Compound interest means your returns earn returns of their own. The effect is small at first and large later, which is why time is the most powerful input. This guide uses a simple example to show how much starting early and contributing regularly can matter.
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Example: $10,000 start, $500 a month, 8.5% a year
| Years | You deposit | Growth | Balance |
|---|---|---|---|
| 10 | $70,000 | $47,396 | $117,396 |
| 20 | $130,000 | $237,912 | $367,912 |
| 30 | $190,000 | $762,278 | $952,278 |
After 10 years, growth is smaller than your deposits. After 30 years, growth is about four times your deposits. The last 10 years add more than the first 20 combined.
These are projections at a constant rate. Real investments rise and fall, and no return is guaranteed.
What changes the result most
Return (20 years, $10,000 start, $500 a month):
| Annual return | Balance |
|---|---|
| 6% | $264,122 |
| 8.5% | $367,912 |
| 10% | $452,965 |
Monthly deposit (20 years, 8.5%):
| Monthly deposit | Balance |
|---|---|
| $250 | $211,162 |
| $500 | $367,912 |
| $750 | $524,662 |
Compounding frequency matters far less. Over 20 years the same plan reaches $352,524 with annual compounding, $367,912 with monthly and $369,380 with daily.
Inflation: today's dollars
A dollar buys less in the future. If inflation averages 3% a year, the $367,912 after 20 years has the buying power of about $203,700 in today's dollars. Use the inflation field in the calculator to see this for your own plan.
The rule of 72
Divide 72 by the annual return to estimate how many years it takes money to double. At 8.5% that is about 8.5 years.
Practical takeaways
- Starting earlier is worth more than a slightly higher return later.
- Automating a monthly deposit removes the need to decide each month.
- Deposits at the start of the month grow slightly more than deposits at the end: $370,133 vs $367,912 in this example.
- Fees and taxes reduce real-world results. They are not modeled here.
Frequently asked questions
Is 8.5% a realistic return?
It is an example. Actual returns vary by investment and year, and may be lower or negative. Try several rates to see the range.
Does the calculator include taxes?
No. Taxes and fees are not modeled.
Should I use monthly or daily compounding?
Use the one that matches your account. The difference is small compared with the effect of the rate and the years.
This article is for general education and uses estimates. It is not financial, tax or legal advice. Your actual numbers depend on your lender, employer, tax situation and location. See our disclaimer.