What your result means
The calculator amortizes the same fixed-rate balance twice: once with the scheduled payment and once with the entered extra principal every month.
Formula and assumptions
Each month, interest = remaining balance × monthly rate. The scheduled payment plus extra amount pays interest first and then principal until the balance reaches zero.
- The rate is fixed and the extra amount is paid every month.
- The servicer applies the extra amount directly to principal.
- There is no prepayment penalty or recast.
Worked example
On a $320,000 balance at 6.5% with 30 years remaining, an extra $300 each month reduces principal faster. The exact time and interest savings appear in the result.
Limitations
Real servicing dates, daily interest, escrow changes, payment timing, skipped payments, recasts, and prepayment penalties can change the outcome. Confirm principal-only instructions with the servicer.