Mortgage Extra Payment Calculator

How to use this calculator

Enter the current principal balance, note rate, and remaining term from the latest statement. Add any recurring extra principal and an optional lump sum, then choose the month when the lump sum would be applied. Enter zero for either extra-payment field if it does not apply.

When this calculator is useful

• Testing a recurring extra-principal amount before changing autopay
• Comparing a lump sum now with the same payment later
• Estimating payoff time and interest savings
• Checking whether a planned prepayment still leaves enough cash for other priorities

Important limitations

The model assumes a fixed rate, monthly amortization, no missed payments, and no loan recast. It does not include escrow, fees, tax effects, investment returns, or a prepayment penalty. Confirm the servicer’s application rules and your note before sending extra money.

For best results, run at least three cases: a conservative case, a middle case, and a stretch case. If a small change in inputs creates a big change in the answer, that is a sign the decision may be sensitive to rates, fees, timing, or market performance.

What this extra-payment calculator measures

The calculator builds two monthly amortization paths from the balance, note rate, and remaining term you enter. The baseline makes only the scheduled principal-and-interest payment. The accelerated path adds the recurring extra principal and applies the one-time payment after the scheduled payment in the selected month.

The scheduled payment is not increased permanently by the one-time amount. The model keeps the normal payment constant afterward, which is why the loan ends sooner instead of producing a lower required payment. A lender-approved recast is a different transaction.

Worked example: $250 monthly plus a $5,000 lump sum

Consider a hypothetical $325,000 remaining balance at 6.5% with 27 years left. The estimated scheduled principal-and-interest payment is $2,130.55. With no prepayment, the remaining interest is about $365,299 over 324 payments.

Add $250 of principal every month and a $5,000 one-time principal payment after month 12. Under this simplified schedule, payoff takes about 242 months, or 20 years and 2 months. That is 82 payments sooner, with about $110,202 less interest than the baseline.

Those figures are outputs from the stated assumptions, not a quote or promise. A different posting date, daily-interest method, late payment, rate adjustment, fee, recast, or servicer rule can change the actual schedule.

Why timing changes the result

A principal payment made earlier removes balance before more monthly interest accrues. The same $5,000 generally saves more interest in month 12 than in month 120. That does not automatically make the earlier payment the better household decision. Cash used for the mortgage is harder to retrieve and may be needed for repairs, income loss, higher-rate debt, or other goals.

Test timing directly by changing the one-time-payment month. If the chosen month falls after the accelerated loan is already paid off, the calculator ignores the lump sum because there is no remaining balance to reduce.

Use the current principal balance and note rate

The original loan amount will overstate what remains. Copy the unpaid principal balance from a recent statement. Use the mortgage note rate rather than today’s advertised rate or the loan’s APR. APR can include certain costs and is not the monthly interest rate used by this simplified amortization model.

Enter the remaining term, not the original term. A 30-year mortgage that has already received three years of payments has roughly 27 years left unless prior prepayments, a modification, or another change altered the schedule.

Tell the servicer to apply extra money to principal

The Consumer Financial Protection Bureau advises borrowers to check whether the loan permits extra payments and to make sure the money is applied to principal. A partial payment can be handled differently from a full scheduled payment. Review the next statement to confirm that the unpaid principal balance fell by the expected amount.

Check the note for a prepayment penalty. CFPB says these penalties usually apply to a full payoff within a specified early period, and in some cases to a large lump sum, rather than to small recurring principal payments. The contract controls the actual loan.

Interest savings are not cash in hand

The interest-saved result is interest that would otherwise accrue over the remaining life of the modeled loan. It is not a rebate deposited today. The saving arrives gradually because future interest charges never occur.

The model also excludes tax effects. IRS Publication 936 explains that deductible home mortgage interest depends on the debt, property, date, use of proceeds, and whether the taxpayer itemizes. Paying less interest can reduce a deduction for a household that qualifies, but spending a dollar of interest to obtain a partial deduction does not make the interest free.

Compare prepayment with the rest of the plan

• Keep enough liquid savings for irregular costs and income interruptions.
• Compare the mortgage rate with higher-rate debt before choosing which balance to attack.
• Account for any employer retirement match that would be lost by redirecting contributions.
• Do not compare certain mortgage interest savings with an assumed investment return as though both were guaranteed.
• Ask whether a recast, refinance, or ordinary prepayment matches the goal. They change different parts of the loan.

Use the emergency fund calculator to test the cash reserve and the debt payoff calculator for higher-rate balances. The mortgage payment calculator estimates the full payment when taxes, insurance, PMI, and HOA dues also matter.

Sources and method notes

• Consumer Financial Protection Bureau: mortgage terms — amortization, biweekly payment plans, and loan terminology.
• Consumer Financial Protection Bureau: mortgage-servicer rights — payment application, statements, and extra-principal guidance.
• Consumer Financial Protection Bureau: prepayment penalties — when a penalty may apply and why borrowers should check the loan.
• Internal Revenue Service: Publication 936 — federal home mortgage interest deduction rules.

Method reviewed September 16, 2026. The calculator runs in the browser and does not store loan inputs.

Educational only. This calculator provides general information, not personalized financial, tax, legal, lending, or investment advice.

Frequently asked questions

How does an extra mortgage payment save interest?

Interest is calculated from the outstanding principal. An extra principal payment lowers that balance sooner, so less interest accrues in later months under a standard fixed-rate amortization schedule.

Should I enter the original mortgage amount?

No. Enter the current unpaid principal balance, the note rate, and the remaining term from a recent mortgage statement.

Can I compare monthly and one-time extra payments?

Yes. Enter either or both. Set an unused extra-payment field to zero. The one-time payment is applied after the scheduled payment in the month you choose.

Will extra principal lower my required monthly payment?

Usually not unless the loan is formally recast. This calculator keeps the scheduled principal-and-interest payment unchanged and uses prepayment to shorten the payoff period. Ask the servicer whether recasting is available and what it costs.

Can a mortgage have a prepayment penalty?

Some loans can. CFPB says small extra-principal payments do not normally trigger a prepayment penalty, but a large payoff can in some cases. Check the note and ask the lender or servicer.