Compound Interest Calculator
Compound interest adds each period's growth to your balance, so later growth is earned on a larger amount. Enter a starting amount, a monthly contribution, an assumed annual return and a time period to see the projected balance and how much of it comes from growth rather than contributions.
Results
Final balance
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Total contributions
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Estimated growth
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Total return on contributions
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Projected balance over time
| Year | Contributions | Growth | Ending balance |
|---|
How the calculation works
The annual rate is converted to an equivalent monthly rate for your chosen compounding frequency: (1 + r/n)n/12 − 1. Each month the balance grows by that rate and the contribution is added at month end.
Example (hypothetical)
$10,000 invested with $500 added monthly at an assumed 7% annual return, compounded monthly, for 20 years. Change any input to model your own assumptions. A return you type in is an assumption, not a prediction.
Frequently asked questions
Is compound interest guaranteed?
No. Investment returns are not fixed. This tool applies the constant return you enter; real returns vary and can be negative in any year.
How does monthly investing affect growth?
Each contribution starts compounding from the month it is added, so regular investing raises the final balance, particularly over long periods.
How does time affect compounding?
The longer money compounds, the larger the share of the ending balance that comes from growth rather than from contributions.