Home Loan Prepayment Calculator: Estimate Interest Savings

A home loan prepayment calculator estimates how extra principal payments may change your payoff time and total interest. Enter your loan balance, annual interest rate, remaining term, and planned extra monthly payment to see an estimated payoff period and interest savings.

Formula

This estimate compares the scheduled-payment loan with a loan receiving the same extra principal amount every month. It assumes monthly payments and a fixed rate. A payment is rounded to the nearest cent before totals are calculated.

• Monthly rate, r = APR ÷ 1,200, where APR is the annual percentage rate entered as a percentage.
• Number of scheduled payments, n = remaining years × 12.
• Scheduled monthly payment, basepmt = round(balance × r ÷ [1 − (1 + r)⁻ⁿ], 2).
• Monthly payment including the extra amount, newpmt = basepmt + extra monthly principal.
• Estimated months to payoff, newmonths = ceil(round[−ln(1 − r × balance ÷ newpmt) ÷ ln(1 + r), 6]).
• Interest for each case is total payments minus the starting balance. Estimated interest saved is scheduled-case interest minus extra-payment-case interest.

Balance and payment amounts are in dollars, APR is a percentage per year, and term is in years. The calculator’s extra-payment model treats the extra amount as part of the same fixed monthly payment through the final month. It does not model a separate, smaller final payment. This can make the payoff month and final interest an approximation.

Worked Example

Using the calculator’s defaults: balance $300,000, APR 6%, remaining term 25 years, and extra monthly principal of $200:

1. Monthly rate: 6 ÷ 1,200 = 0.005. Scheduled months: 25 × 12 = 300.
2. Scheduled payment: 300,000 × 0.005 ÷ [1 − (1.005)⁻³⁰⁰] = $1,932.90 after rounding.
3. Payment including the extra amount: $1,932.90 + $200 = $2,132.90.
4. Using the payoff formula gives 227.2 months before rounding up, or 228 monthly payments.
5. Scheduled-case interest: ($1,932.90 × 300) − $300,000 = $279,870.00.
6. Extra-payment-case interest: ($2,132.90 × 228) − $300,000 = $186,301.20.
7. Estimated interest saved: $279,870.00 − $186,301.20 = $93,568.80.

The headline result is an estimate, not a lender payoff quote. The model uses the same payment amount in each of the 228 months, including the last one; a real final payment may be lower because the remaining balance is smaller.

Assumptions

• The interest rate stays fixed for the entire remaining period.
• Payments are monthly, made at the end of each monthly period; the estimate uses monthly compounding at APR ÷ 12.
• The extra amount is applied to principal every month, without interruption.
• Fees, taxes, insurance, mortgage insurance, escrow, prepayment charges, and lender-specific payment handling are not included.

When to Use a Prepayment Calculator

Use this calculator to explore a regular extra-payment scenario, such as adding a fixed amount to each monthly payment. It helps show the mathematical relationship between a faster principal reduction, a shorter estimated payoff period, and reduced interest under the stated assumptions. It can also help you compare scenarios by changing only the extra amount while keeping the other inputs constant.

Enter the current principal balance—not the home’s purchase price or original loan amount—along with the rate and remaining term that apply now. If the loan has a variable rate, multiple rate periods, or a payment that changes over time, one fixed APR and term cannot represent every future payment. Use figures from your loan documents or current account information; obtain actual rates, charges, and payment-processing details from the lender.

How to Read the Results

“Months to payoff” is the number of monthly periods estimated by the formula, rounded up to a whole payment. Divide it by 12 for an approximate number of years and months. “Interest with extra payment” is the model’s total payment amount above principal. “Estimated interest saved” is a difference between two modelled interest totals; it is not a guaranteed saving or a statement about the exact amount due on a particular date.

For a quick illustration of the payment effect, a $200 extra amount added to a $1,932.90 scheduled payment produces a modelled payment of $2,132.90. That does not mean the lender automatically treats all additional money as principal. Payment instructions and account processing can affect how an extra amount is credited, so check the lender’s information before relying on the estimate.

Common Questions and Limitations

A common misconception is that every extra payment immediately reduces future interest by a fixed amount. The actual interest calculation depends on the loan’s balance, rate, payment dates, and servicing method. This calculator uses a simplified monthly schedule. It does not account for daily interest accrual, irregular payment dates, skipped payments, rate resets, or changes to the required payment.

• Use consistent units: enter APR as a percent, such as 6 for 6%, not 0.06. Enter the remaining term in years, not months.
• Check the balance: the current principal is the relevant starting figure; a statement’s total amount due can include other items.
• Understand rounding: payments are rounded to cents, while the calculated payoff period is rounded up to a whole month. Actual servicing can produce a different final payment.
• Confirm extra-payment handling: the estimate assumes extra money reduces principal. The lender’s instructions determine how payments are applied.

The model is for educational arithmetic. It does not account for personal circumstances or determine whether making extra payments is appropriate for a particular borrower.

Frequently Asked Questions

Does a home loan prepayment calculator show the exact interest I will save?
No. It estimates interest under the inputs and assumptions shown. Actual results depend on payment dates, rate changes, loan terms, rounding, and how the lender credits extra payments.

Should I enter my original loan amount or current balance?
Enter the current principal balance for an estimate from today forward. The original loan amount is not the same as the amount still owed after prior payments.

Does an extra payment reduce my monthly instalment?
Not necessarily. This calculator assumes the scheduled payment continues and extra principal is added to it. A lender’s treatment of additional payments may differ; check the loan terms and servicing instructions.

Can I use this for a variable-rate home loan?
You can make a rough estimate using a single assumed APR, but the calculation holds that rate constant. It will not forecast changes to a variable rate or the resulting payment schedule.

Why does the calculator round the payoff time up?
Payoff is counted in whole monthly payment periods, so a result of 227.2 months is shown as 228 months. The actual last payment may be smaller than the regular payment assumed by the simplified total.

This result is an estimate; 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 principal | $ | 200 | 0 | 100000000
out saved | Estimated interest saved | $ | (basepmt * yrs * 12 - bal) - (newpmt * newmonths - bal) | 2
out months | Months to payoff with extra payment | months | newmonths | 0
out interest | Interest with extra payment | $ | newpmt * newmonths - bal | 2