Tool · Money
Type in what you still owe, your rate and the years left, and the calculator shows when the loan is paid off and what the interest adds up to — then what paying a little extra each month takes off both.
Where does each number come from?
- Amount still owed
- The principal balance on your latest mortgage statement — not the purchase price and not the original loan. On a loan that’s been running a few years it’s smaller than both.
- Interest rate
- The rate on your note, which is also on every statement. The box starts at Freddie Mac’s weekly average for a thirty-year loan; the survey’s scope is further down.
- Years left
- The term remaining. A new loan has thirty; a loan you’ve been paying seven years has twenty-three. Your statement usually shows the maturity date.
- Extra each month
- Money on top of the scheduled payment, applied to principal the month you pay it. It moves exactly two things: the payoff date and the total interest.
What moves the answer?
Let’s say you owe $300,000 at 7% with thirty years to go. The payment on the loan itself is about $2,000 a month, and over thirty years you’d pay about $420,000 in interest — more than the loan itself. Add $200 a month and the loan is gone in about twenty-three years instead of thirty, with about $117,000 less interest. Add $500 a month and it’s about seventeen years and about $200,000 less.
Every extra dollar goes to principal, and every dollar of principal you retire early is a dollar that stops earning interest for the bank for the rest of the loan, which is why the interest saved is so much larger than the extra money you paid.
What does the calculator take as given?
- The rate
- Starts at 6.95%, Freddie Mac’s average for a thirty-year fixed loan for the week of 17 September 2026. It’s a weekly average of new purchase loans and it moves every Thursday.
- What the survey covers
- Freddie Mac’s own words: rates collected from loan applications lenders submit through its system for conventional, single-family loans within the conforming limits. A refinance, a jumbo loan or an FHA or VA loan is priced differently.
- The fifteen-year comparison
- 6.26% the same week. A shorter loan is a different loan with a different rate, which is why it sits in one comparison line and not on a slider.
- How the payment is figured
- The standard formula for a fixed loan, printed in full in the panel: the balance times the monthly rate, divided by one minus one-plus-the-monthly-rate raised to the power of minus the number of months.
- What's left out
- Property tax, homeowners insurance and any HOA dues, which are usually collected with your payment but aren’t part of the loan. Mortgage insurance. Points and closing costs. Adjustable rates, interest-only periods and balloon payments. Prepayment penalties. And how your servicer applies extra money — on many loans an extra payment has to be marked “principal” or it sits as a prepayment of next month, and your servicer’s website says how.
Three hundred thousand at seven percent for thirty years is a made-up loan to show what an extra payment does; your balance, your rate and your maturity date are on your latest statement, and your servicer's site says how an extra payment gets applied.
Sources
- Freddie Mac, Primary Mortgage Market Survey — week of 17 September 2026: 30-year fixed 6.95% (from 6.76% the week before, 6.26% a year earlier); 15-year fixed 6.26%.
- Freddie Mac, Primary Mortgage Market Survey methodology — “results are based on the mortgage rate collected from thousands of loan applications submitted to Freddie Mac through Loan Product Advisor from lenders across the country”; criteria “include weekly conventional, single-family originations with conforming loan limits as set by FHFA.”