Compound Interest Calculator
Work out how much an investment grows with compound interest from a starting amount, monthly contributions, rate, and term.
- Total invested
- —
- Interest earned
- —
- Return
- —
Step by step
Projects the future value of an investment that earns interest on interest, combining a starting balance with fixed monthly contributions at a constant monthly rate. It shows how much you actually put in, how much came from returns, and the final balance at the end of the term. The rate is treated as monthly and the result ignores inflation and income tax.
How to use
Formula
- Enter the starting amount (P) you already have invested.
- Enter the monthly contribution (PMT) you plan to invest each month.
- Enter the monthly interest rate (i) as a percentage — use the monthly rate, not the annual one.
- Enter the term (n) in months.
- Read the final balance, the total invested (no interest), and how much came from returns.
Example
R$1,000 to start, R$200 per month, a 1% monthly rate, over 12 months.
- Compounded starting amount: 1,000 × 1.01¹² = R$1,126.83.
- Compounded contributions: 200 × (1.01¹² − 1) / 0.01 = R$2,536.50.
- Final balance: 1,126.83 + 2,536.50 = R$3,663.33.
- Total invested: 1,000 + 200 × 12 = R$3,400.00.
- Interest earned: 3,663.33 − 3,400.00 = R$263.33.
A final balance of R$3,663.33, of which R$263.33 is return.
Common mistakes
- Entering the annual rate in the monthly rate field — it badly inflates the result. Convert the annual rate to monthly first.
- Forgetting the balance is gross: on fixed income, regressive income tax still applies to the interest.
- Comparing against a savings account without adjusting the rate: savings earns about 0.5% per month, well below the CDI.
Glossary
- Final balance
- The ending value of the investment: everything you contributed plus all the accumulated interest.
- Contribution
- Each recurring deposit you make into the investment — here, a fixed amount every month.
- Equivalent rate
- A rate for one period converted to another while keeping the same return. The monthly equivalent of an annual rate is (1 + annual)^(1/12) − 1.
Reference table
Final balancebyStarting amount — assuming Monthly contribution: 10 R$; Interest rate: 1 %; Term: 2 months.
| Starting amount | Final balance |
|---|---|
| 1 R$ | R$ 21.12 |
| 2 R$ | R$ 22.14 |
| 5 R$ | R$ 25.20 |
| 10 R$ | R$ 30.30 |
| 20 R$ | R$ 40.50 |
| 50 R$ | R$ 71.11 |
| 100 R$ | R$ 122.11 |
Sources and review
Reviewed by Equipe Calculadora do Mundo.
Sources
Estimativa para fins informativos; não substitui orientação contábil ou jurídica.
FAQ
What's the difference between simple and compound interest?
With simple interest, the return is always calculated on the starting amount. With compound interest, each month earns on the accumulated balance — including the interest already added. That's why compounding grows faster and faster over time. This calculator uses compound interest.
Is the rate I enter monthly or annual?
Monthly. If you only know the annual rate, convert it first: the equivalent monthly rate is (1 + annual_rate)^(1/12) − 1. For example, 12.7% per year is about 1% per month, not 1.06%.
Do the monthly contributions count?
Yes. The final balance adds the compounded starting amount to every monthly contribution from the month it goes in. The 'Total invested' figure shows the sum of what came out of your pocket, before any return.
Is income tax already deducted from the result?
No. The balance is gross. On Brazilian fixed income, tax is regressive and applies only to the interest: 22.5% up to 180 days, falling to 15% beyond 720 days. For the net figure, deduct that tax from the 'Interest earned' amount.