See how savings and investments grow when returns earn returns. Add monthly contributions to see the habit that matters most.
Compound Interest Calculator
Free and instant. Try changing the years — time is the most powerful input.
Illustration only — real returns vary and are never guaranteed.
How compound growth works
Compounding means each period’s return is added to your balance, so the next period earns returns on a bigger pot. The formula is A = P × (1 + r/n)nt, plus the future value of your monthly contributions. Two levers dominate: time (starting 10 years earlier usually beats saving more later) and consistency (regular monthly contributions).
Example
$10,000 plus $300 a month at 7% for 25 years grows to about $300,000. You contributed $100,000 of that — roughly $200,000 is pure growth. That gap is compounding doing the heavy lifting in the later years.
Frequently asked questions
What return rate should I assume?
Long-run US stock market returns have averaged around 7% a year after inflation (about 10% before). Cash savings accounts pay far less. Use 5–7% for a cautious plan and never treat any figure as guaranteed.
How often should interest compound?
More frequent compounding grows money slightly faster, but the difference is small next to your rate, contributions and years. Monthly is the common default for savings and investing.
Why does the growth explode in later years?
Because the base keeps getting bigger. In the final years, annual growth on a large balance can exceed your annual contributions — which is exactly why starting early matters so much.