Cost of Credit Card Debt Calculator: Estimate Payoff Cost

The cost of credit card debt calculator estimates how much interest you will pay and how long payoff may take when you enter your balance, APR, and monthly payment. It returns estimated months to repay, total payments, and interest; the result assumes no new purchases or fees.

Formula

The calculator estimates the number of monthly payments needed to reduce the starting balance to zero using a constant payment and a fixed monthly interest rate. It rounds the calculated payoff time up to a whole month, then estimates total payments as that month count multiplied by the entered monthly payment.

• Monthly rate, r: APR ÷ 1,200, expressed as a decimal per month.
• Months, n: ceil(−ln(1 − r × balance ÷ payment) ÷ ln(1 + r)). The result is rounded up because a partial final month is treated as one full payment period.
• Total payments: monthly payment × n.
• Total interest: total payments − starting balance.

Here, APR is the annual percentage rate entered as a percentage, balance is the current amount owed in dollars, and payment is the amount paid each month in dollars. The formula requires the payment to exceed the first month’s interest charge. The calculator checks that condition; if the payment is too small to reduce the balance, it displays “Cannot compute.”

Worked Example

Using the calculator’s defaults, the balance is $5,000, APR is 20%, and the monthly payment is $200.

1. Convert the APR to a monthly rate: 20 ÷ 1,200 = 0.0166667, or about 1.66667% per month.
2. Calculate the first month’s interest: $5,000 × 0.0166667 ≈ $83.33. The $200 payment is larger than this, so it reduces the balance.
3. Estimate months: ceil(−ln(1 − 0.0166667 × 5,000 ÷ 200) ÷ ln(1.0166667)) = ceil(37.19) = 38 months.
4. Estimate total payments: 38 × $200 = $7,600.
5. Estimate interest: $7,600 − $5,000 = $2,600.

The calculator therefore shows approximately 38 months, $7,600 in total payments, and $2,600 in interest. Actual card statements commonly calculate interest using daily balances and may have a smaller final payment, so these figures are estimates rather than a statement-level payoff schedule.

Assumptions

• The APR stays fixed for the full payoff period.
• You make one equal payment at the end of each monthly period.
• Interest compounds monthly using APR ÷ 12; the calculation does not model daily balance methods.
• No new purchases, cash advances, late charges, annual fees, balance-transfer fees, or other transactions are added.
• The entered payment remains unchanged, including in the final month; total payments therefore uses whole months of that amount.

When to Use the Calculator

Use this estimate to understand the relationship between a card balance, its APR, and a planned fixed payment. It can help answer practical questions such as whether a payment is large enough to lower the balance and how much interest could accumulate under the stated assumptions. It does not determine what payment you should make, and it does not account for other financial obligations.

For a useful estimate, enter the balance currently subject to the APR you are modelling and the APR shown in the relevant account disclosure or statement. If separate balances have different rates—for example, purchases and cash advances—one rate may not represent the whole account. Calculate each balance separately or use the rate and balance that apply to the portion you want to examine.

Reading the Results and Common Mistakes

“Months to repay” is a count of monthly payment periods, not a calendar date. The total payment and interest figures are based on paying the same entered amount for every counted month. Since that model does not reduce the last payment to the exact remaining balance, it can slightly overstate what a real payoff would cost.

• Entering a minimum payment as if it were fixed: Card minimums may change as the balance changes. A fixed-payment estimate does not reproduce that schedule.
• Confusing APR with a monthly rate: Enter the annual percentage as shown, such as 20 for 20%, not 0.20 and not 1.67. The formula converts it to a monthly rate.
• Ignoring new charges: Continued spending can add to the balance and invalidate the payoff estimate.
• Assuming every dollar has the same rate: Different transaction categories or promotional terms can result in different interest calculations.
• Expecting the final payment to equal the regular payment: A real account may be paid off with a smaller final amount. This calculator counts whole payments at the entered amount.

If the APR changes, the payment is variable, or the account adds fees, the real payoff time and interest may differ materially. For a more detailed estimate, a period-by-period amortization schedule would need to represent payment dates, daily balances, rate changes, and new transactions.

What the Estimate Does Not Include

This is a simplified calculation, not a reconstruction of a credit card issuer’s billing system. Many card interest calculations use a daily periodic rate and a daily balance, while this formula applies a monthly rate to a simplified balance. Payment dates within a billing cycle, grace-period treatment, and the timing of new transactions can also affect interest. Those details are not inputs here.

The model also assumes that the payment is applied to the balance in a single simplified way. It does not model how payments are allocated among balances with different APRs, promotional periods, or transaction types. If these features apply, consult the account’s disclosed terms or contact the card issuer for an account-specific payoff figure.

Frequently Asked Questions

How much interest will I pay on my credit card debt?
The estimate is the calculated total payments minus the starting balance. It depends on the balance, APR, payment amount, and the assumptions listed above; it is not an exact statement quote.

Why does the calculator say “Cannot compute”?
The monthly payment may not cover the first month’s estimated interest. If payment is no greater than balance × APR ÷ 1,200, the simplified balance will not decline enough to produce a payoff estimate.

Does this calculator use the minimum payment?
No. It uses the fixed monthly payment you enter. If your card’s minimum payment changes each month, the result will not match that changing-payment schedule.

Does paying more each month reduce the estimated interest?
With the same balance and APR, a larger payment generally shortens the estimated payoff period and reduces the number of payment periods in this model. Actual results still depend on account terms, payment timing, and new charges.

Why is my statement’s interest different from this estimate?
Statements may use daily balances, transaction-specific rates, and precise payment dates. This calculator uses a simplified monthly-rate model and excludes fees and new activity.

This result is an educational estimate, not financial advice; the card issuer’s disclosure and account-specific payoff information provide the applicable figures.

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