Paying One Extra Mortgage Payment a Year: Calculator

This calculator estimates how much interest a fixed-rate loan would cost if you made one extra payment equal to your regular monthly payment each year, compared with making only the scheduled payments. Enter the loan balance, APR, and term to see the monthly payment and modeled total interest; the extra-payment effect is estimated by comparing payoff schedules.

Formula

The calculator uses the standard fixed-payment loan formula. Let P be the starting principal in dollars, a the annual percentage rate (APR) in percent, r = a / 1200 the monthly rate as a decimal, and n = 12 × years the number of monthly payments. The monthly payment is M = P × r / [1 − (1 + r)⁻ⁿ]. Total scheduled interest is I = M × n − P. The displayed payment and interest are rounded to cents for display; the total-interest formula uses the unrounded payment result.

This page’s required calculator block computes the ordinary scheduled payment and interest; it does not simulate an annual extra payment. The title’s subject needs an explicit extra-payment schedule and comparison, so its results should not be mistaken for the savings or payoff date from making that extra payment. A true comparison requires month-by-month amortization, applying each additional amount to principal and recalculating the remaining balance and interest. The supporting example below explains that method and illustrates why a plain level-payment formula cannot give the extra-payment result.

Worked Example

Using the exact defaults: P = $20,000, a = 8%, and n = 5 × 12 = 60 months. The monthly rate is r = 8 / 1200 = 0.0066667. Substitute these values into the payment formula:

M = 20,000 × 0.0066667 / [1 − (1.0066667)⁻⁶⁰] ≈ $405.53.

Total scheduled payments are approximately $405.53 × 60 = $24,331.80, so the displayed formula’s interest result is approximately $4,331.80 when using the rounded payment. The calculator’s expression specifically uses the unrounded payment internally, so its output may differ by a few cents from multiplying the displayed monthly payment by 60.

For an annual-extra-payment illustration, the extra amount in this example would be one regular payment, about $405.53, applied to principal once each year. To calculate the actual savings, a schedule must apply the normal monthly payment, accrue monthly interest, then apply the extra payment at the selected point in the year. Because timing and lender servicing affect the result, this ordinary-payment calculator does not produce an extra-payment savings figure.

How One Extra Payment Changes a Loan

On a typical amortizing loan, each scheduled payment first covers interest accrued since the prior payment; the remainder reduces principal. When a separate additional payment is credited directly to principal, the balance falls faster. Later interest is then calculated on a smaller balance, which can shorten the payoff period and reduce total interest.

“One extra payment a year” often means paying an additional amount equal to the monthly payment, either as a lump sum or by splitting it across the year. Those approaches are not always identical. Splitting an extra payment into twelve installments can reduce principal earlier if the servicer applies each amount promptly. A lump sum applied near year-end generally affects the balance later. The payment amount, timing, rate, and balance all affect the outcome.

How to Estimate the Extra-Payment Effect

A proper estimate uses an amortization schedule rather than simply subtracting one payment from the term. For each month, calculate interest on the current balance, subtract the scheduled payment, and apply any extra principal. Stop when the remaining balance reaches zero; the final payment may be smaller than a full monthly payment.

1. Find the current principal balance, APR, required monthly payment, and remaining term from the loan statement or disclosure.
2. Choose the additional amount and when it will be paid. State clearly whether it is one full monthly payment annually or smaller installments.
3. For every month, calculate interest as the opening balance multiplied by the monthly rate, then apply the regular payment and any extra principal.
4. Compare total interest and payoff timing with a baseline schedule that has no extra payments.

If the loan has already been in repayment, using its original loan amount and original term will not represent the current situation. Start with the current balance and remaining term instead. A calculator that only accepts original amount and term cannot reflect past payments.

What to Check Before Making an Extra Payment

Confirm how the servicer handles additional money. It may be necessary to identify the amount as principal-only rather than letting it count as an advance against a future installment. Check the account’s payment instructions and any contract terms for applicable prepayment charges or restrictions; this article does not determine whether any apply. Also verify that the regular payment remains due when an extra amount is submitted.

• Payment allocation: An extra amount must reduce principal for the expected interest effect. If it is held or treated as a future scheduled payment, the result can differ.
• Loan structure: Variable rates, interest-only periods, balloon balances, and irregular payments do not follow the simple fixed-rate model.
• Other charges: Fees, taxes, insurance, escrow, and account-specific charges are not part of the principal-and-interest calculation.
• Rounding and timing: A lender may accrue interest daily or use different rounding and posting conventions. A month-by-month estimate is not an official payoff quote.

Common Misconceptions and Limits

An extra payment does not automatically lower the required monthly payment. In many standard amortizing loans, it reduces the balance and can shorten the time to payoff, while the scheduled installment remains unchanged unless the loan is formally recast or otherwise adjusted. The availability and conditions of any such process depend on the loan agreement and are not modeled here.

Another common mistake is assuming that “one extra payment” always means exactly one-twelfth more each month. That payment pattern may total a full extra installment over a year, but its timing differs from a single annual lump sum. It also matters whether the stated monthly payment includes only principal and interest or includes other items. Use the principal-and-interest portion when comparing amortization schedules; a total housing payment may include amounts that do not reduce loan principal.

Frequently Asked Questions

Does paying one extra mortgage payment a year reduce interest?
It can, if the extra amount is applied to principal. A lower principal balance generally means less interest accrues afterward, but the exact reduction depends on the rate, balance, payment timing, and loan terms.

Does one extra payment a year pay off a mortgage years early?
It may shorten the repayment period, but there is no fixed number of years saved. The result depends on the remaining balance and term, APR, and how and when the extra payment is credited.

Is it better to pay monthly or make one annual extra payment?
This calculator does not decide which approach is preferable. For the same total extra amount, payments made earlier can reduce principal sooner, but the actual impact depends on posting rules and the loan’s interest method.

Will an extra payment lower my monthly mortgage bill?
Usually, an extra principal payment changes the balance and potential payoff date rather than automatically changing the required monthly installment. Check the loan terms for how payment changes are handled.

Can I use the calculator for an existing mortgage?
The input structure shown here uses a loan amount and original term. For an existing mortgage, an accurate estimate needs the current principal balance and remaining term, plus the current payment and extra-payment schedule.

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

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in p | Loan amount | $ | 20000 | 1 | 100000000
in apr | Interest rate (APR) | % | 8 | 0.01 | 100
in yrs | Loan term | years | 5 | 0.0833 | 50
out pmt | Monthly payment | $ | p * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)) | 2
out int | Total interest | $ | pmt * yrs * 12 - p | 2