Mortgage Calculator

The formula

M = P × [ r(1 + r)ⁿ ] ÷ [ (1 + r)ⁿ − 1 ]

M is the monthly principal and interest, P is the amount you borrow, r is the annual rate divided by twelve, and n is the number of monthly payments. That's the whole thing. Taxes, insurance, PMI and HOA get added on top afterwards, and they're not part of the loan.

A worked example, run through the same code the calculator uses. Take a $425,000 house with 20% down, so you borrow $340,000. At 6.69% over 30 years the principal and interest come to $2,192 a month. Keep that loan to term and you pay $449,009 in interest, which is 132% of what you borrowed on top of paying it back.

Reading the amortization schedule

The payment doesn't change but its composition does, and that surprises people. In the first year of the example above, most of each payment is interest and only a small slice touches the balance. By the last few years that's reversed. Nothing shady is going on. You owe the most at the start, interest is charged on what you owe, so the interest portion is biggest when the balance is.

That's also why an extra payment early is worth so much more than the same payment late. It removes principal that would otherwise have accrued interest for decades. If you already have a mortgage and want to see that effect on your own numbers, the mortgage payoff calculator does exactly that. For the payment-by-payment table with real dates, or for a car or student loan rather than a mortgage, use the amortization calculator.

Why the rate box starts where it does

Most mortgage calculators open with an empty rate field or a round number someone picked years ago. This one starts at 6.69%, Freddie Mac's national average for the week ending 2026-08-06. Over the past year that average has run between 5.98% and 6.69%, and today it sits at the top of that range. A tenth of a point sounds trivial and isn't: on the example loan it's roughly $8,133 more interest over thirty years.

Your own quote will differ from the survey, and it should. The average describes a borrower with strong credit putting twenty percent down on a conforming loan. Change any of those and the price changes. Use the average to sanity-check a quote, not to predict one.

What this leaves out

Closing costs aren't here, and they typically run a few percent of the price. Neither are maintenance, utilities, or the repairs that arrive the month after you move in. The property tax and insurance figures are editable estimates rather than sourced data, because both are intensely local and we'd rather label a guess than dress it up. For the wider question of whether the whole thing fits, the budget and cash flow tracker and the emergency fund calculator are the two worth running before you commit.