Compound Interest Calculator: See How Your Money Grows
Compound interest is the reason a small amount invested today can become a large sum decades later. It is also the reason people who start investing in their 20s end up dramatically wealthier than those who start in their 40s — even if the late starter invests more per month. Understanding compounding is the single most important financial concept for building wealth.
How compound interest works
Simple interest pays you a percentage on your original investment only. If you invest $10,000 at 7% simple interest, you earn $700 every year, forever — totalling $31,000 after 30 years.
Compound interest pays you a percentage on your original investment plus all accumulated interest. That same $10,000 at 7% compound interest earns $700 in year one, then $749 in year two (7% of $10,700), then $801.43 in year three, and so on. After 30 years: $76,123 — more than double the simple interest result.
The difference between $31,000 and $76,123 is entirely due to earning interest on your interest. This effect accelerates over time — which is why the second decade of investing grows your money faster than the first, and the third decade faster still.
The Rule of 72
The Rule of 72 is a quick mental shortcut to estimate how long it takes to double your money. Divide 72 by your annual return rate:
| Annual return | Years to double | $10,000 becomes |
|---|---|---|
| 4% | 18 years | $20,000 |
| 7% | 10.3 years | $20,000 |
| 10% | 7.2 years | $20,000 |
| 12% | 6 years | $20,000 |
At 7% returns, your money doubles roughly every 10 years. So $10,000 becomes $20,000 after 10 years, $40,000 after 20 years, and $80,000 after 30 years — without adding a single dollar. With regular monthly contributions, the numbers become much more impressive.
Why starting early matters more than investing more
This is the most powerful and counterintuitive lesson about compounding. Let us compare two investors:
| Investor | Starts at | Monthly | Total contributed | Value at 65 (7%) |
|---|---|---|---|---|
| Anna (starts early) | Age 25 | $300 | $144,000 | $790,000 |
| Ben (starts later) | Age 35 | $300 | $108,000 | $365,000 |
| Carlos (starts late, invests more) | Age 35 | $600 | $216,000 | $730,000 |
The impact of contributions over time
Monthly contributions are where compounding really shows its power. Here is what $500/month grows to at 7% annual return:
| Years invested | Total contributed | Interest earned | Portfolio value | Interest as % of total |
|---|---|---|---|---|
| 5 years | $30,000 | $5,800 | $35,800 | 16% |
| 10 years | $60,000 | $26,500 | $86,500 | 31% |
| 20 years | $120,000 | $140,500 | $260,500 | 54% |
| 30 years | $180,000 | $427,000 | $607,000 | 70% |
| 40 years | $240,000 | $1,060,000 | $1,300,000 | 82% |
After 40 years, 82% of your portfolio is interest — money the market generated for you. Only 18% is money you actually contributed. This is the magic of compounding over long periods: the market does most of the heavy lifting, but only if you give it enough time.
What rate of return should you expect?
The return rate you use matters enormously in projections. Here are reasonable expectations for different asset types:
Stock market (S&P 500 or total market index): Historically about 10% nominal return (7% after inflation) over long periods. Individual years vary wildly (-37% to +52%), but over 20+ year periods, the average has been remarkably consistent.
Balanced portfolio (60/40 stocks/bonds): Historically about 8% nominal (5% real). Lower volatility but lower long-term returns.
High-yield savings account: Currently 4-5% nominal (1-2% real). Safe but will not build significant wealth over time.
Term deposits: Similar to savings accounts. Useful for short-term goals but not for long-term wealth building.
For long-term projections (20+ years), using 7% is a reasonable and conservative assumption for a diversified stock portfolio. For shorter periods or more conservative portfolios, use 5-6%.