How the Extra Payment Loan Calculator works
When you put a lump sum toward your loan's principal, that money comes off your balance immediately — before your lender calculates next month's interest. Because interest on an amortized loan is charged on whatever balance remains, a smaller balance means less interest every single month for the rest of the loan, even though your regular payment doesn't change.
This calculator compares two schedules side by side: your loan as originally scheduled, and the same loan after your one-time extra payment is subtracted from the balance. Both are re-amortized using your same fixed monthly payment, so you can see exactly how many months you'd shave off and how much interest you'd avoid paying.
The earlier in the loan you make the extra payment, the more you tend to save — because interest is compounding on a larger balance for longer at the start of a loan. Even a modest one-time payment can meaningfully shorten a long-term loan.
A worked example
You owe $18,000 at 6.5% with a $350 monthly payment. Left alone, the loan runs about 61 months and costs $3,155.66 in interest. Make a single $1,000 extra payment now and you cut roughly 4 months and $368 in interest — from one payment.
| Scenario | Payoff time | Total interest |
|---|---|---|
| No extra payment | 61 months | $3,155.66 |
| One $1,000 extra now | 57 months | $2,787.80 |
The earlier in the loan you make the extra payment, the more interest it wipes out, because it removes principal that would otherwise be charged interest for years.
How to read your results
Interest saved and months saved compare the loan with and without your extra payment. The tool assumes the extra goes entirely to principal and that your regular payment continues unchanged.
When to use this calculator
Use it when you have a one-off sum — a tax refund, a bonus, a gift — and want to know what applying it to a loan actually buys you versus saving or spending it.
Common mistakes to avoid
- Letting the servicer “advance” your due date. That skips future payments instead of shrinking the balance. Ask for the extra to be applied to principal.
- Comparing the wrong alternative. If the loan rate is 6.5% and a safe account pays more after tax, the math can favor saving instead.
- Making the extra payment before an interest-rate reset. On a variable loan, timing matters less than you think — principal reduction still helps.