Compound Interest Calculator
See how a balance could grow over time as returns compound on your past returns.
Compound Interest Calculator
Compound interest is the engine behind long-term investing: you earn returns on your past returns, not just on what you put in. Enter a starting amount, a regular monthly contribution, an expected annual return, and a time horizon to see a year-by-year projection of how a balance could grow. Every figure is your own assumption and the result is a hypothetical illustration in nominal dollars, not a prediction. It fetches no live data and everything stays in your browser.
For reference, the US stock market has historically averaged roughly 7 to 10 percent a year over long periods, before inflation. Past performance does not guarantee future results.
Fill in the boxes above to see how your money could grow.
How this is worked out
Compounding: The balance compounds monthly: your annual return is divided by twelve and applied each month, and any monthly contribution is added at the end of the month. Monthly compounding is a common convention; real accounts may compound daily, quarterly, or annually.
Return: The annual return you enter is a fixed assumption, not a forecast. Real returns vary every year and can be negative. US stocks have averaged roughly 7 to 10 percent a year over long periods before inflation, with large swings along the way.
Contributions: Contributions are assumed to be the same every month for the whole period. This tool does not step them up over time; for rising contributions, try the Portfolio Growth Calculator.
Not modeled: Taxes, fees, and inflation are left out, so the future value is shown in nominal dollars. The result is a hypothetical illustration, not a prediction or financial advice.
Common questions about compound interest
How does compound interest work?
Compound interest is the return you earn on both your original money and the returns it has already produced. Over time the growth builds on itself, so a balance can grow faster in later years than in early ones. This calculator shows that effect year by year, based on the figures you enter.
How is the future value calculated?
The tool compounds your balance monthly: it divides your annual return by twelve, applies it each month, and adds any monthly contribution at the end of the month. The final figure is a hypothetical projection from your own inputs, not a guaranteed outcome.
What rate of return should I use?
There is no single correct number. Many people use long run averages as a reference point; for example, US stocks have averaged roughly 7 to 10 percent a year before inflation, while bonds and cash have averaged less. Returns are not guaranteed and vary widely year to year, so it can help to test a few different rates.
Does the calculator account for inflation?
No. The result is shown in nominal dollars and is not adjusted for inflation, taxes, or fees, so its future spending power would be lower. To see a value adjusted for inflation over the long term, try the Portfolio Growth Calculator.
What is the difference between simple and compound interest?
Simple interest is earned only on your original amount. Compound interest is earned on your original amount plus the returns already added, which is why a long time horizon matters so much. This tool models compound growth.
Is this calculator financial advice?
No. It is an educational tool that illustrates how compounding could work using the numbers you enter. It does not predict real returns and is not financial advice. Consider speaking with a licensed financial professional about your own situation.
These results are estimates, not predictions. They are based on the inputs you provide and assumed rates of return, which are not guaranteed. Real markets rise and fall every year. For educational purposes only, not financial advice.
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