Investing in the stock market can feel complicated, especially when interest rates are high and markets are volatile. But one of the simplest long-term strategies does not require predicting the next market move.
It consists of investing the same amount of money every month.
This approach is known as dollar-cost averaging: investing equal amounts at regular intervals regardless of whether the market is rising or falling. According to Investor.gov, this means buying more shares when prices are lower and fewer when prices are higher.
But what could actually happen if you invested $500 every month in an S&P 500 fund for 20 years?
$500 a month: the numbers
Over 20 years, you would personally contribute:
$500 × 12 × 20 = $120,000
The final amount, however, could be substantially higher because investment returns themselves have time to compound.
Using three hypothetical average annual return scenarios:
| Average annual return | Approx. value after 20 years |
|---|---|
| 5% | $205,500 |
| 7% | $260,500 |
| 9% | $333,900 |
At a hypothetical 7% annual return, investing $500 per month could therefore grow to roughly $260,000 after 20 years.
Around $120,000 would come from your own contributions and the remainder from investment growth.
These calculations are illustrations, not forecasts. Actual stock-market returns can be significantly higher or lower, especially over shorter periods.
What about 10 or 30 years?
Time makes an enormous difference.
At an illustrative 7% annual return, approximately:
- 10 years: $86,500
- 20 years: $260,500
- 30 years: $610,000
The difference between 20 and 30 years is particularly important. You contribute another $60,000 during that additional decade, but the portfolio could potentially increase by roughly $350,000 because a larger balance has more time to compound.
What if you can invest less—or more?
The same principle works with different monthly amounts.
At a hypothetical 7% average annual return:
| Monthly investment | 10 years | 20 years | 30 years |
|---|---|---|---|
| $100 | $17,300 | $52,100 | $122,000 |
| $250 | $43,300 | $130,200 | $305,000 |
| $500 | $86,500 | $260,500 | $610,000 |
| $1,000 | $173,100 | $520,900 | $1.22 million |
The important variable is not only how much you invest. How long you remain invested can be even more important.
Why this matters after the Fed’s latest decision
The Federal Reserve raised its target range to 3.75%-4.00% on September 16, 2026. Higher rates can make cash, savings accounts and Treasury bills more attractive in the short term. They can also affect borrowing costs and stock valuations.
But short-term interest rates and long-term investing solve different problems.
Money needed soon may require a different approach from money intended for retirement or another goal decades into the future.
That is why the question is not necessarily:
“Should I choose T-Bills or the S&P 500?”
A more useful question may be:
“When will I need this money?”
The risk investors should not ignore
The S&P 500 does not produce a fixed return.
There will be years when the market rises sharply and years when it falls. An investor who may need the money during a market decline could be forced to sell at an unfavorable time.
Dollar-cost averaging does not eliminate market risk and does not guarantee a profit.
Its main advantage is behavioral simplicity: instead of attempting to identify the perfect moment to buy, the investor follows a predetermined schedule.
The key takeaway
Investing $500 per month may not appear dramatic at the beginning.
But over decades, consistency and compounding can potentially transform relatively modest monthly contributions into a substantial portfolio.
At a hypothetical 7% annual return:
$500/month → about $86,500 in 10 years → $260,500 in 20 years → $610,000 in 30 years.
The lesson is not that the S&P 500 will deliver exactly 7%.
It is that time, regular contributions and compounding can matter more than trying to predict the market every month.
Continue the US Watch series
Read the previous articles in the series: Fed Rate Hike 2026: What It Means for Savings, T-Bills, Mortgage Rates and Your Money and Mortgage Rates Near 7%: Buy a Home Now or Wait?
This article is for informational purposes only and does not constitute investment advice.
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