Home Loan Yearly Prepayment Calculator

A home loan yearly prepayment calculator estimates how an extra payment made once each year changes the loan’s payoff time and total interest. Enter the loan balance, annual percentage rate, remaining term, and planned yearly prepayment to estimate the revised payoff and interest saved.

Formula

This estimate models the yearly prepayment as an equivalent monthly extra amount: yearly prepayment divided by 12. It assumes that the borrower makes this extra amount along with each scheduled monthly payment. The monthly rate is the annual percentage rate divided by 1,200, and the scheduled payment is calculated using the standard fixed-payment loan formula.

• r = monthly interest rate, calculated as APR ÷ 1,200.
• n = original remaining term in months, equal to years × 12.
• p = starting loan balance in dollars.
• m = scheduled monthly payment, rounded to cents, plus yearly prepayment ÷ 12.
• k = estimated months to payoff, rounded up to a whole month: ceil(−ln(1 − rp/m) ÷ ln(1 + r)).
• Interest without extras = scheduled monthly payment × n − p.
• Interest with extras = scheduled monthly payment × k − p.
• Interest saved = interest without extras − interest with extras.

Using an equivalent monthly extra is a simplifying convention, not a literal simulation of a single payment posted once a year. It allows a compact estimate using the standard formula for a constant monthly payment. The result therefore should not be interpreted as the exact effect of the timing or posting of a real annual lump sum.

Worked Example

With the defaults, the balance is $200,000, APR is 6%, the remaining term is 20 years, and the yearly prepayment is $5,000. The APR as a monthly decimal rate is 0.06 ÷ 12 = 0.005. The scheduled term is 20 × 12 = 240 months.

1. Calculate the scheduled payment: $200,000 × 0.005 ÷ (1 − 1.005⁻²⁴⁰) = approximately $1,432.86 per month, rounded to cents.
2. Convert the yearly prepayment to a monthly equivalent: $5,000 ÷ 12 = $416.6667. The modeled payment including the extra is therefore about $1,849.5267 per month.
3. Apply the payoff formula: ceil(−ln(1 − 0.005 × $200,000 ÷ $1,849.5267) ÷ ln(1.005)) = 171 months.
4. Estimated interest without extra payments: $1,432.86 × 240 − $200,000 = $143,886.40.
5. Estimated interest with extra payments: $1,432.86 × 171 − $200,000 = $45,019.06. Estimated interest saved: $143,886.40 − $45,019.06 = $98,867.34.

The 171-month result is an estimated payoff duration in this equivalent-monthly model. The displayed interest calculation uses the scheduled payment rounded to cents, while the payoff-duration calculation uses the payment formula before that display rounding. This distinction can create small differences from a full month-by-month lender schedule.

Assumptions

• The APR and monthly payment remain fixed throughout the modeled period.
• Payments occur monthly; the extra is represented as one-twelfth of the yearly prepayment each month.
• Interest accrues monthly at APR ÷ 12.
• The estimate excludes fees, taxes, insurance, mortgage insurance, escrow, and lender-specific payment allocation or prepayment rules.

How to Use and Read the Estimate

Use the current principal balance, not the property’s value or the original amount borrowed. Enter the remaining loan term rather than the term at origination. For the interest rate, use the loan’s stated APR as a percentage: enter 6 for 6%, not 0.06. Enter the amount you expect to prepay over a year in dollars. A zero extra payment provides a baseline estimate, although rounding may affect displayed totals.

• Estimated months to payoff is the modeled time to clear the balance, rounded up to a whole month.
• Estimated interest saved compares modeled interest with the extra payment against interest over the original scheduled term.
• Scheduled monthly payment is the fixed principal-and-interest payment calculated from the balance, APR, and term.
• Estimated interest with prepayments is based on the scheduled payment and estimated payoff months.

Try a few extra-payment amounts to see how the estimate changes, but treat those outputs as illustrations rather than promises. If the loan has a variable rate, the future payment or interest rate can change, so a fixed-rate calculation cannot predict the actual payoff schedule.

Yearly Lump Sum Versus Monthly Extra Payments

A yearly prepayment is a lump sum made once during the year. Paying the same total in monthly installments is not mathematically identical: each installment reaches principal at a different time, and earlier principal reduction generally changes subsequent interest calculations. This calculator deliberately treats the yearly amount as one-twelfth added to every monthly payment. It is useful as a simplified comparison, but it does not model the exact date or size of a yearly lump sum.

For a closer estimate of a real annual payment, a month-by-month amortization schedule needs to apply each payment on its actual date, recalculate interest and principal, and account for the lender’s posting rules. Ask the loan servicer how extra payments are applied and whether instructions are needed to direct them to principal. Do not assume that an extra amount automatically changes the required monthly payment; the contractual payment and the payoff schedule are separate concepts.

Common Limits and Input Checks

The formula assumes that the combined modeled payment is greater than the monthly interest due. If a payment does not cover accrued interest, this payoff formula cannot produce a finite payoff time. The inputs here use a positive APR and nonnegative prepayment, and the term is limited to a realistic calculation range. Results may differ from a lender’s figures because of daily interest accrual, payment dates, rounding conventions, rate changes, fees, or rules about principal-only payments.

Interest saved is an arithmetic comparison, not a forecast of the complete financial consequences of paying early. It does not account for alternative uses of money, changes in expenses, or tax treatment. Use the actual current balance, rate, payment, and remaining term from loan documents or the servicer’s records; if they differ from the calculator’s assumptions, the estimate will differ too.

Frequently Asked Questions

How does a yearly home loan prepayment affect interest?
In a standard amortizing loan, interest is calculated on the outstanding balance. Reducing principal can reduce future interest, but the exact amount depends on when the lender receives and applies the extra payment and on the loan’s terms.

Should I enter the original loan amount or today’s balance?
Enter the current principal balance for an estimate starting now. The original loan amount does not reflect principal already repaid.

Does this calculator model one lump-sum payment each year?
No. It converts the yearly amount into an equivalent monthly extra payment. A literal annual-lump-sum schedule can produce a different result because the timing differs.

Why might the result differ from my lender’s payoff figure?
A lender’s calculation may use exact payment dates, daily interest, updated balances, payment posting rules, fees, or a changing rate. This calculator uses a simplified fixed-rate monthly model.

Does an extra payment lower my required monthly payment?
Not necessarily. The calculator estimates a shorter payoff timeline under its assumptions; it does not determine whether a lender will recast or otherwise change the required payment.

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

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in p | Loan balance | $ | 200000 | 1 | 100000000
in apr | Interest rate (APR) | % | 6 | 0.01 | 100
in yrs | Remaining loan term | years | 20 | 0.0833 | 50
in extra | Yearly prepayment | $ | 5000 | 0 | 100000000
out months | Estimated months to payoff | months | ceil(round(-ln(1 - (apr / 1200) * p / (p * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)) + extra / 12)) / ln(1 + apr / 1200), 6)) | 0
out saved | Estimated interest saved | $ | (p * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)) * yrs * 12 - p) - (p * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)) * months - p) | 2
out pmt | Scheduled monthly payment | $ | round(p * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)), 2) | 2
out int | Estimated interest with prepayments | $ | p * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)) * months - p | 2