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Mortgage Prepayment Calculator

Estimate interest saved from recurring and lump-sum mortgage prepayments across monthly, bi-weekly, accelerated bi-weekly, weekly, and accelerated weekly schedules.

Published

Interest saved
Estimated interest saved
$51,378.19
Original interest
$115,838.19
Prepaid interest
$64,460.00
Original payoff
360 payments
New payoff
228 payments
Time saved
11 years
Total extra principal
$27,800.00

$599.55 monthly scheduled payment plus $100.00 extra and a $5,000.00 lump sum.

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Mortgage Prepayment Calculator

Mortgage prepayment is a broad acceleration strategy: send extra principal before the loan requires it. This calculator estimates the effect of recurring extra principal and a first-period lump sum across different payment frequencies and compounding methods. It reports estimated interest saved, original interest, prepaid interest, original payoff payments, new payoff payments, time saved, and total extra principal.

This page is distinct from the mortgage with extra payments calculator, which uses monthly and annual extras only. It is also different from the biweekly mortgage calculator, which focuses specifically on the classic 26-half-payments-per-year strategy. The prepayment calculator is the flexible version: it lets you test monthly, bi-weekly, accelerated bi-weekly, weekly, or accelerated weekly schedules, plus monthly or semi-annual compounding. For a baseline payment, use the mortgage calculator; for replacing the loan instead, use the mortgage refinance calculator.

Inputs and timing assumptions

Mortgage amount is the principal balance being modeled. Interest rate is the annual nominal rate. Loan term is the remaining or original amortization term, depending on what you want to study. Choose monthly compounding for a typical US or UK-style estimate, or semi-annual compounding when that matches the mortgage contract. Payment frequency controls how many payments occur per year and how the scheduled payment is scaled.

The periodic prepayment is added to every payment period. If you choose weekly, it is added weekly; if you choose bi-weekly, it is added every two weeks. The lump-sum prepayment is applied with the first simulated payment. That is an important assumption because earlier principal reduction saves more interest than later principal reduction. If your real lump sum will be paid in a future month, this calculator will overstate savings slightly; use the mortgage payoff calculator when you need a specific one-time payment month.

Payment-frequency mechanics

The calculator first computes a monthly payment from the annual rate converted to an effective monthly rate. It then converts that monthly payment to the selected frequency. Regular monthly uses the full monthly payment 12 times per year. Regular bi-weekly spreads 12 monthly payments over 26 periods. Accelerated bi-weekly uses half the monthly payment every two weeks, which creates 26 halves, or 13 monthly-payment equivalents, per year. Weekly and accelerated weekly follow the same idea with 52 periods.

For each simulated period, interest is charged on the current balance, then the scheduled payment, recurring prepayment, and first-period lump sum reduce principal. The loop stops when the balance reaches zero. If the payment is too small to cover interest, the calculator marks the scenario invalid rather than showing a misleading payoff.

Formula

The periodic rate is converted from the nominal annual rate and the compounding convention:

periodic rate=(1+annual ratecompounds per year)compounds per yearpayments per year−1\text{periodic rate} = \left(1 + \frac{\text{annual rate}}{\text{compounds per year}}\right)^{\frac{\text{compounds per year}}{\text{payments per year}}} - 1

The scheduled monthly payment is:

monthly payment=P×r(1+r)n(1+r)n−1\text{monthly payment} = P \times \frac{r(1+r)^n}{(1+r)^n - 1}

For each selected payment period:

interest for period=balance×periodic rate\text{interest for period} = \text{balance} \times \text{periodic rate}

principal reduction=scheduled payment+periodic prepayment+first-period lump sum−interest for period\text{principal reduction} = \text{scheduled payment} + \text{periodic prepayment} + \text{first-period lump sum} - \text{interest for period}

Interest saved is the original simulated interest minus the prepaid simulated interest.

Checking a mortgage prepayment scenario

Use the default scenario: a $100,000 mortgage, 6% interest, 30-year term, monthly interest method, monthly payment frequency, $100 extra each period, and a $5,000 one-time lump sum. The calculated scheduled monthly payment is $599.55.

