Home Loan Extra Payment Calculator: See the Payoff Impact

A home loan extra payment calculator estimates how additional principal payments may shorten a mortgage and reduce interest. Enter the loan balance, annual interest rate, remaining term, and extra amount you plan to pay each month; it returns an estimated payoff time and interest savings compared with scheduled payments.

Formula

The calculator first finds the scheduled monthly principal-and-interest payment, then treats the extra amount as part of each monthly payment. It estimates the number of payments needed to clear the balance and compares interest paid under the two schedules.

• Scheduled payment: M = P × r / [1 − (1 + r)⁻ⁿ].
• Monthly rate: r = APR / 1,200, where APR is entered as a percentage per year.
• Scheduled payment count: n = remaining years × 12.
• Payment with extra: A = M + E, where E is the extra monthly payment.
• Estimated payoff months: k = ceil[−ln(1 − rP/A) / ln(1 + r)].
• Interest saved: (A × k − P) − (M × n − P).

P is the remaining principal in dollars, M and A are monthly payments in dollars, E is the extra monthly amount in dollars, r is the monthly rate as a decimal, and n and k are months. The calculator rounds the payoff period up to a whole payment month and compares totals using the unrounded formula payment. Actual statements may use cent-rounded payments and a different final payment.

Worked Example

Using the defaults, P = $300,000, APR = 6%, remaining term = 25 years, and E = $200 per month:

1. Convert the rate: r = 6 / 1,200 = 0.005 per month. The scheduled term is n = 25 × 12 = 300 months.
2. Calculate the scheduled payment: M = 300,000 × 0.005 / [1 − (1.005)⁻³⁰⁰] ≈ $1,933.28 per month.
3. Add the extra: A ≈ $1,933.28 + $200 = $2,133.28 per month.
4. Calculate payoff time: k = ceil[−ln(1 − 0.005 × 300,000 / 2,133.28) / ln(1.005)] = ceil(about 223.8) = 224 months.
5. Compare interest: with the extra, approximately $2,133.28 × 224 − $300,000 = $177,454.72. On the scheduled schedule, approximately $1,933.28 × 300 − $300,000 = $279,984.00. Estimated savings are about $102,529.28.

The displayed outputs are 224 months and $102,529.28. Because the formula uses the scheduled payment without cent-rounding internally, hand calculations using a rounded payment can differ slightly; real payment schedules also adjust the last installment.

Assumptions

• The interest rate remains fixed for the modeled period.
• Payments are made monthly, with each extra amount applied alongside the scheduled payment.
• Interest is compounded monthly at APR divided by 12.
• Fees, taxes, insurance, PMI, escrow, late charges, and payment-processing rules are not included.

How to Use the Estimate

Use the current unpaid principal balance, not the home’s value or the original amount borrowed. Enter the remaining term rather than the original mortgage term. Use the rate that applies to the balance you are modeling, and enter only the recurring extra amount in addition to the scheduled principal-and-interest payment.

The first result is an estimated number of monthly payments until payoff. The second is a modeled interest difference against making only scheduled payments for the full remaining term. It is not a promise of savings: the actual amount depends on when payments post, how the servicer applies extra funds, and whether the loan terms or rate change.

An extra payment generally needs to reduce principal for this calculation to represent the intended effect. Check the loan statement or ask the servicer how to designate additional funds. An amount applied to a future installment, fees, or another balance will not reduce principal in the same way. Also verify whether the loan has a prepayment charge or other relevant condition by checking the loan documents or contacting the lender; this calculator does not model those terms.

Common Errors and Limits

• Entering the wrong balance: use the remaining principal, not the original loan amount or total monthly housing cost.
• Mixing payment components: the model concerns principal and interest. It does not add property-related costs, insurance, or escrow to the payment.
• Confusing one-time and recurring extra payments: this calculator assumes the extra is paid every month. A lump-sum payment needs a separate schedule calculation.
• Assuming exact timing: payment posting dates, daily-interest methods, rounding, skipped or missed payments, and rate changes can affect real payoff figures.

The formula assumes a positive monthly rate and a level scheduled payment. It does not account for an adjustable rate, changing extra amounts, refinancing, a payment holiday, or a final partial payment. If an extra payment is made only occasionally, the recurring-payment result will not match that pattern.

Other Ways to Compare Scenarios

Run separate estimates with different monthly extra amounts to see how the modeled payoff period and interest difference change. Keep the balance, rate, and term the same between runs so the comparison isolates the extra payment. You can also compare a planned extra amount with zero extra to understand the baseline assumptions.

For a one-time principal payment, request an updated amortization schedule or use a calculator designed for lump-sum extra payments, entering the payment date and amount. For a changing-rate loan, model each rate period separately only if you have the applicable terms; a single fixed-rate calculation cannot forecast a future rate.

Frequently Asked Questions

Does an extra mortgage payment reduce principal?
It can if the lender applies it to principal. Payment instructions and servicing procedures vary, so confirm how to make and designate an additional principal payment.

Does paying extra every month reduce total interest?
In this fixed-rate model, paying principal faster reduces the balance on which later interest is calculated, so modeled interest is lower. The amount in practice depends on payment application, timing, and loan terms.

Should I enter my original loan amount or current balance?
Enter the current remaining principal balance. The original amount would overstate the balance if you have already made payments.

Can I use this calculator for a one-time extra payment?
No. Its extra-payment input represents the same additional amount every month. A lump sum requires a model that includes its date and one-time amount.

Why does my lender’s payoff estimate differ?
The lender may account for payment dates, accrued interest, fees, rate changes, rounding, escrow, or other loan-specific details that this simplified calculation excludes.

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

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in bal | Remaining loan 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 monthly payment | $ | 200 | 0 | 100000000
out months | Months to repay with extra | months | ceil(round(-ln(1 - (apr / 1200) * bal / (bal * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)) + extra)) / ln(1 + apr / 1200), 6)) | 0
out saved | Estimated interest saved | $ | (bal * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)) + extra) * ceil(round(-ln(1 - (apr / 1200) * bal / (bal * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12)) + extra)) / ln(1 + apr / 1200), 6)) - bal - ((bal * (apr / 1200) / (1 - (1 + apr / 1200) ^ -(yrs * 12))) * yrs * 12 - bal) | 2