How Mortgage Payments Work

Most home loans in the US are fixed-rate, fully amortizing loans. That means the payment stays the same every month and, if you pay on schedule, the balance reaches exactly zero on the last day of the term. The payment comes from this formula:

M = P × r(1 + r)ⁿ / ((1 + r)ⁿ − 1)

• P is the amount you borrow (the price minus your down payment).
• r is the monthly interest rate: the annual rate divided by 12. A 6.5% loan has r = 0.065 / 12, about 0.5417%.
• n is the number of monthly payments: 360 for a 30-year loan, 180 for a 15-year loan.

Take a $300,000 loan at 6.5% over 30 years. The formula gives a principal-and-interest payment of about $1,896.20 a month. Over 360 payments you would hand over roughly $682,600, which means about $382,600 of that is interest. The loan costs more than twice what you borrowed in interest because you are paying interest on a large balance for a very long time.

You do not need to do this by hand. Our mortgage calculator runs the same formula and also shows the month-by-month table. The value of knowing the formula is understanding which levers matter: the rate, the term and the amount borrowed.

Each month the lender charges interest on whatever you still owe. The rest of your payment goes to principal. Because the balance is highest at the start, so is the interest charge.

On the same $300,000 loan, the first month's interest is $300,000 × 0.065 / 12 = $1,625.00. Your payment is $1,896.20, so only about $271.20 reduces the balance. Next month the balance is a little lower, so the interest is a little lower and slightly more goes to principal. This table of how the split shifts is called an amortization schedule.

The effect is gradual at first and then speeds up. In the final years almost the entire payment is principal. This is why selling or refinancing after only a few years leaves you with surprisingly little equity from payments alone: most of what you paid was the cost of borrowing. Any equity you built early came largely from your down payment or from the home rising in value.

A shorter term raises the monthly payment but cuts the total interest sharply. Using the same $300,000 at 6.5%:

• 30 years: about $1,896 a month, roughly $382,600 total interest.
• 15 years: about $2,613 a month, roughly $170,400 total interest.

The 15-year loan costs around $717 more each month but saves more than $200,000 of interest. In practice 15-year loans usually carry a somewhat lower rate than 30-year loans, which widens the gap further. The trade-off is flexibility: a bigger required payment is a bigger commitment if your income dips.

The rate matters too, but its effect is easier to feel in the payment than in the headline rate. On $300,000 over 30 years, the payment is about $1,799 at 6.0% and about $1,996 at 7.0%. That one percentage point is about $197 a month, or more than $70,000 over the life of the loan. Try your own figures in the calculator rather than relying on rules of thumb.

The formula above covers principal and interest only. The amount you actually pay each month is often higher, because many lenders collect other costs in an escrow account and pay them for you:

• Property taxes are set locally and can change when your home is reassessed.
• Homeowners insurance is required by lenders. Premiums vary widely by location and can rise at renewal.
• Private mortgage insurance (PMI) typically applies to conventional loans when you put down less than 20%. It is usually removable once your balance falls far enough relative to the home's value; ask your servicer about the exact rules for your loan.
• HOA dues are billed separately but belong in your budget.

Because taxes and insurance change, an escrowed payment can go up even on a fixed-rate loan. Budget for the full monthly housing cost, not just principal and interest. A loan offer that looks affordable on the formula alone can look tight once these are added.

Any money you send above the required payment, if the lender applies it to principal, reduces the balance immediately. A lower balance means less interest next month, and that compounding effect shortens the loan.

On the $300,000, 6.5%, 30-year example, adding $200 a month to principal pays the loan off in about 23 years and 1 month instead of 30, and cuts total interest from roughly $382,600 to roughly $279,200: a saving of about $103,000. The extra payments add up to far less than the interest they remove, because the earliest dollars have the longest to work.

Before you do this, check three things. First, confirm the loan has no prepayment penalty (uncommon on standard home loans, but check). Second, tell the servicer to apply the extra to principal, since some default to treating it as an early payment of the next installment. Third, compare the guaranteed return of paying down a 6.5% loan against other uses of the money, such as an emergency fund or retirement contributions with an employer match. There is no single right answer; it depends on your rate, your cushion and your other debts.

1. Compare the APR as well as the rate, since it folds in some fees.
2. Ask for the full monthly cost including escrow, not only principal and interest.
3. Run each offer through the calculator at the same term so you compare like with like.
4. Look at closing costs and how long you plan to stay. Paying points to lower the rate only helps if you keep the loan long enough to recoup them.
5. Check how much total interest you pay and whether a shorter term is affordable.

These figures are for illustration and general information, not financial advice. Your lender's official Loan Estimate is the document to rely on for your actual numbers.