The monthly number comes from one formula plus taxes and insurance. Here's how it works, with an example.
A fixed mortgage payment is calculated with: M = P × [ r(1+r)n ] / [ (1+r)n − 1 ], where P is the loan amount, r is the monthly interest rate (annual rate ÷ 12), and n is the number of payments (years × 12). It's the same amortization math behind every fixed-rate loan.
For a $300,000 loan at 6.5% over 30 years: r = 0.065/12 = 0.005417 and n = 360. Plugging in gives a principal-and-interest payment of about $1,896 a month. Over the full term you'd pay roughly $382,600 in interest — more than the loan itself, which is why rate and term matter so much.
The formula above covers principal and interest. Your real bill usually includes PITI — Principal, Interest, Taxes, and Insurance — plus PMI if you put down less than 20%, and HOA dues if applicable. These extras can add several hundred dollars a month.
A lower rate or a shorter term changes the math a lot. The same $300,000 at 5.5% drops the payment to about $1,703. A 15-year term raises the monthly payment but slashes total interest dramatically. Compare scenarios before you commit.
Extra principal payments shorten the loan and cut total interest. Even one extra payment a year can take years off a 30-year mortgage. Check whether your loan has any prepayment penalty first, then see the effect with the calculator.