Credit Card Debt Calculator in Excel: Formula and Example

A credit card debt calculator in Excel estimates how many months it will take to repay a balance and how much interest you will pay when you enter your balance, APR, and fixed monthly payment. This calculator uses a monthly-rate payoff model; its outputs are estimated payoff time and total interest, not a lender statement.

Formula

The calculator treats the card balance as a debt that accrues interest monthly and receives the same payment at the end of each month. It first finds the fractional number of payments implied by the balance, monthly rate, and payment, then rounds up to a whole month:

Months = ceil(round(−ln(1 − r × B ÷ M) ÷ ln(1 + r), 6))

• B = starting balance, in dollars.
• APR = annual percentage rate, entered as a percentage such as 20, not as 0.20.
• r = monthly interest rate, calculated as APR ÷ 1200.
• M = fixed monthly payment, in dollars.
• ceil = round upward to the next whole month; ln is the natural logarithm.

Total interest is estimated as M × months − B. This method assumes the stated payment is made for each full month, including the final month. It therefore approximates the last payment as the same size as the others rather than reducing it to the precise remaining balance and interest. The result is a simple planning estimate, not a transaction-by-transaction ledger.

Worked Example

Using the default values—$5,000 balance, 20% APR, and a $200 monthly payment—the monthly rate is 20 ÷ 1200 = 0.0166667, or about 1.6667%.

1. Substitute into the payoff formula: −ln(1 − 0.0166667 × 5,000 ÷ 200) ÷ ln(1 + 0.0166667).
2. The expression inside the first logarithm is 1 − 0.4166667 = 0.5833333. The unrounded quotient is about 35.6 months.
3. Round the quotient to six decimal places and round upward to a whole payment month: 36 months.
4. Estimate interest using the calculator’s stated convention: $200 × 36 − $5,000 = $2,200.

The values are model outputs, not a promise that a card account will be paid off on that date or incur exactly that interest. The actual final payment can be smaller, and account calculations can use daily balances and specific posting dates.

Assumptions

• The APR remains fixed throughout the repayment period.
• Payments are equal, made once per month at the end of each monthly interest period.
• Interest is modelled monthly using APR divided by 12; the calculator does not reproduce daily balance calculations.
• No new purchases, cash advances, balance transfers, fees, late charges, taxes, or payment changes are included.

Building the Same Estimate in Excel

To recreate the calculation in a spreadsheet, place the balance in B1, the APR as a percentage in B2 (for example, enter 20%), and the monthly payment in B3. One way to calculate months is:

=ROUNDUP(ROUND(-LN(1-(B2/12)*B1/B3)/LN(1+B2/12),6),0)

When APR is stored as 20%, Excel represents it internally as 0.20, so dividing by 12 gives the monthly rate. If you instead store APR as the number 20, use B2/1200. Do not mix these conventions: doing so can multiply or shrink the rate by a factor of 100.

If the resulting months are in B4, a simple interest estimate matching this calculator’s convention is =B3*B4-B1. Label the result as an estimate. To model a smaller last payment accurately, a month-by-month table is more suitable: calculate interest on the opening balance, add it to the balance, subtract that month’s payment, and stop when the balance reaches zero. A daily-balance method may be needed to mirror a card issuer’s statement more closely.

How to Read the Results and Common Errors

The month figure is rounded up because a fraction of a monthly payment period does not correspond to a complete payment month. The interest figure uses the full fixed payment for every counted month, so it can overstate interest slightly when the final payment would be smaller. For a more exact estimate, calculate each month’s interest and payment separately, and adjust the last payment to the amount still owed.

• Payment too small: A payment must exceed the monthly interest on the starting balance for this formula to produce a finite payoff estimate. If it does not, the balance will not decline under the model. The calculator rejects such cases as “Cannot compute.”
• Changing APR: A variable rate or promotional period breaks the fixed-rate assumption. Model each rate period separately or use a schedule that allows the rate to change.
• New charges: Continued card spending adds to the balance and can extend repayment. This estimate assumes no new charges.
• Payment timing and rounding: Issuers may calculate interest daily and round statement amounts according to account terms. Small differences in dates and cents can change the actual result.

What This Calculator Does Not Show

This tool answers a narrow question: how long a fixed balance may take to repay under an assumed constant APR and monthly payment. It does not decide how payments are allocated among balances with different rates, estimate the effect of fees or missed payments, or account for interest-free grace periods and newly posted transactions. If your account has multiple balances or changing terms, a single-balance calculator cannot represent the full account.

For a more detailed Excel workbook, use one row per month with columns for opening balance, interest, payment, and closing balance. A basic row can calculate interest as opening balance × APR ÷ 12, then closing balance as opening balance + interest − payment. Ensure the final payment does not exceed the amount due; otherwise, the model will show a negative balance rather than an exact payoff. Check your card statement for the balance, applicable APR, fees, and payment dates that reflect your account.

Frequently Asked Questions

Can Excel calculate how long it will take to pay off credit card debt?
Yes. Enter the balance, APR, and payment into a formula or use a month-by-month schedule. The result depends on assumptions about rate, payment timing, and new charges.

Why does the calculator say “Cannot compute”?
The payment may be no greater than the estimated monthly interest on the starting balance. In that case, the formula cannot find a finite payoff time because the assumed payment does not reduce the debt.

Should I enter APR as 20 or 20% in Excel?
Either can work if the formula matches the entry. For 20%, divide the stored rate by 12; for the number 20, divide by 1200. Do not apply both percentage conversions.

Does this estimate include new purchases or card fees?
No. It assumes no additional charges or fees and a steady payment and APR. Add those items to a month-by-month model if you need to examine their effect.

Why might my statement show a different payoff amount?
Statements can reflect daily balances, transaction and payment dates, changing rates, fees, and a smaller final payment. Those details are outside this simplified monthly estimate.

This result is an estimate for educational purposes, and your card issuer’s disclosures and account statement provide the exact figures.

Related

• New Car Payments Under $200 a Month?
• New Car Payments: Estimate Your Monthly Cost
• New Car Payment Estimator: Calculate Monthly Costs

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 mos | Months to repay | months | ceil(round(-ln(1 - (apr / 1200) * bal / pmt) / ln(1 + apr / 1200), 6)) | 0
out int | Total interest | $ | pmt * mos - bal | 2