THE NUMORIX GUIDE
How to use the Mortgage payoff calculator
Last reviewed September 14, 2026
What this calculator does
The calculator establishes an original monthly payment either from original loan, original term, and rate or from an entered unpaid balance and monthly payment.
Formula and method
The calculator establishes an original monthly payment either from original loan, original term, and rate or from an entered unpaid balance and monthly payment. It simulates the original path, then a second path that can add monthly, yearly, or one-time money and can split the base monthly payment into biweekly amounts. Interest is charged on the current balance before principal is reduced.
Variables and inputs
Choose known remaining term or unknown remaining term. The first mode uses original loan, original term, rate, and optional remaining-term fields; the second uses unpaid balance, monthly payment, and rate. Then choose no extra, extra per month, extra per year, or a one-time payment, enter its amount, and choose normal or biweekly repayment.
Worked example
For a $300,000 30-year loan at 6.5%, the monthly rate is 0.065 / 12 = 0.0054167 and the regular payment is about $1,896.20. In month 1, interest is 300000 x 0.0054167 = $1,625.00 and regular principal is about $271.20; adding $500 extra makes the balance about 300000 - 271.20 - 500 = $299,228.80.
How to interpret the result
Interest saved is the difference between the two simulated paths, and time saved is expressed in years and months. A biweekly setting divides the base payment into half-payments and can create an extra annual-payment effect, but the exact result depends on how payments are applied in the real servicing system.
Common mistakes to avoid
Select the mode that matches the information you actually have. Do not enter the original loan as the current unpaid balance when many payments have already been made. Treat a yearly extra as an annual amount, not as a monthly amount repeated twelve times.
Assumptions and limitations
The simulation uses monthly interest and applies a one-time extra in month 1. It does not include escrow, taxes, insurance, servicing fees, lender-specific biweekly processing, or contractual restrictions. The result is a comparison under the entered assumptions, not a payoff statement.