanguard S&P 500 ETF (VOO -0.27%) or iShares Core S&P 500 ETF (IVV -0.26%) -- is a fund that tracks the S&P 500 index itself. This means it includes around 500 stocks from the largest and strongest companies in the U.S.

Because this ETF contains so many stocks from healthy companies in a wide variety of industries, that substantially lowers your risk. Your investment is instantly diversified, and strong stocks like these are the most likely to recover from periods of market volatility.

While it can be daunting to invest when the future is uncertain, history shows that there's never been a bad time to invest in the S&P 500 -- as long as you keep a long-term outlook.

Analytics firm Crestmont Research examined the rolling 20-year total returns of the S&P 500 starting in 1900. It found that in all 103 years analyzed (1919 to 2022), the S&P 500 saw positive returns.

In other words, if you had invested in an S&P 500 fund at any point after 1900 and held that investment for 20 years, you would have earned positive total returns.

A long-term outlook is key, as any investment can experience volatility in the near term -- the S&P 500 included. But if you hold this ETF for at least a couple of decades, you're all but guaranteed to make money.

Becoming a stock market millionaire

Despite being one of the safer investment options, the S&P 500 ETF can also help you make a lot of money over time.

Historically, the index itself has earned an average rate of return of around 10% per year. While you likely won't see these returns each and every year, the annual highs and lows should average out to around 10% per year over time.

If you have a goal of reaching $1 million and you're earning a 10% average annual return, here's approximately how much you'd need to invest each month depending on how many years you give your money to grow:

Number of Years Amount Invested per Month Total Savings
40 $200 $1.062 million
35 $325 $1.057 million
30 $525 $1.036 million
25 $900 $1.062 million
20 $1,500 $1.031 million

Source: Author's calculations via Investor.gov

Time is your greatest ally when it comes to building wealth in the stock market, so the sooner you get started, the better.

Downsides to consider

While the S&P 500 ETF can be a fantastic fit for many investors, it's not right for everyone.

One downside to consider is the fact that it can only earn average returns. This type of fund is designed to follow the market, so by definition, it can't beat the market. For many people, that's a worthwhile trade-off for the ease and safety of this ETF. But if you're looking to earn above-average returns, you might be better off with individual stocks.

Also, this is a hands-off type of investment. All the stocks in the fund are chosen for you, and you never need to do any research on individual companies.

Again, for many investors, that's not necessarily a negative. But if you want to be more involved in your portfolio or if there are certain stocks in the S&P 500 you'd prefer to avoid, this type of investment may not be the best fit.

There's no right or wrong answer as to how you should invest, as it will depend on your personal preferences. But by weighing the advantages and disadvantages, it will be easier to decide whether the S&P 500 ETF is the right path for you toward becoming a stock market millionaire.