How to use the extra mortgage payment calculator
- Enter your current loan balance, interest rate and remaining term in years and months (from your latest mortgage statement).
- Add an extra monthly amount, a yearly extra payment, and/or a one-time lump sum with the payment number it is made with.
- Compare the new payoff date and total interest with your current schedule on the chart and the yearly table.
- Download both schedules as a CSV file if you want to keep them.
Worked example
$250,000 balance at 6.75% with 27 years remaining, plus $150 a month
The regular principal and interest payment is $1,679.00 over 324 payments, with $293,997.30 of interest still to pay. Adding $150 a month clears the loan in 262 payments instead of 324 (5 years 2 months sooner) and cuts total interest to $227,608.30, saving $66,389.00 for $39,150 of extra payments.
How it works
Two schedules are built with the same fixed-rate amortization engine. The regular payment is recalculated from your balance, rate and remaining months with M = P × r / (1 − (1 + r)−n), where r is the annual rate ÷ 12. Each month interest is the balance × r, rounded to the cent. The comparison schedule applies your extra amounts to principal in the months they are paid. Interest saved is the difference in total interest between the two schedules.
Assumptions
- Extra payments are applied to principal immediately. Confirm with your servicer that prepayments are applied this way and that there is no prepayment penalty.
- The regular payment stays the same after extra payments; the loan simply ends sooner.
- The yearly extra payment is made with every 12th payment.
Frequently asked questions
Is it better to pay extra monthly or once a year?
Paying earlier saves slightly more interest because the balance drops sooner, but the difference between monthly and annual extras of the same yearly total is usually small. Use the calculator to compare.
Do biweekly payments help?
Paying half your monthly payment every two weeks results in 26 half-payments, the equivalent of 13 monthly payments a year. You can model this as one extra monthly payment per year in the yearly extra field.
Does an extra payment lower my monthly payment?
Usually not. On most fixed-rate US mortgages the payment stays the same and the loan ends earlier. Some servicers offer a recast, which re-amortizes the lower balance for a fee.
Should I pay extra or invest?
That depends on your rate, other debts, emergency savings, and tax situation. This tool shows the interest saved; it does not give financial advice.
Limitations
- Does not account for tax effects or the opportunity cost of the extra cash.
- Taxes, insurance and PMI are not included; use the mortgage payment calculator for the full payment.