The basic formula: principal, interest, taxes, and insurance
Your monthly mortgage payment is built from four separate pieces, often called PITI: principal, interest, taxes, and insurance. The lender calculates the first two based on your loan amount and interest rate. Your local government sets the property tax portion. Your homeowners insurance company sets the last piece. Together, they make up what you owe each month.
The principal is the actual money you borrowed. The interest is what the lender charges you for lending it. These two parts stay roughly the same every month for a 30-year fixed-rate loan — the math is locked in when you sign. Taxes and insurance can shift year to year, so your payment may go up or down even if the loan itself does not change.
Key Takeaways
- Principal and interest are calculated together using a formula that spreads your loan across the number of months you have to repay it, with interest charged on the remaining balance each month.
- Property taxes and homeowners insurance are added on top and can change annually, which is why your total payment may shift even on a fixed-rate loan.
- If you put down less than 20 percent, mortgage insurance (PMI) is added to your payment until you build enough equity in the home.
- A shorter loan term (15 years instead of 30) means higher monthly payments but much less interest paid over the life of the loan.
- You can use an online calculator with your loan amount, interest rate, and loan term to see what your payment would be before you explore.
How principal and interest are calculated together
The lender uses a standard formula to divide your loan amount across all the months you have to repay it. On a 30-year loan, that is 360 monthly payments. The formula accounts for the fact that you pay interest on whatever balance remains — so early payments are mostly interest, and later payments are mostly principal.
Here is a simplified example: if you borrow $300,000 at 7 percent interest over 30 years, your principal and interest payment would be roughly $1,996 per month. In your first payment, about $1,750 goes to interest and $246 goes to principal. By payment 360, almost all of it goes to principal because so little is left to charge interest on. The formula ensures that by the end of month 360, you have paid back the full $300,000 plus all the interest.
The interest rate matters enormously. The same $300,000 loan at 5 percent interest would be about $1,610 per month — nearly $400 less. At 9 percent, it jumps to about $2,414. Even a 1 percent difference adds up to tens of thousands of dollars over 30 years.
Property taxes and homeowners insurance
Property taxes are set by your city or county and are based on the assessed value of your home. They vary widely by location — some areas charge less than 0.5 percent of home value per year, while others charge over 2 percent. Your lender will estimate the annual tax, divide it by 12, and add that amount to your monthly payment.
Homeowners insurance protects your home against fire, theft, and weather damage. The cost depends on the home's age, location, construction type, and the coverage level you choose. Your lender requires you to carry it and will add the monthly premium to your payment. If your estimate was too low, your payment may increase when the bill renews.
Both taxes and insurance go into an account called an escrow that your lender manages. When your property tax bill or insurance premium comes due, the lender pays it from that account using the money you have been sending each month. This protects the lender's investment in the home.
Mortgage insurance if you put down less than 20 percent
If your down payment is less than 20 percent of the home price, the lender requires you to pay mortgage insurance, usually called PMI (private mortgage insurance). This protects the lender if you stop paying — it is not insurance for you. The cost is typically 0.5 to 1.5 percent of your loan amount per year, added to your monthly payment.
On a $300,000 loan with 10 percent down, mortgage insurance might add $125 to $300 per month. You can stop paying it once you have paid down the loan to 80 percent of the original home value, though you have to request it — the lender will not stop automatically. Some loans let you remove it sooner if your home value rises.
How loan term changes your payment
A shorter loan term means a higher monthly payment but far less interest paid overall. A 15-year loan on $300,000 at 7 percent would cost roughly $2,997 per month — about $1,000 more than a 30-year loan. But over 15 years, you pay roughly $239,460 in total interest instead of $418,560 over 30 years. You save nearly $180,000.
The choice depends on your budget and goals. If you can afford the higher payment and want to own the home free and clear sooner, a 15-year loan makes sense. If you need the lower payment to fit your monthly budget, a 30-year loan is the right choice — you can always pay extra toward principal when you have the money.
What changes your payment after you close
On a fixed-rate loan, your principal and interest payment never changes. But your total payment can shift because property taxes and insurance are re-estimated each year. If your home is reassessed and taxes go up, or if your insurance premium increases, your lender will adjust your escrow payment upward. If either goes down, your payment may drop.
Some lenders also allow you to refinance, which means taking out a new loan to pay off the old one. You might refinance to get a lower interest rate, shorten the loan term, or switch from an adjustable-rate loan to a fixed-rate loan. Refinancing resets the calculation — you get a new principal amount, new interest rate, and new term, which changes your monthly payment.
Using a calculator to estimate your payment
Before you explore for a mortgage, you can use an online calculator to see what different loan amounts and interest rates would cost. You need three pieces of information: the loan amount (home price minus down payment), the interest rate, and the loan term in years. Most calculators also let you enter estimated property taxes and insurance to see your full PITI payment.
Keep in mind that a calculator shows an estimate, not a may provide. Your actual interest rate depends on your credit score, income, and the lender you choose. Property taxes and insurance are estimates based on averages for your area. The real numbers come from the lender's quote and your local assessor's office. But a calculator gives you a realistic starting point for budgeting.
Frequently Asked Questions
Why does my payment go toward interest first instead of principal?
The lender charges interest on the balance you still owe. Early in the loan, you owe the full amount, so interest is large. As you pay down the principal, the interest portion shrinks and the principal portion grows. This is built into the formula — it is not a choice the lender makes each month.
Can I pay extra toward principal without refinancing?
Yes. Most lenders let you send extra money with your payment and specify that it goes toward principal. This shortens your loan term and saves you interest. Check your loan documents or call your lender to confirm they do not charge a penalty for early repayment.
What happens to my payment if interest rates drop after I close?
Your payment stays the same on a fixed-rate loan — the rate is locked in. But you can refinance to a new loan at the lower rate, which would lower your payment. Refinancing costs money upfront, so it only makes sense if the savings are large enough to cover those costs within a few years.
How much of my payment goes to principal versus interest?
Early in the loan, most goes to interest. On a $300,000 loan at 7 percent, the first payment is about 88 percent interest and 12 percent principal. By year 15, it flips — most goes to principal. Your lender sends an annual statement showing the breakdown, or you can ask for an amortization schedule that shows every payment.
Does my credit score affect my monthly payment?
Your credit score does not change the formula, but it affects the interest rate the lender offers you. A higher score usually gets a lower rate, which lowers your monthly payment. A lower score gets a higher rate, which raises it. The difference can be hundreds of dollars per month.