Where an extra dollar actually lands
Every month, each debt first picks up interest, then gets its minimum payment. Minimums are mostly set to cover that interest plus a slice of principal. An extra payment is different: once the minimums are covered, every extra dollar goes straight to principal on one focus debt.
A smaller principal means a smaller interest charge the next month, and the month after that. So an extra $50 does more than lower the balance by $50. It also trims every interest charge that would have followed on that $50.
How extra payments stack with rollover
The effect grows once the first debt is gone. When the focus debt hits zero, its old minimum joins the extra and both move to the next debt in line. Any leftover from a partial final payment moves over the same month.
That is why an extra amount tends to pay off more than one debt early. It clears the first debt sooner, which starts the rollover sooner, which clears the second debt sooner, and so on down the list. The calculator tracks that chain month by month, and the debt-free date calculator shows where it ends for each debt.
Reading the ladder: when the first $50 does the most
The ladder often shows diminishing returns in months. Going from no extra to $50 can cut a lot of time, while going from $250 to $500 may cut fewer months than you might expect. The reason is simple: once the plan is already short, there are fewer months left to remove.
The pattern is not always smooth, though. A few situations can produce a bigger jump than the step before:
- Minimums that barely cover interest, where even a small extra finally gets the balance moving.
- A 0% promo that ends, where a slightly larger extra clears the debt before the higher rate starts.
- An extra that finishes a debt one month earlier, which pulls the whole rollover chain forward.
Interest saved and months saved can also tell different stories. Some people watch the months because the end date is what keeps them going; others watch the dollars because that is money that stays theirs.
Monthly extra vs a one-time amount
This calculator assumes the same extra every month. A tax refund or bonus works differently because it lands once, then the plan carries on at its normal pace. The lump sum calculator tests where a single amount moves the payoff most.
Working the other way is also possible. Instead of picking an extra and seeing the date, the pay off by date calculator starts from a target month and solves for the extra it would take.
What these numbers assume
Results are estimates. The model holds interest rates steady, adds no new charges, and treats each month as one interest charge at APR divided by 12. Real statements are figured on daily balances, so actual totals can land a little differently. The order the extra follows comes from the strategy chosen; the debt payoff calculator covers how those orders compare.