This free mortgage calculator shows your monthly principal-and-interest payment, the total interest you will pay over the life of the loan, and your exact payoff date. Its most useful feature is modelling extra payments — a set amount added to every month, or a lump sum once a year — so you can see precisely how much interest and time they save. Enter your home price, down payment, interest rate, and term; drag the sliders or type exact numbers. Everything is calculated locally in your browser.
A fixed-rate mortgage is repaid through amortization: each level monthly payment first covers the interest due that month, and whatever is left reduces the principal. Because interest is charged on the outstanding balance, early payments are mostly interest and later payments are mostly principal. The standard monthly payment is:
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 (years × 12).
This is where the tool earns its keep. Because every extra dollar goes straight to principal, it erases all the future interest that balance would have generated. On a $350,000 loan at 6.5% over 30 years:
| Extra per month | Payoff time | Approx. interest saved |
|---|---|---|
| $0 | 30 years | — |
| $100 | ~26.5 years | ~$55,000 |
| $200 | ~24 years | ~$95,000 |
| $500 | ~19.5 years | ~$165,000 |
Even a modest $100–$200 a month can remove five or six years and tens of thousands in interest. The yearly lump-sum field lets you model bonuses or tax refunds on top.
At the same rate, a 15-year loan roughly halves the total interest of a 30-year loan — but the required monthly payment is much higher. A popular compromise is to take the 30-year for flexibility and use the extra-payment sliders to pay it on a 15-year schedule voluntarily, keeping the option to pause if money gets tight.
| Term | Monthly P&I (on $350k at 6.5%) | Total interest |
|---|---|---|
| 30 years | ~$2,212 | ~$446,000 |
| 15 years | ~$3,049 | ~$199,000 |
Yes. A larger down payment reduces the principal, which lowers both the monthly payment and the total interest, and a down payment of 20% or more typically lets you avoid private mortgage insurance (PMI). Try moving the down-payment slider to see the loan amount and interest fall.
It depends on your rate versus your expected after-tax investment return, and on your appetite for risk. Above roughly 6.5%, prepaying is a strong guaranteed return; below about 4%, investing the difference has historically won. We break down the full framework, with the math for both sides, in Pay Off the Mortgage Early or Invest?.
What interest rate should I enter? Use the rate from your loan estimate or current statement. Recent 30-year fixed averages have generally ranged between about 6% and 7%.
Does this include property taxes and insurance? No — it calculates principal and interest only, which is the part determined by your loan terms. Add your local tax and insurance separately for a full housing budget.
Is my information private? Yes. The calculation runs entirely in your browser; nothing you enter is sent or stored.
This tool is Step 3 of our master guide — The DIY Financial Plan: From First Dollar to Financial Independence.
Explore our other tools — Retirement Calculator, FIRE Calculator, and Portfolio Stress-Tester. WealthDeck provides educational tools only, not financial advice — see our Terms & Disclaimer.