With no prepayment, the monthly schedule lasts 360 payments and accumulates about $115,838.19 of interest. With the $5,000 lump sum applied during the first simulated payment and $100 added to each monthly payment, the loan is paid off in 228 payments. Prepaid interest is about $64,460.00. Interest saved is therefore $51,378.19. The calculator reports 132 payments saved, which equals 11.0 years at a monthly frequency. Total extra principal is $27,800.00, made up of the first $5,000 lump sum plus $100 for each of the 228 prepaid payments.

The example shows how a lump sum and a recurring habit reinforce each other. The lump sum lowers the base on which future interest is calculated, and the recurring add-on keeps pushing the balance down before later interest accrues.

When prepayment helps

Prepayment can help when the goal is reducing interest, shortening debt duration, or lowering retirement risk without replacing the mortgage. It can also be attractive when refinance costs are high or when the existing rate is good enough that a new loan is unnecessary. The more expensive the mortgage rate and the earlier the prepayment, the larger the interest reduction tends to be.

Prepayment may be less attractive when the mortgage rate is low, cash reserves are thin, or other debt carries a higher rate. It can also conflict with flexibility: once money is sent to principal, accessing it may require selling, refinancing, or using a home-equity product. Check for prepayment penalties, partial-payment rules, and whether extra amounts post immediately to principal.

This calculator is informational, not financial advice. It does not include tax deductions, investment returns, escrow changes, fees, or lender-specific daily interest. Use the estimate to understand direction and magnitude, then confirm operational details with the servicer before making large prepayments.

Method scope and source version

Jurisdiction-neutral loan mathematics; lender contracts, disclosures, taxes, insurance, PMI, and compounding conventions vary. Evergreen method only; defaults/examples must not be represented as current market, legal, tax, or institutional data. The sources below support the stated method and definitions; they do not supply a live rate, quote, legal conclusion, lender offer, or institution-specific policy.

Sources

Frequently asked questions

How is this different from the extra payments calculator?
This calculator adds payment frequency and interest-compounding choices, including monthly, bi-weekly, accelerated bi-weekly, weekly, and accelerated weekly schedules. The extra payments calculator focuses on a simpler monthly and annual extra-principal plan for a standard fixed-rate mortgage.
What happens to the lump-sum prepayment?
The lump sum is applied with the first simulated payment. Because it reduces principal immediately, it can create more interest savings than the same amount paid later. If the lump sum is larger than the mortgage amount, the calculator only counts principal up to the mortgage amount.
Why does the interest calculation method matter?
The calculator can convert the annual rate using monthly compounding, often used in the United States and United Kingdom, or semi-annual compounding, commonly associated with Canadian mortgages. The choice changes the effective periodic rate used in the simulated schedule.
Do accelerated bi-weekly payments equal regular bi-weekly payments?
No. Regular bi-weekly payments in this calculator scale the monthly payment across 26 periods. Accelerated bi-weekly payments use half the monthly payment every two weeks, creating the equivalent of about one extra monthly payment per year before any added prepayment.
Can I rely on this as a lender payoff quote?
No. It is an educational estimate. Actual payoff depends on the lender's posting date, daily interest rules, fees, penalties, escrow handling, and the exact remaining balance. Request an official payoff statement before closing a mortgage or sending a large final payment.

Related calculators

Biweekly Mortgage CalculatorEstimate interest saved by making 26 accelerated half-payments per year, with optional extra principal, compounding choices, payoff time, and total paid.Balloon Payment CalculatorEstimate the fixed monthly payment, remaining balloon balance, total repayment, and interest on a loan with a large final payment.Bi-Weekly Mortgage Payment CalculatorEstimate a true 26-payment bi-weekly mortgage plan, including the half-payment amount, accelerated payoff time, interest saved, and extra principal impact.Mortgage Acceleration CalculatorCompare monthly overpayments, accelerated biweekly payments, and accelerated weekly payments to estimate mortgage interest saved and payoff time shortened.Mortgage Payoff CalculatorEstimate how extra monthly payments and one-time principal payments change a mortgage payoff date, total interest, months saved, and new payment path.Mortgage Penalty CalculatorEstimate a mortgage prepayment penalty using the larger of three months' interest or an interest rate differential after applying your annual prepayment privilege.

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