Compound Interest Calculator
Starting balance, monthly deposit, and interest rate. See where it lands.
How it is calculated
Interest compounds monthly at one-twelfth of the annual rate, and each monthly deposit is added at the end of the month. That matches how most savings accounts and many investment projections are presented. The year-by-year table shows how much of the balance is your own deposits and how much is growth.
Saving $200 a month
| Annual rate | 10 years | 20 years | 30 years |
|---|---|---|---|
| 3% | $27,948 | $65,660 | $116,547 |
| 5% | $31,056 | $82,207 | $166,452 |
| 7% | $34,617 | $104,185 | $243,994 |
| 9% | $38,703 | $133,577 | $366,149 |
Over 30 years at 7%, deposits total $72,000 and interest adds $171,994. The longer the period, the larger the share that comes from growth rather than deposits.
Starting early
Someone who saves $200 a month for 10 years at 7% and then stops, leaving it to grow for another 30 years, ends with about $280,968 from $24,000 in deposits. Someone who starts 10 years later and saves $200 a month for the full 30 years ends with about $243,994 from $72,000. Time does a lot of the work.
The rule of 72
Divide 72 by the annual rate to estimate how many years it takes money to double with no extra deposits. The shortcut is close for typical rates:
| Rate | Rule of 72 | Exact (annual compounding) |
|---|---|---|
| 2% | 36.0 years | 35.0 years |
| 4% | 18.0 years | 17.7 years |
| 6% | 12.0 years | 11.9 years |
| 8% | 9.0 years | 9.0 years |
| 10% | 7.2 years | 7.3 years |
| 12% | 6.0 years | 6.1 years |
Inflation and real returns
These figures are in future dollars. Inflation reduces what that money will buy, so a rough way to see growth in today’s money is to subtract an expected inflation rate from the interest rate before entering it. Investment returns also vary from year to year; a fixed rate is a simplification. This is a projection tool, not financial advice.