The S&P 500 is one of the most widely used benchmarks for understanding how the U.S. stock market is performing. It’s an index of the 500 leading publicly traded companies in the country.

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As an individual investor, you can purchase shares in index funds that are invested in these 500 companies and aim to mirror the returns of the S&P 500. Investment returns vary, though, and when you invest in the market can significantly impact your results. So, what if you had invested in the S&P 500 20 years ago? How much would you have today?

On Dec. 1, 2005, the S&P 500 closed at 1,248.29. This isn’t a dollar value, like the value of individual stocks, but rather an index value measured in points. What if you invested $1,000 in an S&P 500 index fund on this date?

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A few years later, on Feb. 1, 2009, in the middle of the housing market crash and Great Recession, the S&P 500 closed at 735.09. That’s down over 40% from the initial investment, leaving you with around $590 of your original $1,000. But if you decide to leave your money in the index fund, the value could still change.

Ten years after your initial investment, on Dec. 1, 2015, the S&P 500 closed at 2,043.94. That’s up 63.7% from when you first invested. So, your initial $1,000 stake would be worth $1,637. But that’s only halfway through the 20-year investment period.

Flashing forward until Dec. 3, 2025, the S&P 500 closed at 6,849.72. The index has grown by 448.7% since 2005, when you made your initial investment. So, your original $1,000 would now be worth $4,487, minus inflation adjustments. Over just 20 years, your money would have more than quadrupled.

You can’t go back and invest in the past, though. The real question is: How much can you grow your money by making investments now?

If you put $1,000 in an S&P 500 index fund today, there’s no way to know for sure how much you’ll have in 2045. Markets are unpredictable, and past performance is no guarantee you’ll see those same returns again. However, you can use historical data to make some predictions.

Over the last 100 years, the S&P 500 has averaged a return of around 10% before inflation. If this remains true, and you put $1,000 in an S&P 500 index fund now, your money would grow to $6,727.50 in 20 years. The sooner you invest, the more time your money will have to grow and benefit from compound returns.