How Mortgage Payments Are Calculated

Finance · 8 min read

A mortgage is usually the largest loan a person will ever take, and yet the way the monthly payment is calculated is a mystery to most borrowers. Understanding it demystifies a lot of confusing behavior — like why your balance barely moves in the early years, and why shaving a fraction of a percent off your rate can save a small fortune.

The amortized payment formula

Most mortgages are "amortized," meaning you pay a fixed amount each month that gradually pays off both the interest and the principal over the loan term. The monthly payment M 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 (the annual rate divided by 12), and n is the total number of monthly payments (years × 12).

The formula guarantees that if you make exactly this payment every month, the balance reaches zero on the final payment. Every payment is the same size, but what happens inside each payment changes over time.

Why early payments are mostly interest

Each month, interest is charged on the remaining balance. Early in the loan, the balance is large, so most of your payment goes to interest and only a little to principal. As the balance slowly shrinks, the interest portion shrinks with it, and more of each fixed payment attacks the principal. This is why a mortgage feels like it barely moves for years and then accelerates near the end. The schedule that shows this shift, payment by payment, is called an amortization table.

A worked example

Consider a 300,000 loan at 6% annual interest over 30 years. The monthly rate is 0.06 / 12 = 0.005, and n is 360. Running the formula gives a monthly payment of about 1,799. Over 30 years you would pay roughly 647,500 in total — meaning about 347,500 in interest alone, more than the original loan. That single fact surprises many first-time buyers.

How the term changes everything

Shorten that same 300,000 loan to 15 years and the monthly payment rises to about 2,532 — higher, because you are paying it off faster. But the total interest drops to roughly 155,700, less than half. The trade-off is stark: a longer term lowers your monthly payment but dramatically raises the lifetime cost. A shorter term costs more each month but saves enormously overall.

How the rate changes everything

Interest rate has a similarly outsized effect. On the 30-year, 300,000 loan, dropping the rate from 6% to 5% cuts the monthly payment from about 1,799 to about 1,610 and saves tens of thousands over the life of the loan. This is why buyers shop aggressively for rates and why even a quarter-point matters.

What to do with this knowledge

When you understand the formula, three strategies become obvious. Making extra principal payments early, when the balance is largest, saves the most interest. Choosing the shortest term you can comfortably afford minimizes lifetime cost. And securing the lowest possible rate is worth real effort. Before signing anything, model the scenarios so you can see the trade-offs in actual numbers rather than vague feelings.

The mortgage calculator on this site lets you plug in your own loan amount, rate, and term to see the monthly payment and total interest instantly. Try comparing a 15-year and 30-year version of the same loan to feel the difference.