Last updated: August 22, 2026
Free Compound Interest Calculator
Enter a starting amount, a monthly contribution, an interest rate, and a timeframe to see how much your money could grow. This calculator illustrates the same principle behind dollar-cost averaging and automated retirement contributions — consistent contributions plus time are what compound growth actually rewards.
| Total contributed: | $0 |
| Total interest earned: | $0 |
| Final balance: | $0 |
How to Use This Calculator
- Enter a starting amount, if any — this can be $0 if you're starting from scratch.
- Enter a monthly contribution you plan to add consistently.
- Enter an expected annual interest rate — this is an assumption, not a guarantee, since actual investment returns vary year to year.
- Enter the number of years you plan to let the money grow.
Why Time Matters More Than the Starting Amount
This calculator makes visible what's often described but rarely seen directly: the earlier you start, the more of your final balance comes from growth rather than your own contributions. Try entering the same monthly contribution with 10 years versus 30 years — the difference in total interest earned, not just total contributed, illustrates why starting early is repeatedly emphasized in our retirement planning guide.
| Years | Total Contributed | Growth Earned | Final Balance |
|---|---|---|---|
| 10 | $46,000 | $26,022 | $72,022 |
| 20 | $82,000 | $114,665 | $196,665 |
| 30 | $118,000 | $329,156 | $447,156 |
Between year 10 and year 30, total contributions only grow by about 2.6x — but growth earned grows by more than 12x. That's the mechanic worth internalizing: the same monthly habit, given more time, does dramatically more of the work on its own.
A Note on the Interest Rate Assumption
Unlike a savings account with a stated rate, investment returns aren't fixed or guaranteed — a 7% annual rate is a commonly used long-term historical average assumption for diversified stock market investments, but any individual year (or even decade) can vary significantly above or below that average. Use this calculator to understand the mechanics of compound growth, not as a guaranteed projection of actual future returns. A more conservative planning approach is to run the numbers at a few different rates — say 5%, 7%, and 9% — to see a realistic range of outcomes rather than anchoring to a single optimistic assumption.
Frequently Asked Questions
What interest rate should I use for a retirement account?
There's no single correct answer, since actual returns depend on your specific investments and market performance over your timeframe. Many long-term retirement projections use a conservative-to-moderate historical average as a planning assumption, but it's worth running the calculator at a few different rates to see a range of possible outcomes rather than relying on a single number.
Does this calculator account for taxes?
No — it shows raw compound growth without factoring in taxes, which vary based on account type (a Roth account, a traditional account, or a taxable account) and your individual tax situation. See our Traditional vs. Roth IRA guide for how account type affects the tax treatment of this growth.
Does this calculator save my data?
No — it runs entirely in your browser, and the numbers you enter aren't sent to or stored on our servers.
How much of my final balance actually comes from growth versus my own contributions?
It depends heavily on your timeframe — the longer the money compounds, the larger the share that comes from growth rather than contributions. In the 10,000-dollar starting balance example above, growth makes up about 36% of the total at 10 years but roughly 74% of the total at 30 years, with the same monthly contribution the whole time.
Why does a small change in interest rate make such a big difference over time?
Because compound growth builds on itself — a higher rate doesn't just earn more each year, it earns more on a larger base each subsequent year too. Try running the same numbers at 5% versus 8% over 30 years in the calculator above to see how much that few-point difference compounds into.
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Where to Go Next
Related guides on ClearCents:
Start the Habit This Month
The numbers in this calculator only become real through consistent contributions over years, not a single lump sum or a plan you intend to start "someday." If you're not yet contributing regularly to a retirement or investment account, that's the actual next step.
Ready to open an account? Our guide to investing apps for beginners covers where to actually start.