How this calculator works
For a fixed term, the payment is loan × r ÷ (1 − (1 + r)^−n), with r the monthly rate and n the months.
For a fixed payment, the number of months is −ln(1 − loan × r ÷ payment) ÷ ln(1 + r).
If the payment is no more than the first month’s interest, the loan is never repaid; the calculator warns you.
Use the APR, which includes most fees, for a realistic total cost.
Worked example
$15,000 at 8%
- Over 48 months: $366.19 a month and $2,577 of interest.
- Paying $400 a month instead: about 44 months.
- The larger payment saves several months and some interest.
Questions people ask
Is this the same for personal, boat and business loans?
Yes, if they are fixed-rate installment loans. Only the rate and term typically differ.
How can I lower my payment?
A longer term or lower rate. A longer term costs more interest overall.
Do lenders charge prepayment penalties?
Some do, especially on business and auto loans. Check your loan agreement before paying early.
What about variable-rate loans?
Their payments change when rates move. Use the current rate for an estimate and test a higher one.
Last reviewed October 2, 2026