Compound Interest Calculator with Monthly Contributions
Enter your initial capital, monthly contribution, interest rate and term to see how your money grows.
Compound interest is interest on interest: one month's return starts earning a return of its own the next month. It's why a small amount invested for long enough grows far more than intuition suggests — this calculator shows month by month how that happens.
As a % per month. Don't know the monthly rate? Use the button below to convert from an annual rate.
Mathematical simulation with a fixed interest rate — it doesn't account for inflation, income tax on returns, or real-world rate changes over time. Actual investment returns vary and are never guaranteed.
Frequently asked questions
What is compound interest?
It's when the interest earned in one period becomes part of the capital and starts earning interest itself in the next period — unlike simple interest, which is always calculated only on the original amount.
Why does the monthly contribution make such a big difference?
Every new contribution starts compounding from the moment it's added — the earlier and more consistent the contributions, the more time they have to compound before the end of the period.
Does this rate already account for income tax?
No. The calculator simulates the gross return. Fixed-income investments in Brazil have a regressive income tax (from 22.5% down to 15% depending on the holding period), which reduces the final net amount.
How do I find my monthly rate if my investment's return is quoted per year?
Use the built-in converter in this calculator: it applies compound equivalence (not just dividing by 12) to find the monthly rate that matches the annual rate you enter.