How Extra Mortgage Payments Reduce Interest

Take a $300,000, 30-year loan at 6.75%. Scheduled P&I is about $1,945.79. Add $100 extra principal every month from the first payment, and the modeled payoff drops to about 311 months instead of 360 — roughly 4 years and 1 month sooner — with about $65,000 less interest in that scenario. Those figures match this site’s extra mortgage payment calculator for that example. Your loan will differ.

Why the balance drop matters

Each month, interest is charged on whatever principal is left. Extra money that actually reduces that balance means less interest next month. On a standard fixed-rate loan, the required P&I payment usually stays the same; after the balance falls, more of each payment goes to principal. The amortization calculator shows that shift payment by payment.

Tell the servicer it is for principal

Extra payments only help if they hit principal, not prepaid future installments. Say so when you pay. Some loans have prepayment restrictions; most closed-end consumer mortgages allow prepayment, but you still verify. Biweekly drafts, “skip a payment” offers, and escrow shortages are different animals. Recasting — lowering the required payment after a lump sum — is a separate servicer product and is not what the extra-payment calculator models.

When the mortgage is not the first place for cash

High-interest credit cards, a thin emergency fund, and uninsured risks often deserve money before optional mortgage principal. Extra payments cut a specific interest cost; they do not automatically beat every other use of cash. Choosing between extras on a 30-year loan and a shorter original term? Compare 15-year vs 30-year mortgage differences. Thinking about refinancing soon? Extra principal still lowers the balance you refinance, but the lifetime-interest story changes — see refinance break-even.