Credit Card Debt Calculator: Interest and Payoff

A credit card debt calculator estimates how much interest a balance may accrue and how long it may take to repay it using your balance, annual percentage rate (APR), and monthly payment. Enter those figures to see an estimated payoff time and total interest; the result assumes a fixed APR and the same payment each month.

Formula

The calculator estimates the number of equal monthly payments needed to reduce a balance to zero, then subtracts the original balance from the sum of those payments to estimate interest.

• Monthly rate, r: APR ÷ 1,200, converting a percentage per year to a decimal rate per month.
• Months, n: round up [−ln(1 − r × B ÷ M) ÷ ln(1 + r)], where ln is the natural logarithm.
• Estimated interest: M × n − B.
• B: starting balance in dollars; APR: annual percentage rate in percent; M: monthly payment in dollars; n: estimated number of monthly payments.

The months formula comes from the standard balance equation for equal payments and monthly compounding. Rounding up means a partial final month is treated as one full payment period. Because this simple estimate applies the entered payment for every month, its interest total does not model a smaller final payment; actual interest depends on the issuer’s balance calculations and payment schedule.

Worked Example

Using the calculator’s defaults: balance B = $5,000, APR = 20%, and monthly payment M = $200.

1. Convert APR to a monthly decimal rate: r = 20 ÷ 1,200 = 0.0166667, or about 1.66667% per month.
2. Substitute into the payoff formula: n = ceil[−ln(1 − 0.0166667 × 5,000 ÷ 200) ÷ ln(1.0166667)].
3. The expression inside the first logarithm is about 0.583333. The unrounded result is about 33.4 months, so the estimate rounds up to 34 months.
4. Estimate total interest using the displayed rounded month count: $200 × 34 − $5,000 = $1,800.

The calculator therefore displays 34 months and $1,800 in estimated interest. The latter is a simplified estimate based on 34 full payments of $200; a real final payment may be smaller, so an account’s actual total may differ.

Assumptions

• The APR stays fixed throughout the estimated payoff period.
• Payments are made monthly and on time, with no new purchases or cash advances.
• Interest compounds monthly at APR ÷ 12. Actual card interest may be calculated using a daily balance method, so this monthly approximation can differ.
• There are no fees, promotional-rate changes, late payments, taxes, or other balance adjustments in the calculation.

How to Use the Result

Use the balance and APR shown on your account statement, and enter the amount you expect to pay each month. If the balance or APR changes, rerun the estimate. The payoff-month figure is a planning approximation, not a promised date. Interest is the estimated amount paid above the starting balance, subject to the simplified assumptions above.

The calculator cannot produce a finite payoff estimate when the payment does not cover the month’s interest. For example, at a $5,000 balance and a 20% APR, the first month’s approximate interest is $83.33. A payment of $80 would be less than that amount, so the balance would not fall under this model. The expression also cannot calculate a payoff if the payment is zero.

What Changes Credit Card Interest

Credit card interest is commonly tied to an APR, but a statement’s interest charge is not always exactly the balance multiplied by APR divided by 12. Issuers may calculate interest using daily balances and a daily periodic rate; transactions made during a billing cycle, payment dates, and the number of days in that cycle can affect the result. Check the card agreement and statement for the method and rates that apply to your account.

A card can also have different APRs for different balance categories, such as purchases, transfers, or cash advances, and a promotional APR may later change. This calculator takes one APR and one starting balance, so it cannot separately model those categories, an introductory period, or a variable-rate change. To estimate one portion of a balance, use the applicable rate for that portion and treat the result as a separate approximation.

Common Errors and Limits

• Entering APR as a decimal: enter 20 for 20%, not 0.20. The formula divides the percentage by 1,200.
• Using a minimum payment as a fixed amount: many minimum payments can change as the balance changes. A fixed monthly payment estimate will not reproduce that schedule.
• Ignoring new charges: purchases or fees added while paying down a balance can extend payoff and increase interest.
• Assuming the last payment is full size: the estimate rounds months upward and applies the entered payment to every period. Actual payoff accounting may have a smaller final payment and different interest.
• Mixing balances with different rates: a single-rate calculation cannot accurately represent multiple APR categories at once.

For an account-specific figure, compare the estimate with statements and the issuer’s payoff information. The calculator is useful for understanding the relationship among balance, APR, and payment, but it does not reproduce every detail of a card’s billing system.

Frequently Asked Questions

How do I calculate interest on credit card debt?
For a rough monthly estimate, multiply the balance by APR ÷ 12, using APR as a decimal. For example, $5,000 at 20% APR gives about $83.33 for one month before considering payment timing and the issuer’s exact calculation method.

Why does my credit card interest differ from this calculator?
The calculator uses monthly compounding and a fixed APR. Your issuer may use daily balances, different APRs for transaction types, changing balances, or a variable rate, all of which can change the statement’s interest charge.

What payment should I enter?
Enter the monthly amount you want to model. If that amount varies, or if the required minimum changes over time, the result will not represent the exact payment schedule.

Can I pay off a balance if my payment is less than monthly interest?
Not under this model: a payment that does not cover accrued interest will not reduce the balance. The calculator returns no valid payoff estimate for that case.

Does paying more reduce interest?
With the same balance and APR, a higher regular payment generally reduces the time a balance remains outstanding and the interest estimated by this fixed-rate model. Actual results depend on payment timing, new charges, and account terms.

This result is an estimate for education, not financial advice; your card issuer’s disclosure and account records give the exact figures.

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in bal | Credit card balance | $ | 5000 | 0 | 100000000
in apr | Interest rate (APR) | % | 20 | 0.01 | 100
in pay | Monthly payment | $ | 200 | 0.01 | 100000000
out mon | Months to repay | months | ceil(round(-ln(1 - (apr / 1200) * bal / pay) / ln(1 + apr / 1200), 6)) | 0
out int | Total interest | $ | pay * mon - bal | 2