Extra House Payment Calculator: See Interest Savings

An extra house payment calculator estimates how additional principal payments change a mortgage’s payoff time and total interest. Enter your current balance, annual interest rate, remaining term, and extra amount per month; the calculator below shows the resulting monthly payment amount including the extra, plus estimated interest over the payoff period.

Formula

The calculator first finds the scheduled monthly principal-and-interest payment for a fixed-rate loan, adds the extra principal amount, then estimates how many monthly payments at that combined amount are needed to repay the starting balance.

• Scheduled payment: P × r / [1 − (1 + r)⁻ⁿ]
• Payment with extra: scheduled payment + extra monthly principal
• Months to repay: ceil[−ln(1 − rP/M) / ln(1 + r)]
• Estimated interest: (payment with extra × estimated months) − P

Here, P is the current mortgage balance in dollars, r is the monthly interest rate (APR ÷ 1,200), n is the remaining scheduled term in months (years × 12), and M is the payment with extra in dollars. The payoff-time formula assumes that the same payment is made each month. The calculator rounds the payment with extra to cents and rounds estimated payoff time up to a whole month.

Worked Example

Using the defaults above: a $300,000 balance, 6% APR, 25 years remaining, and $200 in extra principal per month:

1. Convert the annual rate to a monthly rate: 0.06 ÷ 12 = 0.005.
2. Convert the term to months: 25 × 12 = 300.
3. Calculate the scheduled payment: $300,000 × 0.005 ÷ [1 − (1.005)⁻³⁰⁰] = about $1,933.28.
4. Add the monthly extra: $1,933.28 + $200 = $2,133.28.
5. Apply the payoff formula: −ln[1 − (0.005 × $300,000 ÷ $2,133.28)] ÷ ln(1.005) = about 215.8 months; rounded up, that is 216 months.
6. Estimate interest using the rounded payment and whole-month estimate: ($2,133.28 × 216) − $300,000 = $160,788.48.

Assumptions

• The interest rate remains fixed for the calculation period.
• Payments are made monthly at the end of each period; extra principal is included with each payment.
• Interest compounds monthly at APR ÷ 12.
• Fees, taxes, homeowners insurance, mortgage insurance, escrow, and payment-processing timing are not included.
• The final payment may be smaller than the regular amount. The displayed interest is an estimate because the calculator uses a whole-month payoff estimate and a constant payment.

How to Use an Extra House Payment Calculator

Use the current principal balance, rather than the home’s purchase price or original loan amount. A recent mortgage statement is one place to find the balance and interest rate; check the lender’s figures if your statement reports a different rate or balance basis. Enter the remaining time on the loan, not necessarily the original term.

Enter only the additional amount intended for principal. The required scheduled mortgage payment generally includes more than principal and interest when taxes, insurance, or other items are collected with it. Those charges do not pay down principal, so this calculator does not add them to its payment result. It also does not model a one-time lump-sum payment; to estimate a recurring extra, enter the amount paid each month.

How to Read the Results

“Monthly payment with extra” is the estimated principal-and-interest payment used in the calculation: the scheduled principal-and-interest amount plus the extra principal entered. It is not the full amount due on a mortgage statement. “Estimated payoff time” counts months from the current balance, while “estimated interest paid” covers the modelled payoff period—not interest already paid before the starting balance date.

To judge the effect of an extra-payment scenario, run the calculation once with $0 extra and again with the proposed monthly amount. Compare estimated months and interest, keeping the balance, APR, and remaining term the same. The difference is an arithmetic comparison, not a prediction of a lender’s exact payoff quote.

Common Errors and Limitations

• Using the wrong balance: the calculator starts from the outstanding principal now, not the original amount borrowed.
• Mixing annual and monthly rates: enter APR as a percentage per year, such as 6, not 0.06. The formula converts it to a monthly decimal rate.
• Confusing the extra with the total: enter $200 for an extra $200, not the scheduled payment plus $200.
• Assuming all payment components reduce principal: escrowed taxes and insurance are not principal payments and are excluded.
• Ignoring changing terms: adjustable rates, skipped or late payments, fees, payment changes, and one-off principal payments are not represented.

The result is not usable if the payment does not cover monthly interest. In that case, the balance would not amortize under the formula. Real loan contracts may also specify how additional funds are applied, so confirm with the servicer that an intended additional amount is credited to principal and check how it should be submitted.

Frequently Asked Questions

Does an extra mortgage payment go directly to principal?
Not necessarily. The calculator treats the entered extra as principal, but actual handling depends on the loan’s payment instructions and servicer processing. Check how to designate additional principal with the servicer.

Does this calculator include property taxes and homeowners insurance?
No. It estimates principal and interest only. Taxes, insurance, mortgage insurance, and escrow can affect the amount billed but do not enter this payoff calculation.

Can I enter a one-time extra house payment?
This version models a recurring monthly extra. A lump-sum payment changes the balance at a specific date, so it needs a calculator that accounts for the payment’s timing.

Why does the calculator show a different payoff date from my lender?
The lender may use a precise daily interest calculation, actual payment dates, rounding rules, fees, or a different balance. This calculator uses monthly compounding and a simplified, level-payment estimate.

What if I can only pay extra in some months?
A fixed monthly-extra calculator cannot represent an irregular schedule. Use a month-by-month amortization calculation with each payment and its date for a closer estimate.

This result is an estimate; the lender’s disclosure gives the exact figures.

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in bal | Mortgage balance | $ | 300000 | 1 | 100000000
in apr | Interest rate (APR) | % | 6 | 0.01 | 100
in yrs | Remaining term | years | 25 | 0.0833 | 50
in extra | Extra principal per month | $ | 200 | 0 | 100000000
out pmt | Monthly payment with extra | $ | round((bal * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)) + extra), 2) | 2
out months | Estimated payoff time | months | ceil(round(-ln(1 - (apr / 1200) * bal / (pmt)) / ln(1 + apr / 1200), 6)) | 0
out int | Estimated interest paid | $ | pmt * months - bal | 2