Extra Payment Mortgage Payoff Calculator

How extra payments shorten a mortgage

Every regular mortgage payment is split into two parts: interest, which is charged on whatever balance you still owe, and principal, which actually reduces the debt. In the early years of a 30-year loan, most of each payment goes to interest. On a $300,000 loan at 6.5%, the first payment of $1,896.20 includes $1,625.00 of interest and only $271.20 of principal. That ratio is the reason extra payments are so powerful: any dollar you add on top of the scheduled payment goes 100% to principal, which lowers the balance that next month's interest is calculated on. The savings then compound for the rest of the loan.

This calculator models exactly that process month by month. It computes your regular payment from the loan amount, rate and term, then applies any extra money you specify in the month you specify it, recalculating interest on the reduced balance each period until the loan reaches zero.

The formula behind the calculator

Your scheduled monthly payment M on a fixed-rate loan comes from the standard amortization formula:

where P is the principal, r is the monthly interest rate (annual rate ÷ 12) and n is the number of monthly payments. For $300,000 at 6.5% over 30 years, r = 0.065 / 12 = 0.0054167 and n = 360, which gives M = $1,896.20.

Each month the calculator then runs these steps:

Interest is rounded to the cent each period, the way loan servicers do it, and the final payment is reduced so you never overpay. Note that your required payment stays the same when you prepay: lenders do not lower your monthly bill after extra principal payments (unless you formally recast the loan). Instead, the loan simply ends earlier.

Worked examples: $300,000 at 6.5% for 30 years

Without any extra payments, this loan costs about $382,637 in interest over 360 months. Here is how three common strategies change that:

• $200 extra every month: the loan is paid off in 277 months — about 23 years and 1 month instead of 30 years — and total interest falls to roughly $279,187. That is a saving of about $103,000 for $200 a month.
• $2,400 once a year (the same annual amount, paid as a single December payment): payoff in about 279 months with roughly $99,700 saved. Slightly less than monthly, because the money sits idle in your account for up to eleven months before it reduces the balance.
• A single $10,000 lump sum after year one: payoff in about 329 months (2 years 7 months early) and around $50,000 of interest saved — five dollars saved for every dollar prepaid, because the money works for almost three decades.

The pattern is consistent: earlier and more frequent extra payments save the most, because each dollar of principal removed early avoids interest for every remaining month.

Monthly, yearly or lump sum: which is best?

Mathematically, a dollar paid sooner always beats a dollar paid later. But the best option is the one you will actually keep doing. A small automatic monthly extra is easy to sustain and lands in your budget like any other bill. A yearly payment works well if your income is lumpy — for example a tax refund or annual bonus. A lump sum fits windfalls such as an inheritance or the proceeds of selling another asset. You can combine all three in the calculator to model your real plan.

Should you pay extra on your mortgage at all?

Prepaying a mortgage is effectively a guaranteed, risk-free return equal to your interest rate. At 6.5%, every extra dollar "earns" 6.5% by avoiding interest. Whether that beats the alternatives depends on your situation:

• Emergency fund first. Money sent to principal is hard to get back out. Most planners suggest keeping three to six months of expenses in cash before prepaying.
• High-interest debt first. Credit cards at 20%+ should be cleared before a 6.5% mortgage.
• Employer retirement match. A 401(k) match is an instant 50–100% return, which nothing beats.
• Low-rate loans. If your mortgage rate is 3%, investing may out-earn prepaying over long periods, though with more risk.
• Tax considerations. If you itemize, mortgage interest may be deductible, which slightly reduces the effective benefit of prepaying. Most households now take the standard deduction, so this matters less than it used to.

How to make sure extra payments count

When you send more than your scheduled payment, tell your servicer to apply the extra to principal only. Otherwise some servicers hold it as an advance on next month's payment, which saves you nothing. Most online portals have a dedicated "additional principal" field. Check your next statement to confirm the balance dropped by the full extra amount. Also review your loan documents for a prepayment penalty; they are rare on modern conforming US mortgages but do exist on some non-QM and older loans.

Limitations of this estimate

The calculator assumes a fixed interest rate and that each payment is credited on its due date. It does not include property taxes, homeowners insurance, PMI or HOA dues, because those escrow items do not affect how fast principal is repaid. If your loan is an adjustable-rate mortgage, results are only accurate until the first rate adjustment. Small differences of a few dollars versus your servicer's statement are normal due to day-count conventions and payment timing.

Frequently asked questions

Sources

1. Consumer Financial Protection Bureau: Mortgages
2. Consumer Financial Protection Bureau: What is a prepayment penalty?
3. Freddie Mac: Primary Mortgage Market Survey (PMMS), weekly average mortgage rates

Last reviewed: Oct 17, 2026. See how we calculate and test. Calculations use the standard fixed-rate amortization formula with interest rounded to the cent each period.