Debt payoff, in short
This calculator works the same way for any fixed-rate debt — a personal loan, a medical bill on a payment plan, a card balance — projecting your payoff date and total interest, then showing exactly what an extra monthly payment buys you. On $15,000 at 18% APR, a $400 payment takes 56 months and costs $7,210.43 in interest; adding just $100 more a month cuts that to 41 months and $5,077.47.
Key takeaways
- $15,000 at 18% APR, $400/month: 56 months (4.67 years) to debt-free, $7,210.43 total interest.
- Add $100/month extra ($500 total): 41 months (3.42 years), $5,077.47 total interest — 15 months faster and $2,132.96 less interest.
- The savings curve bends: the first $50 of extra payment saves about $1,259.62 in interest, but the next $50 only adds about $873.34 more in savings — real money either way, just with diminishing returns.
- Even a modest $50/month extra shaves 9 months off the payoff timeline on this example — small, consistent increases add up.
What determines your payoff timeline
Three numbers decide everything: balance, rate, and payment. Each month, interest accrues on whatever's left, and whatever your payment doesn't cover in interest goes toward shrinking the principal. A bigger payment relative to the balance means more of it hits principal each month, which is why the relationship between payment size and payoff time isn't a straight line — it curves, accelerating as the payment grows relative to the debt.
The $100 extra payment, worked out
Using the calculator's defaults — $15,000 balance, 18% APR:
$400/month: 56 months to payoff, $7,210.43 total interest
$500/month ($100 extra): 41 months to payoff, $5,077.47 total interest
A 25% increase in the payment (from $400 to $500) buys a 27% reduction in payoff time and a 30% cut in total interest — the extra payment does more, proportionally, than its size alone would suggest, because it shrinks the balance faster and reduces the interest that balance would otherwise keep generating.
Why extra payments have diminishing (but still real) returns
Run the same $15,000/18% debt across a range of extra payments:
+$0: 56 months, $7,210.43 interest
+$50: 47 months, $5,950.81 interest
+$100: 41 months, $5,077.47 interest
+$150: 36 months, $4,435.31 interest
+$200: 32 months, $3,941.89 interest
The first $50 saves $1,259.62; the next $50 (getting to +$100) saves a smaller $873.34; the $50 after that saves $642.16. Each increment still helps — there's no point where extra payments stop working — but the biggest jump in value comes from the first dollars you add above the minimum.
What happens if your payment doesn't cover the interest
If the monthly payment is smaller than the interest accruing that month, the balance grows instead of shrinking — you're not just failing to make progress, you're actively losing ground. This calculator will flag that scenario rather than pretend a payoff date exists; if you see it, the fix is either a larger payment or a lower rate (through negotiation, a balance transfer, or consolidation), not sticking with the current schedule.
Related calculators
For credit-card-specific minimum payment mechanics, see the credit card calculator. If you're juggling several debts at once, the credit cards payoff calculator compares avalanche and snowball ordering, and the debt consolidation calculator checks whether rolling everything into one lower-rate loan beats paying them off separately.