A mortgage is one of the largest financial commitments most people will ever make. Knowing how the monthly payment is calculated — not just guessing at it — gives you real leverage when comparing offers, deciding on a term, or asking whether a bigger down payment is worth it.

The formula

A standard fixed-rate, fully amortizing mortgage is calculated with one tidy formula:

M = P × r(1 + r)^n / ((1 + r)^n − 1)

Where:

M is the monthly payment
P is the loan principal (the amount you borrow)
r is the monthly interest rate (annual rate divided by 12)
n is the total number of payments (years × 12)

That’s it. Every spreadsheet, bank app, and online calculator — including ours — runs that one expression.

A worked example

Say you borrow $300,000 at a 6.5% annual rate for 30 years.

P = 300,000
r = 0.065 / 12 ≈ 0.005417
n = 360

Plug it in and you get about $1,896 / month. Over 30 years you’ll pay roughly $682,000 — meaning about $382,000 in interest. That gap between what you borrow and what you pay back is what makes mortgage math so worth understanding.

Why early payments are mostly interest

In the first month, your bank charges interest on the entire $300,000. At 6.5% that’s $1,625 — almost the whole monthly payment. Only about $271 actually pays down principal.

By year 15, the balance is lower, so less of each payment goes to interest. By the final year, almost the whole payment goes to principal. This shape is called the amortization curve, and it’s why an extra $100 a month in early years saves so much more interest than the same $100 thirty years in.

What the calculator can answer

Use the Mortgage Calculator to play with three variables that change everything:

Loan amount. Doubling the loan roughly doubles the payment, but more importantly doubles the interest you pay.
Interest rate. A jump from 6% to 7% on a $300k 30-year loan adds about $200 a month — and roughly $70,000 over the life of the loan.
Term. A 15-year loan has a higher monthly payment but cuts total interest dramatically. The same $300k at 6.5% for 15 years costs about $159,000 in interest — less than half the 30-year total.

A quick mental model

Before you calculate anything precisely, you can ballpark a mortgage payment with this rule of thumb: for every $100,000 borrowed at ~6.5% over 30 years, plan on about $632 / month. Multiply by your loan size in hundreds-of-thousands and you’re within a few percent of the truth.

That’s enough to glance at a listing and know whether it’s even worth a closer look. For everything else — refinancing, comparing loans, deciding between 15 and 30 years — let the calculator do the precise work.