At the calculator's own defaults — a $6,000 balance at 22% APR — interest alone adds $110 to the balance every single month before your payment even touches it. A $250 monthly payment leaves $140 actually going toward the balance after that $110 is covered, which pays it off in 32 months (2y 8mo) and costs $1,979.05 in total interest. Drop the payment to $180/month and only $70 goes toward the balance each month — payoff stretches to 52 months and costs $3,357.66 in interest, more than half the original $6,000. Same balance, same rate — the payment size alone is the difference between paying about a third extra and paying more than half extra.
The scenarios in the calculator's own results table, worked out in full on the $6,000/22%/$250 default:
| Payment | Payoff time | Total interest | Interest saved |
|---|---|---|---|
| $250/mo (default) | 32 months | $1,979.05 | — |
| $300/mo (+$50) | 26 months | $1,542.87 | $436.18 |
| $350/mo (+$100) | 21 months | $1,269.28 | $709.77 |
| $450/mo (+$200) | 16 months | $944.02 | $1,035.03 |
Notice the pattern: doubling the extra payment from $100 to $200 doesn't double the interest saved — the gains shrink slightly each time, because you're spending less total time accruing interest at all. Even a relatively small permanent increase (+$50/month here) saves several hundred dollars, which is why the standard advice is to add whatever fixed amount is sustainable rather than waiting for a lump sum.
Credit card minimum payments are often calculated as a small percentage of the balance — commonly 1–3% — which can land dangerously close to the interest floor. On this $6,000/22% example, a payment near $180/month barely outpaces the $110 in monthly interest, which is why it takes 52 months and $3,357.66 in interest to clear. If a monthly payment on a real balance feels low relative to the total, it's worth checking it against the balance × (APR ÷ 12) math above — if the payment isn't comfortably above that number, payoff will be slow and interest will add up fast.
This tool works backward from a payment amount to tell you how long payoff will take and how much interest you'll pay — useful for credit cards and existing debts rather than new loans.
Extra payments reduce your principal faster, which means less of your balance sits there accumulating interest each month — the effect compounds over the life of the debt.
Run each debt through separately. If you're deciding which to pay off first, prioritizing the highest interest rate (the "avalanche" method) usually saves the most money overall.