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 principal and interest together using a fixed formula. Property taxes and homeowners insurance are added on top, and they change based on where you live and what your home is worth.
The principal is the amount you borrowed. The interest is what the lender charges you for lending it. These two are locked in at the moment you sign the loan documents—they do not change for the life of a fixed-rate mortgage. Taxes and insurance, by contrast, are estimates that get recalculated every year or two, so your payment can shift even if your loan terms stay the same.
Key Takeaways
- Principal and interest are calculated using a fixed formula based on your loan amount, interest rate, and loan term, and they stay the same for the entire loan on a fixed-rate mortgage.
- Property taxes and homeowners insurance are added to your principal and interest payment, and these amounts change annually based on assessed home value and insurance rates.
- The amortization schedule determines how much of each early payment goes to interest versus principal—most of the first payments are interest.
- Your lender holds taxes and insurance in an escrow account and pays those bills on your behalf, which is why your payment includes them.
- Adjustable-rate mortgages recalculate the interest portion when the rate resets, which can raise or lower your payment significantly.
How principal and interest are calculated together
Lenders use an amortization formula to spread your loan across the number of months you have to repay it. The formula takes three inputs: the loan amount, the annual interest rate, and the loan term in months. From those three numbers, it produces a single monthly payment that stays the same every month (on a fixed-rate loan).
The formula is designed so that by the time you make your last payment, you will have paid back every dollar you borrowed plus all the interest owed. Early in the loan, most of your payment goes toward interest because you still owe a large balance. As you pay down the principal, more of each payment goes toward principal and less toward interest. This shift happens automatically—you do not choose it.
For example, on a $300,000 loan at 6.5% interest over 30 years, the principal and interest portion alone is roughly $1,896 per month. In month one, about $1,625 of that goes to interest and $271 to principal. By month 360 (the last payment), almost all of it goes to principal because so little is owed. The total amount you pay in interest over 30 years is roughly $382,000—nearly as much as the original loan.
Where property taxes fit into your payment
Property taxes are assessed by your county or municipality based on the estimated value of your home. The tax rate varies widely by location—some counties charge less than 0.5% of home value per year, while others charge 2% or more. Your lender estimates what your annual tax bill will be, divides it by 12, and adds that amount to your monthly payment.
Because home values change and tax rates can shift, your property tax estimate gets updated periodically—often every year or two. When the estimate goes up, your monthly payment goes up. When it goes down, your payment goes down. This is one reason your mortgage payment can change even though your loan terms have not.
Your lender does not keep this money. Instead, they hold it in an escrow account (sometimes called an impound account) and pay your property tax bill directly to the county when it comes due. You never write a separate check for property taxes—the lender handles it as part of your monthly payment.
How homeowners insurance is added to your payment
Homeowners insurance protects the lender's investment in your home. Your lender requires you to carry it and will not close the loan without proof of coverage. The insurance premium is estimated at the time you lock in your rate, divided by 12, and added to your monthly payment.
Like property taxes, homeowners insurance gets re-estimated periodically. If your insurer raises rates or you change coverage levels, your monthly payment will change. If you live in a flood zone or high-risk area, you may also need separate flood insurance, which is calculated and escrowed the same way.
The lender holds the insurance money in the same escrow account as your property taxes and pays the insurance company directly when your policy renews. You receive the insurance bill, but the lender pays it from the escrow funds you have been contributing each month.
What happens when interest rates change
On a fixed-rate mortgage, your interest rate is locked in at closing and never changes. Your principal and interest payment stays exactly the same for 15, 20, or 30 years. Only the tax and insurance portions can shift.
On an adjustable-rate mortgage (ARM), the interest rate is fixed for an initial period—often 3, 5, 7, or 10 years—and then resets periodically based on market conditions. When the rate resets, the lender recalculates your principal and interest payment using the new rate. This can raise your payment significantly or lower it, depending on whether rates have gone up or down. Some ARMs have rate caps that limit how much the rate can increase at each adjustment, but the payment can still change substantially.
Understanding your escrow account and annual adjustments
Your escrow account is a separate account your lender maintains on your behalf. Each month, you contribute money for property taxes and insurance. The lender pays those bills from the account when they come due. Once a year, the lender reviews what was actually paid out and what you actually contributed, and adjusts your monthly payment if needed.
If you overpaid (contributed more than was spent), you may receive a refund or a credit toward future payments. If you underpaid, your monthly payment increases to make up the difference. This annual adjustment is called an escrow analysis, and your lender is required to send you a statement showing the calculation. The adjustment is not optional—it is a requirement of the loan.
Some lenders allow you to pay property taxes and insurance yourself instead of through escrow, but this is uncommon for mortgages with less than 20% down. If you do pay them separately, your monthly mortgage payment is lower, but you are responsible for making sure those bills are paid on time.
How to read your loan estimate and closing disclosure
Before you close on a mortgage, you receive two documents that show exactly how your payment is calculated. The Loan Estimate is provided within three days of your application and shows the estimated principal, interest, taxes, insurance, and other costs. The Closing Disclosure is provided at least three days before closing and shows the final numbers.
Both documents break down your monthly payment into its four components and show the total amount you will pay over the life of the loan. They also show your interest rate, loan term, and loan amount. If the numbers on the Closing Disclosure differ significantly from the Loan Estimate, ask your lender to explain why before you sign.
The amortization schedule—a month-by-month breakdown of how much principal and interest you pay each month—is usually provided at closing or shortly after. This schedule shows exactly how your balance decreases over time and how the split between principal and interest shifts with each payment.
Frequently Asked Questions
Why do I pay so much interest at the beginning of the loan?
Interest is calculated on your remaining balance each month. At the start, your balance is highest, so the interest charge is highest. As you pay down principal, the interest portion shrinks automatically. This is built into the amortization formula and happens on every mortgage.
Can I pay extra toward principal to shorten my loan?
Yes. Most mortgages allow you to make extra payments toward principal without penalty. When you do, you reduce your balance faster, which means you pay less interest over the life of the loan and can pay off the mortgage years early. Check your loan documents or ask your lender whether extra payments are allowed.
What if my property taxes or insurance go down?
Your lender will recalculate your escrow account during the annual analysis. If taxes or insurance costs drop, your monthly payment will decrease. The lender will either refund the overage or credit it toward future payments.
How is the interest rate determined when I get a mortgage?
Your interest rate depends on market conditions, your credit score, your down payment size, your loan term, and the type of mortgage you choose. Lenders set rates based on what they can borrow money for themselves, plus a margin for profit and risk. You can shop different lenders to compare rates.
Does my payment change if I refinance?
Yes. When you refinance, you take out a new loan with a new interest rate, new term, and possibly a new loan amount. Your new monthly payment is calculated using the same formula but with these new terms. You also restart the amortization schedule, so you begin paying mostly interest again.