How the payoff is modeled
Each month, interest accrues on whatever balance is left (APR ÷ 12 of the balance), and your payment is applied after that. What's left over after covering interest chips away at the principal. Run that forward one month at a time and you get a payoff date, a total interest figure, and a running balance for the chart. It's the same math a credit card statement uses to build its own payoff estimate, just with a payment you control instead of the card's minimum.
How to read the results
"Debt-free in" counts the months until the balance hits zero at your current payment, extra included. The chart plots that balance line alongside a second line showing what would happen at the base payment alone, so you can see the gap the extra payment opens up. "Total interest" and "total paid" summarize the same run in dollars: total paid is just the balance plus every dollar of interest charged along the way. When you set an extra payment above zero, the results also show how many months sooner you'd finish and how much interest that extra payment saves versus paying the base amount alone.
When the balance never shrinks
If your payment doesn't cover the interest that accrues in the first month, the balance grows instead of shrinking, no matter how long you wait. The calculator flags this directly rather than showing a payoff date months or years out that would never actually arrive. Raise the payment past that first month's interest charge and a real payoff timeline appears.
Caveats worth keeping in mind
This model assumes a fixed APR and a constant payment every month, which is a simplification. Credit card minimum payments typically fall as the balance drops, so if you're paying a shrinking minimum rather than a fixed amount, payoff will take longer than shown here. It also doesn't account for new charges, promotional rate periods that expire, or fees. Treat the result as a planning estimate for a fixed payment strategy, not a guarantee from your card issuer or lender.