What happens if you invest $100 a month
Investing $100 a month at an 8% average annual return grows to about $149,036 after 30 years, from just $36,000 of your own money. Here is the full picture.
Growth table: $100 a month
| Time | You put in | At 6% | At 8% | At 10% |
|---|---|---|---|---|
| 5 years | $6,000 | $6,977 | $7,348 | $7,744 |
| 10 years | $12,000 | $16,388 | $18,295 | $20,484 |
| 15 years | $18,000 | $29,082 | $34,604 | $41,447 |
| 20 years | $24,000 | $46,204 | $58,902 | $75,937 |
| 25 years | $30,000 | $69,299 | $95,103 | $132,683 |
| 30 years | $36,000 | $100,452 | $149,036 | $226,049 |
| 35 years | $42,000 | $142,471 | $229,388 | $379,664 |
| 40 years | $48,000 | $199,149 | $349,101 | $632,408 |
Monthly contributions at month end, returns compounded monthly, no taxes or fees. Starting balance $0.
How long until you're a millionaire?
Investing $100 a month from scratch, you would reach $1,000,000 in about 65.8 years at 6%, 52.9 years at 8%, or 44.6 years at 10%.
Why the last decade matters most
Look at the 8% column. Going from 30 to 40 years adds more than the first 20 years combined. That's compounding: in later years, growth on your existing balance far exceeds your contributions. Starting earlier beats investing more later.
Want to add a starting balance or raise your contribution each year? Try the compound interest calculator with $100/month pre-filled.
Where to invest $100 a month
For most people, a low-cost, broadly diversified index fund (a total US market or S&P 500 fund, often paired with an international fund) inside a tax-advantaged account is the simplest place to start. Many brokers let you schedule automatic monthly investments, including fractional shares.
Common questions
How much will $100 a month be worth in 20 years?
About $58,902 at an 8% annual return, $46,204 at 6%, or $75,937 at 10%. You would have contributed $24,000.
Is an 8% return realistic?
The US stock market has returned about 10% a year nominally over the long run, but with big swings and decade-long flat stretches. 6–8% is a common planning assumption for a stock-heavy portfolio.