Compound interest calculator with monthly deposits
Enter what you have today, how much you will add each month, the interest rate and the term. The calculator shows the final amount, how much of it is interest and how the balance grows year by year.
| Total invested | R$65,000.00 |
|---|---|
| Interest earned | R$47,900.64 |
| Final amount | R$112,900.64 |
- Equivalent monthly rate
- 0.7974%
- Equivalent annual rate
- 10.00%
- Share of the amount that is interest
- 42.43%
- Months to double the initial amount
- 88
- Gross values: no income tax, IOF, fees or inflation.
Year-by-year growth
Year by year
| Year | Invested so far | Interest so far | Balance |
|---|---|---|---|
| 1 | R$11,000.00 | R$770.27 | R$11,770.27 |
| 2 | R$17,000.00 | R$2,217.56 | R$19,217.56 |
| 3 | R$23,000.00 | R$4,409.59 | R$27,409.59 |
| 4 | R$29,000.00 | R$7,420.82 | R$36,420.82 |
| 5 | R$35,000.00 | R$11,333.17 | R$46,333.17 |
| 6 | R$41,000.00 | R$16,236.75 | R$57,236.75 |
| 7 | R$47,000.00 | R$22,230.69 | R$69,230.69 |
| 8 | R$53,000.00 | R$29,424.03 | R$82,424.03 |
| 9 | R$59,000.00 | R$37,936.70 | R$96,936.70 |
| 10 | R$65,000.00 | R$47,900.64 | R$112,900.64 |
Estimate only; it does not replace an accountant, an employment lawyer or the official calculation by the employer/Receita Federal.
No official table: you enter the rate from your contract.all tables and sources
How it is calculated
1. The formula
With compound interest each month earns on the previous balance, including interest already earned. With initial amount C, monthly deposit A, monthly rate i and a term of n months:
M = C × (1 + i)^n + A × [((1 + i)^n − 1) ÷ i]
The first term is the initial amount growing on its own; the second adds up every deposit, each compounding for as long as it stays invested. This is the end-of-month case: the first deposit only starts earning the following month. Choose start of each month and every deposit earns one extra month, so the deposit term is multiplied by (1 + i). At a zero rate the result is simply C + A × n.
2. Monthly and annual rates
The calculation always runs monthly. An annual rate is converted by compound equivalence: i = (1 + annual rate)^(1/12) − 1. So 10% a year becomes about 0.7974% a month, not 10% ÷ 12. The reverse figure, the equivalent annual rate (1 + i)^12 − 1, is also shown: 1% a month compounds to 12.68% a year, not 12%. Brazilian banks and the Tesouro quote rates both ways, often as a.m. (per month) or a.a. (per year), so the conversion helps compare offers.
3. Year-by-year table and doubling time
The table shows, at the end of every 12 months, the total deposited, the interest accumulated so far and the balance. If the term is not a multiple of 12, the last row is the final month. Each row is rounded to the cent.
“Months to double” tells you how long the initial amount alone, without deposits, takes to double at the given rate: ceil(ln 2 ÷ ln(1 + i)), rounded up to the next whole month. It is a quick way to feel how much the rate matters.
4. What is not included
All values are gross: no income tax, IOF (Brazil's financial transactions tax), management or custody fees, and no inflation adjustment. The rate is held constant for the whole term, which floating-rate investments rarely do. To compare real Brazilian investments after tax, use the savings vs CDB vs Tesouro Selic calculator. The same maths drives debt; for instalment loans see the SAC vs Price loan calculator.
Results are planning estimates; your bank or broker statement is the official figure. “Copy simulation link” saves or shares the scenario.
Worked example
In the example, someone starts with R$10,000.00 and adds R$1,000.00 at the end of every month at 1% a month for 24 months. After the first year they have deposited R$22,000.00 and earned R$1,950.75 in interest, for a balance of R$23,950.75. In the second year interest speeds up because it applies to a larger balance: after 24 months the total invested is R$34,000.00, interest adds R$5,670.81 and the final amount reaches R$39,670.81. Interest makes up 14.29% of that amount. The annual rate equivalent to 1% a month is 12.68%, and the initial amount on its own would take 70 months to double. These are gross numbers, before any tax or fees.
- Initial amount
- R$10,000.00
- Monthly deposit
- R$1,000.00
- Interest rate (%)
- 1
- Rate period
- per month
- Term (months)
- 24
- Deposit at the
- end of each month
| Total invested | R$34,000.00 |
|---|---|
| Interest earned | R$5,670.81 |
| Final amount | R$39,670.81 |
- Equivalent monthly rate
- 1.0000%
- Equivalent annual rate
- 12.68%
- Share of the amount that is interest
- 14.29%
- Months to double the initial amount
- 70
- Gross values: no income tax, IOF, fees or inflation.
Year-by-year growth
Year by year
| Year | Invested so far | Interest so far | Balance |
|---|---|---|---|
| 1 | R$22,000.00 | R$1,950.75 | R$23,950.75 |
| 2 | R$34,000.00 | R$5,670.81 | R$39,670.81 |
Frequently asked questions
What is the difference between simple and compound interest?
With simple interest the return is always calculated on the original amount, so it grows in a straight line. With compound interest each month's interest joins the balance and earns interest itself in the following months. The gap is small at first but becomes large over long terms. Brazilian investments and debts generally compound.
Can I divide the annual rate by 12 to get the monthly rate?
No, that overstates the monthly rate. By compound equivalence the monthly rate is (1 + annual rate)^(1/12) − 1. At 10% a year, dividing gives 0.8333% a month, but the equivalent rate is about 0.7974%. The calculator converts automatically when you pick “per year” as the rate period.
Does it matter whether I deposit at the start or the end of the month?
Yes, because a start-of-month deposit earns one extra month. In the formula the deposit term is multiplied by (1 + i). Over a short term the difference is small; over many years it adds up. Pick the option that matches your habit: investing right after payday is closer to the start of the month.
Is income tax already deducted?
No. The calculator shows gross values, without income tax, IOF, fees or inflation. On Brazilian fixed-income products such as CDBs and Tesouro bonds, tax on the gain depends on how long you hold them; the poupança (regulated savings account) follows its own rules. To see net results, use the savings vs CDB vs Tesouro Selic calculator.
How is the doubling time worked out?
The calculator solves (1 + i)^n = 2 for n, which gives ln 2 ÷ ln(1 + i), and rounds up to a whole month. Only the initial amount is considered, not the deposits. At 1% a month, for instance, the answer is 70 months. The familiar “rule of 72” is an approximation of the same calculation.
Can I use it to see how a debt grows?
Yes, for a debt with no payments: enter the amount owed as the initial amount, a zero deposit and the contract rate. The final amount shows what the debt would grow to. Loans repaid in instalments follow different logic, since the balance falls with each payment; for those use the loan calculator.
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Updated Nov 13, 2026 · by Juliana Costa