Investing in the S&P 500 index has been a great way for investors to grow their wealth for not just years but decades. Having a position in the top 500 stocks in the world is an easy way for anyone to invest in the stock market. There's no need to pick individual stocks, track the latest stock news and analyze it, or do anything else besides put money into an exchange-traded fund (ETF) that tracks the index.

Whether you're a long-term investor who wants a simple strategy or if you just don't want to have to worry about tracking individual companies, having a position in the broad index can make a lot of sense. And even if you don't have thousands of dollars to invest today, investing money every month can still help you build up a massive portfolio in the long run.

Below, I'll show you how a $500-per-month investment into an S&P 500 ETF can grow over a period of 25-plus years.

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Why investing in the S&P 500 can be an easy decision

Historically, the S&P 500 has averaged annual returns of around 10%. At that rate, it would take a little over seven years for an investment to double in value. And if that average holds up over 25 years, then your investment would grow to close to 11 times its original value.

There's a huge incentive in tracking the S&P 500 simply because it'll always contain the best growth stocks on the market and rebalance on its own. You don't have to worry about making any decisions about which stocks to buy or sell.

An ETF that does an excellent job of tracking the index is the SPDR S&P 500 ETF (SPY 0.03%). It has a low expense ratio of 0.09%, and in return it gives you an easy way to invest in the top stocks on the market. As you can see from the chart below, the difference between the ETF's returns and the S&P 500's returns over the past decade has been negligible.

SPY return versus S&P 500 data by YCharts

The payoff from a $500-per-month investment in the S&P 500

Investing in an S&P 500 fund can be a no-brainer decision for long-term investors. And in the table below, you'll see how much your portfolio might be worth if you invest $500 each month into the SPY ETF. I've included multiple columns for potential returns to account for multiple scenarios, including a possible slowdown in the future. The biggest unknown when it comes to investing is always what that average annual return will be, and unfortunately, that will also have the most significant impact on any projections that you make.

Calculations and table by author.

The table helps to illustrate just how significant even a 1% change in your annual return can impact your portfolio balance, especially in later years. But by investing $500 each month into the SPY ETF, you could be on track to grow your portfolio to $1 million one day.

Through the power of compounding the longer you stay invested, the higher your portfolio is likely to grow. While there will be bad years along the way, staying the course, trusting the process, and remaining invested is the key to ensuring you reach your goals.