Extra Mortgage Payment
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Yes — the interest savings only happen if the extra amount is explicitly applied to the loan's principal balance rather than counted as an early regular payment. Check with your servicer that extra payments are marked 'apply to principal,' since some lenders default to holding it as a future payment credit instead.

Mortgage interest is calculated on the remaining balance, so every dollar of extra principal paid early stops accruing interest for the rest of the loan term. Because early payments have the most years left to compound, extra payments made early in the loan produce disproportionately large savings.

A lump sum paid today saves more interest than the same total spread out over future months, because the money stops accruing interest sooner. That said, consistent smaller extra payments are often easier to sustain and still meaningfully shorten the loan.

It depends on the size of the extra payment relative to the loan, but even modest amounts—an extra $100 to $200 a month on a typical 30-year mortgage—can cut several years off the term. The calculator's amortization comparison shows the exact new payoff date for your numbers.

It depends on your mortgage rate versus your expected investment return, and on your risk tolerance. Paying down a mortgage is a guaranteed, risk-free return equal to your interest rate, while investing carries market risk but historically higher average returns — there's no universally correct answer.