The basic formula: principal, interest, taxes, and insurance

Your monthly mortgage payment is the sum of four separate costs: principal and interest (what you owe the lender), property taxes (what you owe your county or municipality), and homeowners insurance (what you pay to protect the house). Lenders call this bundle PITI. The principal and interest portion is fixed for the life of the loan if you have a fixed-rate mortgage. The tax and insurance portions change year to year.

The lender calculates your payment using the loan amount, interest rate, and loan term (usually 15 or 30 years). They then estimate your annual property taxes and insurance costs, divide by 12, and add those monthly amounts to your principal-and-interest payment. The result is what hits your bank account each month.

If you put down less than 20 percent, your payment also includes mortgage insurance (PMI on conventional loans, or built into the rate on FHA loans). This protects the lender if you stop paying, and it disappears once you reach 20 percent equity in the home.

Key Takeaways

  • Principal and interest are calculated using a fixed formula based on loan amount, interest rate, and term; this portion never changes on a fixed-rate mortgage.
  • Property taxes and homeowners insurance are estimated annually and divided into your monthly payment, so they rise when your tax assessment or insurance premium increases.
  • Mortgage insurance (PMI) is added to your payment if you put down less than 20 percent and can be removed once you reach that equity threshold.
  • The exact payment depends on your down payment, credit score (which affects your rate), loan term, and local property tax rates.

How principal and interest are calculated

The lender uses an amortization formula to spread your loan across the number of months in your term. On a 30-year fixed mortgage, that is 360 monthly payments. The formula accounts for the fact that early payments are mostly interest (because you owe more), and later payments are mostly principal (because you owe less).

The calculation is: M = P × [r(1 + r)^n] / [(1 + r)^n − 1], where M is your monthly payment, P is the loan amount, r is your monthly interest rate (annual rate divided by 12), and n is the total number of payments. You do not need to do this math yourself—your lender or a mortgage calculator does it—but the formula shows why a higher interest rate or longer term raises your payment, and why a larger down payment lowers it.

On a $300,000 loan at 7 percent interest over 30 years, the principal-and-interest payment is roughly $1,996 per month. On the same loan at 6 percent, it drops to about $1,799. On a 15-year term at 7 percent, it rises to roughly $2,996. The interest rate and term are the two biggest levers on your payment size.

Property taxes and homeowners insurance in your payment

Your lender does not collect taxes and insurance directly—your county or municipality collects taxes, and your insurance company collects premiums. But if you have a mortgage, your lender requires you to pay these costs through an escrow account (also called an impound account). Each month, you send the lender an extra amount on top of principal and interest. The lender holds this money and pays your tax bill and insurance premium when they come due.

The lender estimates your annual property tax by looking at your home's assessed value and your local tax rate. They divide this by 12 and add it to your monthly payment. The same happens with homeowners insurance: the lender gets a quote from your insurance company, divides by 12, and adds it to your payment. Once a year, usually in the fall, the lender reviews these estimates. If your taxes or insurance went up, your monthly payment goes up. If they went down, your payment goes down.

Property tax rates vary widely by location. A home worth $400,000 might carry $4,000 in annual taxes in one county and $8,000 in another. Homeowners insurance also varies by location, home age, and coverage level. A newer home in a low-risk area might cost $1,200 per year to insure; an older home in a high-risk area might cost $2,500 or more. These differences are why two identical mortgages in different states can have very different monthly payments.

Mortgage insurance when you put down less than 20 percent

If your down payment is less than 20 percent of the home price, lenders require private mortgage insurance (PMI) on conventional loans. This is an extra monthly cost that protects the lender, not you. On a $300,000 home with a $45,000 down payment (15 percent), PMI might add $150 to $300 per month, depending on your credit score and the size of your down payment.

PMI is not permanent. Once you reach 20 percent equity in the home—either by paying down the principal or by the home appreciating in value—you can request that the lender remove it. Federal law requires lenders to remove PMI automatically once you reach 22 percent equity, but you can ask earlier. Keep track of your equity and request removal when you hit the threshold; do not assume the lender will tell you.

