Compound Interest Calculator
See how an initial amount plus monthly contributions grows over time — final balance, total contributed, total growth, and the full year-by-year path.
Your Numbers
Result after 20 years
Final Balance
142,438
Total Contributed
53,000
Total Growth
89,438
| Year | Contributed | Growth | Balance |
|---|---|---|---|
| 1 | 7,400 | 505 | 7,905 |
| 4 | 14,600 | 3,548 | 18,148 |
| 7 | 21,800 | 9,360 | 31,160 |
| 10 | 29,000 | 18,687 | 47,687 |
| 13 | 36,200 | 32,481 | 68,681 |
| 16 | 43,400 | 51,949 | 95,349 |
| 19 | 50,600 | 78,623 | 129,223 |
| 20 | 53,000 | 89,438 | 142,438 |
Monthly compounding, growth applied after each month's contribution. A constant-rate, long-horizon projection you choose yourself — real interest/return rates vary year to year. For informational purposes only, not financial advice.
For educational and informational purposes only — not financial, investment, trading, legal or tax advice. See our full Financial Disclaimer.
Why compound interest grows faster than it looks
Simple interest pays you a fixed amount every period, always based on the original amount you started with. Compound interest is different: each period's interest is calculated on the CURRENT balance, which already includes every bit of interest paid so far — so the amount you earn grows a little larger each period, even without adding any new money. Over short periods the difference is small; over many years it compounds into a substantially larger number, which is why it's often described as earning “interest on your interest.”
This calculator compounds monthly rather than annually: every month, the running balance grows by one-twelfth of your entered annual rate, and then that month's contribution (if any) is added on top. Repeating that for every month across your chosen time horizon produces the year-by-year table below the headline numbers, so you can see exactly how much of each year's ending balance is your own money versus growth.
The rate you enter is held constant for the entire projection, which is a simplification — real interest rates and investment returns move up and down from year to year. Use this to understand how the mechanics of compounding work and to compare different contribution amounts or time horizons against each other, not as a prediction of what any specific account or investment will actually return.
Formula used:
Each month: Balance = Balance × (1 + Annual Rate ÷ 12) + Monthly Contribution
Key terms:
- Monthly compounding: interest calculated and added every month, not once a year
- Total growth: final balance minus everything you personally contributed
Quick answer
A compound interest calculator projects how an initial amount plus recurring contributions grows over time by applying interest to the running balance every period (here, monthly): Balance = Balance × (1 + Annual Rate ÷ 12) + Monthly Contribution, repeated for every month, so each period's interest is earned on top of all prior growth, not just the original amount.
Compound Interest — Guide & FAQs
How monthly compounding works, and what this isn't.