Calculadora de Taxa de Juros do Cartão
- Created by
- Renato Passos, Eng. de Software
- Reviewed by
- Renato Passos, Eng. de Software
Last updated: Apr 18, 2026
Formula
taxa_anual = (1 + taxa_mensal)^12 − 1
About this calculator
The Credit Card Interest Rate Calculator is a practical tool to convert the monthly credit card interest rate into its effective annual rate. This is useful because banks often disclose only the monthly rate, but the actual annual cost can be much higher due to compound interest. The formula used is: annual rate = (1 + monthly rate)^12 − 1, which considers monthly compounding.
To use the calculator, simply enter the monthly rate (as a percentage). The result is the effective annual rate, representing the true cost of revolving or installment credit. For example, a monthly rate of 10% results in an annual rate of approximately 213.84%. This conversion is essential for comparing credit offers and planning bill payments.
This calculator is especially useful for consumers who want to understand the impact of credit card interest on their finances. It can be used when reviewing bank statements, considering installment purchases, or evaluating debt renegotiation. Remember that the effective annual rate is higher than simply multiplying the monthly rate by 12, due to compound interest.
Cautions: the calculator assumes a constant monthly rate and monthly compounding. In practice, some cards may have variable rates or additional charges (e.g., IOF). Also, not paying the full bill can generate interest on the outstanding balance, further increasing the cost. Use the tool as a reference but always check your contract terms.
Frequently asked questions
Why is the effective annual rate higher than the monthly rate multiplied by 12?
Because interest is compounded monthly, meaning each month's interest is added to the balance and generates further interest. The formula (1 + monthly rate)^12 − 1 captures this effect.
Can I use this calculator to compare credit card offers?
Yes, it is ideal for comparing the effective annual cost of different cards, provided the monthly rates are known.
What does credit card interest rate mean?
It is the rate charged on the outstanding balance if you do not pay the full invoice. Usually expressed monthly, but the actual annual cost is higher.
Does the calculator consider IOF?
No. IOF is a federal tax that may be charged on credit operations and is not included. Check your contract for the full rate.
How can I reduce credit card interest?
Pay the full bill by the due date, avoid revolving credit, and negotiate rates with the bank. Using the calculator shows the real cost, encouraging full payment.