Key Points
The S&P 500 index represents about 80% of the value of the overall U.S. stock market.
It has averaged annual gains of close to 10% over long periods.
If you're looking to invest $100 a month into a low-fee index fund, how much can you expect to earn? It's a good question. You'll see recommendations to invest in a low-fee S&P 500 index fund all over the place, but you won't often see any estimates of how much you might earn investing in it. So let's take a look.
A look at the Vanguard S&P 500 ETF's (NYSEMKT: VOO) past performance offers a clue:
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Period |
Average Annual Gain |
|---|---|
The past three years |
22.2% |
The past five years |
12.9% |
Past 10 years |
15.4% |
Part 15 years |
15.4% |
Source: Morningstar.com as of Aug. 27, 2026.
You might thus expect around a 15% annual return on average from this exchange-traded fund (ETF). But don't think that -- because for one thing, past performance is no guarantee of future returns. And the past 10-plus years have featured above-average gains from the stock market. It's important to understand that the S&P 500 has averaged annual returns close to 10% (ignoring inflation) over many decades.
Here, though, are some examples of how $100 per month might grow in this ETF:
Period |
Growing at 8% Annually |
Growing at 10% Annually |
Growing at 12% Annually |
|---|---|---|---|
Five years |
$7,039 |
$7,326 |
$7,623 |
10 years |
$17,384 |
$19,125 |
$21,059 |
15 years |
$32,582 |
$38,127 |
$44,736 |
20 years |
$54,914 |
$68,730 |
$86,463 |
25 years |
$87,727 |
$118,017 |
$160,000 |
30 years |
$135,940 |
$197,393 |
$289,599 |
35 years |
$206,780 |
$325,229 |
$517,100 |
40 years |
$310,868 |
$531,111 |
$920,510 |
Source: Calculations via Investor.gov.
Image source: Getty Images.
The truth is that no one can know how much you'll earn over time in the S&P 500 -- though we do know that you'll participate in the growth of the U.S. economy and your gains will roughly track those of the overall stock market. (The S&P 500 accounts for about 80% of the U.S. stock market's value.)
So do consider parking some of your long-term dollars in the S&P 500 via a solid index fund.
Selena Maranjian has no position in any of the stocks mentioned. The Motley Fool has positions in and recommends Vanguard S&P 500 ETF. The Motley Fool has a disclosure policy.