How Are Mortgage Payments Calculated?

The three inputs

Only three numbers set your payment: the amount you borrow, the annual interest rate, and the term in years. A bigger loan or a higher rate raises the payment; a longer term lowers it. Everything a mortgage calculator shows flows from these three.

The amortization formula

The payment M = P·i·(1+i)^n / ((1+i)^n − 1) finds the fixed sum that clears the loan in exactly n payments. Convert the annual rate to monthly (÷ 12) and the term to months (× 12). For $250,000 at 3.5% (i = 0.002917) over 240 months, that works out to roughly $1,450 a month.

Principal vs interest over time

Although the payment stays the same, its split changes. Early on, most of each payment is interest and only a little chips away at the balance; near the end, almost all of it is principal. This front-loading of interest is why paying extra early, or overpaying, saves so much — it attacks the balance while interest is highest.

What changes the monthly payment

A lower rate, a longer term or a bigger deposit all shrink the monthly payment — but they are not equal. A longer term lowers the monthly figure while raising the total interest paid, sometimes by a lot; a bigger deposit lowers both. Shopping the rate is usually the cleanest win, since it cuts the payment and the total at once.

Frequently asked questions

Why is a longer mortgage cheaper monthly but more expensive overall?

Spreading the same amount over more months lowers each payment, but you pay interest for more years, so the total climbs.

What is amortization?

It's the schedule that splits each payment between interest and principal, putting most of the interest early and most of the principal late.

Does a bigger deposit lower my payment?

Yes. Borrowing less reduces both the monthly payment and the total interest, and a larger deposit can also unlock a better rate.