Compound Interest Calculator
Compound Interest Calculator is a free finance tool that runs directly in your browser with no account required.
About this tool
This calculator projects how a starting balance plus regular monthly contributions grows over time with compound returns. It supports an annual contribution increase so you can model saving more each year, splits the final value into what you contributed versus what returns earned, and produces a year-by-year growth table with a CSV export.
How to use
- Enter your Initial amount and planned Monthly contribution.
- Set the Annual return % and the number of Years to project.
- Optionally set an Annual contribution increase % to raise contributions each year.
- Read the future value, total contributed, and estimated gain, then use Download growth CSV for the yearly breakdown.
Frequently asked questions
How often is interest compounded?
Monthly. The annual return is divided by 12, the balance grows by that rate each month, and then the monthly contribution is added.
When does the annual contribution increase kick in?
At each year boundary. After every 12 months the monthly contribution is multiplied by (1 + increase %), so a 3% increase turns a $500 contribution into $515 in year two.
What does "Return share" mean in the results?
It is the fraction of your final balance that came from investment gains rather than your own deposits — a quick view of how much compounding did the work.
Can I model a negative return?
Yes, you can enter a negative annual return down to -99% to stress-test a decline, though most people use it for positive growth scenarios.
Where is the calculation performed?
Entirely in your browser. Your amounts never leave your device and nothing is saved between visits.
Will my investments actually grow like this?
No projection can promise that. This tool assumes a constant return with no taxes or fees, so treat the output as an estimate for comparison, not financial advice.