Calculator
Extra mortgage payment calculator
This is for homeowners with a fixed-rate loan who are wondering what a steady extra amount each month would change. You type the balance, the rate, the months left and the extra amount, and the calculator runs the loan month by month with and without the extra. It shows no rates of its own and nothing leaves your device.
Worked example (shown when scripts are off)
A balance of 200,000 at 6% with 360 months left has a payment of about 1,199.10. Paying 200 extra each month ends the loan 108 months (9 years) sooner and saves roughly 79,800 in interest. Turn scripts on to run your own numbers.
Before you decide
Months and interest saved are only one side of the choice. Cash you send to a mortgage is harder to get back than cash in savings, and other debts may cost more. This is general information, not financial advice.
How the calculator works
The monthly rate is the annual rate divided by 12. If you leave the optional payment blank, the regular payment comes from the standard loan formula: payment = balance x r / (1 - (1 + r) to the power of minus n), where r is the monthly rate and n is the months remaining. At a rate of zero it is simply balance divided by months.
Then the tool builds a schedule. Each month, interest is the balance times r, the rest of the payment reduces the balance, and the last payment is capped so the loan ends at zero. It runs once with the regular payment and once with the regular payment plus your extra. Months saved is the difference in length, and interest saved is the difference in total interest.
Worked example
Take 200,000 at 6% with 360 months left. The monthly rate is 0.5% and the payment is about 1,199.10, so total interest over the full term is about 231,676. Adding 200 a month raises the payment to about 1,399.10, and the schedule ends in 252 months, which is 108 months (9 years) sooner and saves about 79,800 in interest. The schedule, not a shortcut, produces those figures, and the test script checks them against an independent simulation.
Limits
- It assumes a fixed rate and a fixed payment. Adjustable-rate loans change over time and are not modeled.
- Your real statement may differ by a few dollars because lenders round, and escrow for taxes and insurance is not included.
- It assumes the extra is paid every month and applied to principal. Ask your lender how to mark extra payments and whether the loan has any prepayment terms, because this tool cannot see your contract.
- If you type a current payment that does not cover the monthly interest, it will say so, since the balance would never fall.
- It ignores what else the same money could do, such as paying a higher-rate card or building savings.
Where this comes from
The payment formula is the standard fixed-rate amortization formula used for installment loans, and the month-by-month schedule is the plain definition of how interest accrues on a declining balance. We did not take a rate, fee or limit from any source, since the visitor supplies them. Correctness was checked with a Node test script covering known payments, zero rates, one-month loans and rising extra amounts.