If you invest $500 a month for 30 years and earn an average of 7% a year, you end up with about $610,000. Only $180,000 of that is money you put in; the other $430,000 is growth.
That gap is compound growth: your returns earn returns of their own, and the effect snowballs the longer you leave the money alone.
$500 a month: growth chart
Deposits are made at the end of each month, and returns are compounded monthly. These are nominal figures, before inflation, fees and taxes.
| Years | You put in | At 5% | At 7% | At 10% |
|---|---|---|---|---|
| 5 | $30,000 | $34,003 | $35,796 | $38,719 |
| 10 | $60,000 | $77,641 | $86,542 | $102,422 |
| 15 | $90,000 | $133,644 | $158,481 | $207,235 |
| 20 | $120,000 | $205,517 | $260,463 | $379,684 |
| 25 | $150,000 | $297,755 | $405,036 | $663,417 |
| 30 | $180,000 | $416,129 | $609,985 | $1,130,244 |
| 40 | $240,000 | $763,010 | $1,312,407 | $3,162,040 |
Notice how little happens early and how much happens late. In the first 10 years at 7%, growth adds about $26,500. Between years 30 and 40 it adds roughly $640,000, even though you contributed just $60,000 more.
Which return should you assume?
No one can promise a return. As a rough guide:
- A broad US stock index has historically averaged around 10% a year before inflation over long periods, with big losing years along the way.
- 7% is a common, more cautious planning figure for a stock-heavy portfolio. It is also close to the historical return after inflation.
- 5% fits a mix with a larger share of bonds, or a pessimistic stock scenario.
- High-yield savings accounts and CDs pay less, but your balance never drops. The Compound Interest Calculator is built for those.
Starting early beats contributing more
Compare two people who each invest $500 a month at 7% until age 65:
- Starting at 25 (40 years): $240,000 contributed, about $1.31 million at 65.
- Starting at 35 (30 years): $180,000 contributed, about $610,000 at 65.
Ten extra years cost the early starter $60,000 in contributions and produce about $700,000 more. If you can't afford $500 today, start with what you can. $250 a month at 7% for 30 years still grows to about $305,000.
Inflation and fees shrink the numbers
Inflation: $610,000 in 30 years won't buy what $610,000 buys today. At 3% inflation it's worth about $251,000 in today's dollars. The Investment Calculator can show results in today's money.
Fees: A fund charging 1% a year turns a 7% return into 6%. Over 30 years that drops the result from about $610,000 to about $502,000, so a 1% fee costs over $100,000. Low-cost index funds often charge less than 0.1%.
Where to put $500 a month
A typical order of priorities (not personal advice):
- Contribute enough to get your full 401(k) employer match. It's an instant return on your money.
- Pay off high-interest debt. Clearing a credit card at 20%+ APR beats almost any investment.
- Keep an emergency fund of a few months' expenses in savings.
- Use tax-advantaged accounts: a Roth or traditional IRA, and more 401(k) contributions. $500 a month is $6,000 a year, which fits within the annual IRA limit.
- Choose diversified, low-cost funds, keep contributing through downturns, and avoid selling in a panic.
The rule of 72
For a quick mental check, divide 72 by the annual return to find roughly how many years it takes money to double. At 7% that's about 10 years; at 10% about 7 years; at 5% about 14 years.
Try your own amounts, yearly increases and time frames in the Investment Calculator.
Frequently asked questions
How much is $500 a month for 10 years?
Is 7% a realistic return?
Should I invest or pay off debt first?
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