Tool · Investor Sam Home

Extra Mortgage Payment Calculator

June 30, 2026 • By the Investor Sam Editorial Team • Reviewed by Berly Sam Varghese, Editor
Adding even a modest amount to your monthly mortgage payment goes straight to principal, which shrinks the balance interest is charged on and compounds into large savings over the life of the loan. This calculator simulates your amortization month by month with the extra payment applied, then compares it to the standard schedule to show how many years and how much interest you cut. It is one of the highest-return moves an ordinary homeowner can make.

Example: Loan balance: 320000 $ · Interest rate (APR): 6.5 % · Loan term remaining: 30 years · Extra monthly payment: 300 $

Months to pay off254
Interest saved$138,446
Months saved106

Worked example

Take a $320,000 balance at 6.5% over 30 years. The standard payment is about $2,023, and over the full term you would pay roughly $408,000 in interest. Add $300 a month and the loan clears in about 24 years instead of 30 — roughly 73 fewer months — and total interest drops by more than $90,000. The extra $300 does far more work than the same money sitting in a low-yield account.

Frequently asked questions

Where does an extra payment go?

Applied correctly, extra money goes entirely to principal, reducing the balance immediately. Tell your servicer to apply it to principal, not to prepay next month, so it actually shortens the loan.

Is paying down my mortgage better than investing?

The answer turns on your rate versus expected investment returns and your risk tolerance. A guaranteed return equal to your mortgage rate is attractive when rates are high, but many people split the difference. This tool shows the guaranteed side of that trade.

Will extra payments lower my monthly bill?

No — your required monthly payment stays the same; you just finish sooner. If you want a lower required payment, you would need to recast or refinance. The benefit here is time and interest saved.

Are there prepayment penalties?

Most modern conventional mortgages have no prepayment penalty, but some loans do. Check your note before making large extra payments so you are not charged a fee for paying early.

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Sources

Berly Sam Varghese · Editor, Investor Sam

Berly Sam Varghese is an engineer who treats money the way he treats any hard problem — something to be engineered, not gambled on. He funded years of education and built real financial stability the patient way, by living below his means and investing rather than borrowing. He writes for the person trying to make a home a sound decision, not just a purchase. He reviews and approves every article on Investor Sam and checks the figures against primary sources before anything is published. More about our standards.