Updated: July 2026
What is the Compound Interest Calculator?
The Compound Interest Calculator is the definitive tool for you to project the growth of your money over time. Known as the "eighth wonder of the world" (a phrase attributed to Albert Einstein), compound interest is the engine behind investment wealth: it generates interest on interest.
While in simple interest the income is calculated only on the initial value, in compound interest the income for one month is added to the capital to calculate the income for the following month. This creates a positive "snowball" effect for those who invest, and a negative one for those who are in debt (as with credit card revolving).
Our free online tool allows you to calculate the final value of an investment, simulating monthly contributions, interest rate and period (in months or years), also showing a proportion graph between what came out of your pocket and what was generated by interest.
How to Calculate Compound Interest?
The standard mathematical formula used in the financial market for compound interest is:
M = C × (1 + i)ⁿ
- M (Amount): The final accumulated value.
- C (Capital): The initial amount invested.
- i (Interest rate): The yield rate per period (ex: 1% per month becomes 0.01).
- n (Time): The number of months or years that the money will be invested.
When we make monthly contributions (putting in more money every month), the formula becomes even more complex, mixing the interest on the initial value with the future value formula of the annuities. But don't worry! Our calculator does all this work instantly.
How to use the Calculator
- Initial Value: The money you already have to start investing (if you are starting from scratch, enter 0).
- Monthly Input: How much you plan to invest every month.
- Rate (% per month): The expected profitability. A realistic rate for Fixed Income is usually between 0.8% and 1% per month, depending on the Selic.
- Period: Choose whether you want to enter the time in Months or Years and enter the number.
As soon as you fill it in, the visual bar will show you the magic: the longer the period, the larger the green slice (Interest Earned) will be in relation to the gray one (Total Invested from your pocket). Use the Month-by-Month Evolution Table below to track the detailed growth of your assets!