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 using a fixed formula based on your loan amount, interest rate, and loan term. Taxes and insurance are estimates added on top, and they change when your property tax bill or insurance premium changes.
The principal and interest portion stays the same every month for a fixed-rate mortgage. The taxes and insurance portions can shift annually or when your local assessor raises your property value or your insurance company adjusts your rate. If you have an escrow account—which most mortgages do—your lender collects all four pieces each month and pays the taxes and insurance on your behalf when they come due.
Key Takeaways
- Principal and interest are calculated using your loan amount, interest rate, and the number of months you have to repay, and this portion never changes on a fixed-rate mortgage.
- Property taxes and homeowners insurance are estimated annually and added to your payment; they rise when your tax assessment or insurance premium rises.
- The amortization schedule shows exactly how much of each payment goes to principal versus interest—early payments are mostly interest, later ones mostly principal.
- An escrow account collects your tax and insurance money each month so the lender can pay those bills when they arrive, protecting the lender's collateral.
How the principal and interest portion is calculated
The lender uses three pieces of information: the loan amount (called the principal), the annual interest rate, and the loan term in months. For a $300,000 loan at 6.5% interest over 30 years (360 months), the lender converts the annual rate to a monthly rate and applies a standard amortization formula that spreads the interest across all 360 payments.
The formula ensures that you pay all the interest owed and fully repay the principal by the final payment. Early in the loan, most of your payment covers interest because you still owe the full amount. As you pay down the principal, more of each payment goes toward principal and less toward interest. By the final years, the split reverses—most of your payment reduces what you owe.
You can see this shift on your amortization schedule, a month-by-month breakdown that your lender provides. Month 1 might show $1,625 going to interest and $200 to principal; month 360 might show $15 to interest and $1,810 to principal. The total payment stays the same, but the split changes every single month.
Property taxes: the variable part of your payment
Your lender estimates your annual property tax and divides it by 12 to add to your monthly payment. The estimate is based on your local tax rate and the assessed value of your home. When your county or municipality reassesses your property—often every year or every few years, depending on where you live—the assessed value may change, and so does your tax bill.
If your assessed value rises, your property tax rises, and your lender adjusts your monthly payment upward at your next annual escrow review. If it falls, your payment goes down. Some states cap how much the assessed value can rise in a single year, which slows the increase in your payment. Others have no cap, so a major reassessment can mean a sharp jump.
You can usually find your assessed value and tax rate on your county assessor's website or your most recent property tax bill. Knowing this number helps you predict whether your payment will rise or fall at the next review.
Homeowners insurance and what it covers
Your lender requires homeowners insurance and collects the premium as part of your monthly payment. The insurance covers damage to the structure from fire, wind, theft, and other named perils. It does not cover flood or earthquake damage—those require separate policies purchased through the National Flood Insurance Program or a private insurer.
Your insurance premium depends on the replacement cost of your home, the deductible you choose, the age and condition of the roof, your claims history, and your location. If you live in a high-risk area for hurricanes or wildfires, your premium will be higher. If you make a claim, your premium usually rises at renewal.
Your lender estimates the annual premium and adds one-twelfth to your monthly payment. At renewal time, if your premium increases, your lender adjusts your payment upward. If you shop for a cheaper policy and switch insurers, tell your lender so they can lower your payment to match the new premium.
How escrow accounts protect both you and the lender
An escrow account is a separate account your lender holds in your name. Each month, you pay principal, interest, taxes, and insurance into this account. The lender then pays your property tax bill and insurance premium from the escrow account when they come due, ensuring these critical bills never go unpaid.
Without escrow, you would be responsible for paying taxes and insurance separately on your own schedule. The lender requires escrow because unpaid property taxes can result in a tax lien on the home, and an uninsured home is at risk of total loss. Both scenarios threaten the lender's collateral, so escrow protects the lender's investment in your loan.
Once a year, usually in the spring, your lender reviews your escrow account. If you have paid more into escrow than was needed, you receive a refund. If you have paid less, your monthly payment rises to make up the shortfall. This annual adjustment is separate from any change caused by rising taxes or insurance premiums.
Adjustable-rate mortgages and how the payment changes
On a fixed-rate mortgage, the interest rate and principal-and-interest payment never change. On an adjustable-rate mortgage (ARM), the interest rate is fixed for an initial period—often 3, 5, 7, or 10 years—then adjusts annually or semi-annually based on a market index plus a margin set by the lender.
When the rate adjusts upward, your principal-and-interest payment rises. When it adjusts downward, your payment falls. Most ARMs have a rate cap that limits how much the rate can rise per adjustment period and over the life of the loan, but the payment can still increase significantly. A $300,000 ARM at 4% for the first 5 years might jump to 6% or higher in year 6, raising your monthly payment by $300 or more.
ARMs are riskier than fixed-rate mortgages because you cannot predict your payment after the initial period ends. They are most common when interest rates are high and borrowers want a lower starting rate, or when a borrower plans to sell or refinance before the rate adjusts.
PMI and other costs that may be added
Private mortgage insurance (PMI) is required when you put down less than 20% of the home's purchase price. It protects the lender if you default, and it is added to your monthly payment. PMI costs vary but typically run 0.5% to 1.5% of the loan amount per year, divided into 12 monthly payments.
Once your home equity reaches 20% of the home's value—through a combination of principal payments and home appreciation—you can request that PMI be removed. Some loans remove it automatically when you hit that threshold. PMI is not the same as homeowners insurance; it does not protect you or your home, only the lender.
Other costs that may appear in your payment include homeowners association (HOA) fees, if your property is in a planned community, and mortgage insurance for FHA or VA loans, which works differently than PMI but serves the same purpose.
Frequently Asked Questions
Why does my payment go up if my interest rate is fixed?
A fixed interest rate means the principal-and-interest portion never changes, but property taxes and insurance can rise. When your county reassesses your home or your insurance company raises your premium, your lender adjusts your payment upward at the annual escrow review. This is separate from the interest rate and is normal.
Can I pay more toward principal without changing my monthly payment?
Yes. You can make extra payments toward principal at any time without affecting your required monthly payment. Some borrowers make one extra payment per year or add $100 to each payment. Extra principal payments reduce the total interest you pay and shorten the loan term, but they do not lower your required monthly payment.
What happens if my property tax bill is higher than the lender estimated?
Your lender will discover the shortfall at the annual escrow review and adjust your monthly payment upward to cover the difference. If the shortfall is large, the lender may spread it over several months or require a lump-sum payment. You can appeal your property tax assessment if you believe it is too high.
Does refinancing change how my payment is calculated?
Refinancing replaces your old loan with a new one, so the calculation starts over with a new loan amount, interest rate, and term. If you refinance a $250,000 balance at a lower rate over 25 years instead of 30, your new payment will be higher even though the rate is lower, because you are paying off the loan faster.
What is the difference between a 15-year and 30-year mortgage payment?
A 15-year mortgage has a higher monthly payment but you pay far less total interest because the loan is paid off in half the time. A 30-year mortgage has a lower monthly payment but costs more in total interest. The choice depends on whether you prioritize lower monthly payments or paying off the home faster.