This free mortgage calculator shows your monthly principal-and-interest payment, the total interest you'll pay over the life of the loan, and your payoff date. Crucially, it also lets you model extra payments — an additional amount added to every monthly payment, or a lump sum paid once a year — and instantly see how many years and how much interest they save. Enter your home price, down payment, interest rate, and term, then experiment. Everything is calculated locally in your browser.
A mortgage is repaid through amortization — each fixed monthly payment covers the interest due that month, and whatever is left reduces the principal. Early on, most of your payment goes to interest; over time the balance tips toward principal. The standard monthly payment is calculated as:
M = P × [ r(1+r)ⁿ ] / [ (1+r)ⁿ − 1 ]
where P is the loan principal, r is the monthly interest rate (annual rate ÷ 12), and n is the total number of monthly payments.
Because interest is charged on the outstanding balance, any extra payment goes straight to principal and removes all the future interest that balance would have generated. On a typical $350,000 loan at 6.5%, an extra $200 a month pays the mortgage off roughly six years early and saves on the order of $100,000 in interest — enter your own numbers above to see the exact figure. The chart compares your standard payoff against the accelerated schedule, and the yearly lump-sum input models bonuses or tax refunds.
Use the term buttons to compare. A 15-year term roughly halves the total interest of a 30-year loan at the same rate, but locks in a much higher required payment. A popular middle path: take the 30-year for flexibility, then use the extra-payment sliders to simulate paying it on a 15-year schedule voluntarily — similar savings, with the option to pause if life happens.
Does a bigger down payment help? Yes — a larger down payment reduces the loan principal, lowering both your monthly payment and total interest, and may help you avoid mortgage insurance.
Should I pay off my mortgage early or invest? It depends on your rate versus expected after-tax returns — and your temperament. We break down the full framework, with the math for both sides, in Pay Off the Mortgage Early or Invest?
This tool is Step 3 of our master guide — The DIY Financial Plan: From First Dollar to Financial Independence.
Explore our other tools — the Retirement Calculator, FIRE Calculator, and Portfolio Stress-Tester. WealthDeck provides educational tools only, not financial advice — see our Terms & Disclaimer.