How the debt payoff calculator works
Each month interest is charged on the remaining balance at the APR divided by 12, and your payment is subtracted. The calculator repeats this until the balance reaches zero and adds up the interest along the way.
New balance = Balance + interest − payment
Months (closed form) = −ln(1 − r × B ÷ P) ÷ ln(1 + r)
If your payment is less than or equal to the first month's interest, the debt never shrinks, and the calculator tells you the minimum needed to make progress. The extra-payment field runs the schedule twice and shows how much interest and time you save. This covers student loans and HELOCs in their repayment phase too; for a HELOC still in its interest-only draw period, enter the payment you plan to make, not the minimum.
Worked example
An $8,000 credit card balance at 22% APR costs about $147 in interest in the first month. Paying $250 a month clears it in 49 months with roughly $4,160 of interest. Adding $100 a month (paying $350) finishes in 30 months and saves about $1,695 of interest. For a $30,000 student loan at 6.5% with a $340 payment, payoff takes about 10 years (121 months).
Frequently asked questions
How is credit card interest calculated?
Most cards charge a daily periodic rate (APR ÷ 365) on the average daily balance. Monthly compounding at APR ÷ 12, used here, gives a very close estimate.
Why does my balance barely move?
Early payments go mostly to interest. At 22% APR, $147 of a $250 payment on an $8,000 balance is interest. Raising the payment even a little shifts much more toward principal.
Does this work for student loans?
Yes. Enter the combined balance, the weighted average rate and your payment. Income-driven plans change payments over time, which this fixed-payment model does not capture.
Should I pay off debt or invest?
A common rule is to pay off debt with a rate above what you expect to earn after tax, and to always capture any employer retirement match first.
Updated October 2026. Results are estimates for education, not financial or medical advice. All math runs in your browser; nothing you type is sent anywhere.