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How to use this calculator
This calculator answers: What does an extra monthly payment do to my payoff date and total interest?
Why this matters: Every extra dollar of principal kills the interest it would have earned the bank — the effect is bigger and earlier than most people guess.
- Enter Current balance ($), Interest rate (%), Years remaining, and “Extra payment per month ($)”.
- Enter your numbers, then tap the button — editing an input clears the result so every calculation is fresh.
- Read “Your payoff plan”: the new payoff date plus “Interest: with vs. without extras”. Try raising the extra payment by $200 to see how many years and dollars it saves.
How the payoff math works
- Baseline: your regular payment comes from the standard amortization formula, and total interest without extras is simply payment × months − balance.
- With extras: the loan is simulated month by month — interest accrues, then your full payment (regular + extra) reduces the balance. Because each extra dollar permanently shrinks the balance, later months accrue less interest; that compounding-in-reverse is where the savings come from.
- Why early dollars matter most: extra principal paid in year 1 saves interest in every remaining year of the loan. The same dollar sent in year 20 saves far less.
- Assumes a fixed rate, extras applied to principal starting next month, and no prepayment penalties or fees.
Frequently asked questions
How much do extra mortgage payments save?
It depends on rate and timing, but extra principal payments are powerful because they permanently shrink the balance that future interest is charged on. On a $400,000, 7%, 30-year loan, an extra $500/month saves roughly $230,000 in interest and pays the loan off about 10¾ years early.
Is it better to pay extra monthly or make one lump sum?
Earlier is better — a lump sum today beats the same dollars spread over months, because it stops interest sooner. But the best schedule is the one you'll actually stick with; consistent monthly extras beat a planned lump sum that never happens.
Do extra payments go to principal automatically?
Not always. Many servicers apply overpayments to future payments or escrow unless you specify principal-only. Tell your servicer in writing that extra amounts should reduce principal, and check a statement to confirm.
Should I pay down my mortgage or invest instead?
Compare your after-tax mortgage rate to your expected after-tax investment return, and weigh the guarantee: extra mortgage payments certainly save interest at your rate, while investing offers higher expected but uncertain returns. Liquidity matters too — home equity is harder to access than a brokerage account.
Are there prepayment penalties?
Most US conventional mortgages have no prepayment penalty, but some loans do — especially certain non-QM or older products. Check your note or ask your servicer before sending large extra payments.
How much interest do extra mortgage payments save?
It depends on the loan size, rate, and extra amount — an extra $200 a month on a $400K loan at 6.5% saves about $112,000 in interest and cuts about 5½ years. Enter your numbers above for the exact figure.
What next?
- Could a lower rate get you there faster? — run the refi breakeven.
- Sitting on equity? — know what your stake is actually worth.
Related guides
- How Long to Save a Down Payment? — how long 3%, 10%, and 20% down actually take at different monthly savings rates, with price growth modeled.
- Extra Mortgage Payments: The Real Math — what extra principal really saves — monthly vs. lump vs. biweekly.
- Pay Off the Mortgage or Invest? — the break-even math: extra principal earns your rate for certain; investing must beat it after tax.
- 15-Year vs. 30-Year Mortgage — the full tradeoff, with the invest-the-difference case.
- Refinancing From 7% to 6% — what a one-point rate drop actually saves, with breakeven math.
- When to Refinance Your Mortgage — break-even math and rate-drop rules of thumb.
- How Bond Yields Affect You — why mortgage rates move the way they do.
- Biweekly Mortgage Payments — $1,297.20 every two weeks saves $126,619 on a $400K loan.
- Mortgage Recast Explained — extra payments cut the payoff date; a recast is what cuts the required payment itself.
- Are Mortgage Discount Points Worth It? — the 60.5-month breakeven, worked out.
- Debt-to-Income Ratio (DTI) Explained — front-end vs back-end DTI and the real lender limits behind your approval.
- How Much Is PMI? — $150/month on a $400K home, and when it drops off.
- HELOC vs Home Equity Loan — drawing on the equity your extra payments build, as a line or a lump sum.
- How Much House Can You Afford on $100K? — the affordability math behind the loan size this calculator pays off.
Last updated: September 28, 2026