FHA loans and VA loans do not use PMI, but they have their own insurance costs built into the rate or added to the monthly payment. An FHA loan typically includes an upfront mortgage insurance premium (1.75 percent of the loan amount, rolled into your balance) and an annual premium added to your payment. A VA loan includes a funding fee at closing but no monthly insurance.

How your credit score affects the interest rate and payment

Your credit score determines the interest rate the lender offers you. A score of 760 or higher typically qualifies for the lowest rates available that day. A score of 700 to 759 might be 0.25 to 0.5 percent higher. A score below 620 might be 1 to 2 percent higher, or you might not may have access to for a conventional loan at all.

On a $300,000 loan over 30 years, a 0.5 percent difference in rate changes your monthly payment by roughly $150. Over the life of the loan, that is $54,000 in extra interest. This is why improving your credit score before you apply—by paying down debt, correcting errors on your report, or waiting for old negative marks to age—can save you tens of thousands of dollars.

Your credit score also affects whether you may have access to for a lower down payment. Borrowers with scores above 740 often may have access to for 3 percent down; those below 620 may need 10 percent or more, or may not may have access to for a conventional loan at all.

Adjustable-rate mortgages and payment changes

An adjustable-rate mortgage (ARM) has a fixed interest rate for an initial period—usually 3, 5, 7, or 10 years—and then the rate adjusts annually or semi-annually based on a market index. During the fixed period, your principal-and-interest payment stays the same. After the adjustment period ends, your payment can rise significantly.

A 5/1 ARM, for example, has a fixed rate for 5 years, then adjusts every year after. If you lock in 6 percent for the first 5 years and the rate jumps to 8 percent in year 6, your monthly payment on a $300,000 loan rises from roughly $1,799 to about $2,203—an increase of $400 per month. ARMs are riskier than fixed-rate mortgages because your payment is not may provide. They make sense only if you plan to sell or refinance before the adjustment period ends, or if you can afford the payment at the highest possible rate.

What changes your payment after you close

Your principal-and-interest payment never changes on a fixed-rate mortgage. But your total monthly payment can rise if property taxes increase, your homeowners insurance premium goes up, or your home's assessed value rises (which increases taxes). Some lenders also adjust your escrow account if they underestimated taxes or insurance; if they collected too little, they raise your payment to catch up.

If you refinance your mortgage, you get a new loan with a new rate, term, and payment. This is a separate transaction from your original mortgage and resets the clock on your loan term. A refinance can lower your payment if rates have dropped, or raise it if you shorten the term or rates have risen.

Your payment also changes if you remove PMI, which lowers it by the amount of the insurance premium. Once you reach 20 percent equity, request removal in writing and provide proof of the home's current value if the lender asks for it.

Frequently Asked Questions

Why is my payment higher than the principal and interest amount I calculated?

Your payment includes property taxes, homeowners insurance, and possibly mortgage insurance, all divided into monthly amounts. The lender adds these to the principal-and-interest figure. Ask your lender for a loan estimate; it breaks down each component so you can see where every dollar goes.

Can I pay just principal and interest without taxes and insurance?

No. If you have a mortgage, the lender requires an escrow account and will not release the loan without it. Once you own the home outright, you pay taxes and insurance separately. Some lenders allow you to remove escrow after you reach 20 percent equity and have a strong payment history, but this is rare and depends on the lender's policy.

What happens if my property taxes go up mid-year?

Your lender reviews your escrow account annually, usually in the fall. If taxes or insurance increased, they recalculate your monthly payment and send you a notice of the new amount. The change takes effect on your next payment. If the increase is large, you may be able to spread it over several months rather than absorbing it all at once.

Does a longer loan term always mean a lower monthly payment?

Yes, spreading the same loan over more months lowers the monthly payment. A 30-year mortgage has a lower monthly payment than a 15-year mortgage on the same loan amount and rate. But you pay far more interest over the life of the loan. A 30-year loan at 7 percent costs roughly $239,000 in total interest on a $300,000 loan; a 15-year loan costs roughly $107,000.

How do I know if my escrow estimate is accurate?

Your loan estimate shows the estimated taxes and insurance. Compare the tax figure to your county assessor's website and the insurance figure to your insurance quote. If either seems low, ask the lender to adjust it upward; it is better to overpay and get a refund than to underpay and face a surprise increase in your monthly payment.