Compound Interest Calculator
See what a starting balance plus steady monthly contributions grows into over time — and how much of the final number is growth rather than money you put in.
Year-by-year growth
| Year | Total contributed | Projected value |
|---|---|---|
| 1 | $16,000 | $16,919 |
| 2 | $22,000 | $24,339 |
| 3 | $28,000 | $32,294 |
| 4 | $34,000 | $40,825 |
| 5 | $40,000 | $49,973 |
| 6 | $46,000 | $59,782 |
| 7 | $52,000 | $70,299 |
| 8 | $58,000 | $81,578 |
| 9 | $64,000 | $93,671 |
| 10 | $70,000 | $106,639 |
| 11 | $76,000 | $120,544 |
| 12 | $82,000 | $135,455 |
| 13 | $88,000 | $151,443 |
| 14 | $94,000 | $168,587 |
| 15 | $100,000 | $186,971 |
| 16 | $106,000 | $206,683 |
| 17 | $112,000 | $227,820 |
| 18 | $118,000 | $250,486 |
| 19 | $124,000 | $274,790 |
| 20 | $130,000 | $300,851 |
Why compounding is so powerful
Compound interest means you earn returns on your returns. $10,000 at 7% becomes $10,700 after one year — but in year two you earn 7% on $10,700, not just the original $10,000. Over decades this snowballs: at 7%, money doubles roughly every 10 years (the Rule of 72).
The biggest factor isn't the rate — it's time. Starting 10 years earlier routinely beats contributing twice as much later, which is why the year-by-year table matters more than the final number.
What return rate should you assume?
The S&P 500 has returned about 10% annually before inflation over the long run (roughly 7% after inflation). High-yield savings accounts pay 4–5% in the current rate environment; bonds sit in between. For long-horizon planning, 6–8% is a common conservative assumption for a diversified stock portfolio — and using after-inflation rates keeps the result in today's dollars.
Frequently asked questions
What is the Rule of 72?
Divide 72 by your annual return to estimate how many years it takes money to double. At 8%, that's about 9 years; at 6%, about 12 years.
How often should interest compound?
More frequent compounding helps slightly, but the difference is small: $10,000 at 5% for 10 years yields $16,289 compounded annually vs $16,470 compounded daily. Rate and time matter far more.
Does this account for inflation?
Enter a real (after-inflation) return — e.g. 7% nominal minus 3% inflation = 4% — and the result is in today's purchasing power.
Are contributions or returns more important?
Early on, contributions dominate. After 15–20 years, growth usually overtakes them — the calculator's breakdown shows exactly when that crossover happens for your inputs.
This calculator provides estimates for educational purposes only and is not financial advice. Actual loan terms, rates, and outcomes depend on your lender and personal situation.