The basic formula for a monthly mortgage payment

Your monthly mortgage payment is calculated using four pieces of information: the loan amount you borrowed, the interest rate, the length of the loan in years, and whether you have an escrow account. The payment itself covers principal (the amount you borrowed) and interest (what the lender charges you to borrow). If you have an escrow account, your payment also includes property taxes and homeowners insurance, which the lender collects and pays on your behalf.

The simplest way to see this number is to use a mortgage calculator — you enter the loan amount, interest rate, and loan term, and it shows you the monthly payment for principal and interest. That number alone is what you owe the lender each month. If your loan includes an escrow account, you add the monthly property tax and insurance amounts to get your true total payment.

If you want to understand how the calculation works, the formula is: M = P [ r(1 + r)^n ] / [ (1 + r)^n – 1 ]. In that formula, M is your monthly payment, P is the principal (loan amount), r is the monthly interest rate (annual rate divided by 12), and n is the total number of payments (years times 12). Most people never need to do this by hand — calculators do it instantly — but knowing the pieces helps you see why a higher interest rate or longer loan term changes your payment.

Key Takeaways

  • Your monthly payment covers principal and interest, and may also include property taxes and homeowners insurance if you have an escrow account.
  • A mortgage calculator requires only the loan amount, interest rate, and loan term to show you the principal-and-interest portion of your payment.
  • A higher interest rate or longer loan term increases your monthly payment, while a larger down payment decreases it by reducing the loan amount.
  • Property taxes and insurance vary by location and coverage level, so your escrow payment is unique to your property and policy choices.
  • Your actual payment may be higher than the calculator shows if you have private mortgage insurance (PMI), which is required when you put down less than 20 percent.

What a mortgage calculator needs from you

A mortgage calculator asks for the loan amount (the price of the home minus your down payment), the interest rate, and the loan term in years. Most calculators default to 30 years, but you can change it to 15 years or another term if that is what you are considering. Some calculators also ask whether you want to include property taxes and insurance, and if so, they ask for your estimated annual tax and insurance costs.

The loan amount is the number that matters most to your payment. If you are buying a $300,000 home and putting down $60,000, your loan amount is $240,000. If you put down $30,000 instead, your loan amount is $270,000, and your payment goes up. The interest rate is the second-biggest driver — a 6 percent rate produces a much lower payment than a 7 percent rate on the same loan.

The loan term is the third factor. A 15-year mortgage has a higher monthly payment than a 30-year mortgage on the same loan amount and interest rate, because you are paying back the money in half the time. However, you pay much less interest overall on a 15-year loan, because interest accrues over fewer years.

How property taxes and insurance change your total payment

If your lender requires an escrow account (which most do when you put down less than 20 percent), your monthly payment includes a portion of your annual property taxes and homeowners insurance. The lender collects this money each month, holds it in an escrow account, and pays the tax bill and insurance premium when they are due. This protects the lender — they know the property taxes and insurance are paid, so their collateral (your home) stays protected.

Property taxes vary dramatically by location. A home worth $300,000 might have annual taxes of $3,000 in one county and $9,000 in another. Homeowners insurance also varies based on your home's age, location, construction type, and the coverage limits you choose. A basic policy might cost $1,200 per year, while a more comprehensive one costs $2,000 or more.

To estimate your escrow payment, add your annual property tax and annual insurance premium, then divide by 12. If your taxes are $6,000 per year and insurance is $1,500 per year, your escrow payment is $625 per month. This amount is added to your principal-and-interest payment to get your total monthly payment to the lender.

Private mortgage insurance (PMI) and how it affects your payment

If you put down less than 20 percent, your lender requires private mortgage insurance (PMI). This is an insurance policy that protects the lender if you stop paying — it is not insurance for you. PMI is added to your monthly payment and typically costs between 0.5 and 1.5 percent of your loan amount per year, depending on your down payment size and credit score.

If your loan is $240,000 and PMI costs 1 percent annually, you pay $2,400 per year, or $200 per month. This amount is separate from your principal, interest, taxes, and insurance. A mortgage calculator that includes PMI will show you this cost, but many basic calculators do not, so you may need to add it yourself or use a calculator that specifically accounts for it.

PMI stays on your loan until you reach 20 percent equity in your home. Once you do — either by paying down the principal or by your home's value rising — you can request that the lender remove it. Some loans have automatic removal at 22 percent equity, but you should not count on this; contact your lender when you think you have reached 20 percent to ask about removal.

The difference between a 15-year and 30-year mortgage payment

A 15-year mortgage has a higher monthly payment but costs far less in total interest. A 30-year mortgage has a lower monthly payment but you pay interest for twice as long. The choice depends on your budget and how long you plan to stay in the home.

On a $240,000 loan at 6.5 percent interest, a 30-year mortgage costs about $1,520 per month (principal and interest only). The same loan on a 15-year term costs about $1,970 per month. The 15-year payment is roughly 30 percent higher, but over 15 years you pay about $115,000 in interest, compared to about $310,000 in interest on the 30-year loan. If you can afford the higher payment and plan to stay in the home, a 15-year mortgage saves you a substantial amount of money.

Some people choose a 30-year mortgage for the lower payment, then pay extra toward principal when they can. This gives you flexibility — you can make the minimum payment in tight months and pay extra when your budget allows. However, you only save interest on the extra payments you actually make, so this approach only works if you follow through.

How interest rates change your payment

Interest rate changes have a large effect on your monthly payment. On a $240,000 loan over 30 years, a 5.5 percent rate produces a payment of about $1,361 per month. At 6.5 percent, the payment rises to about $1,520. At 7.5 percent, it is about $1,686. A one-percentage-point increase adds roughly $125 to your monthly payment on this loan size.

This is why shopping for the best interest rate matters. Even a 0.25 percent difference in rate can save you $30 to $50 per month, which adds up to thousands of dollars over the life of the loan. Interest rates change based on market conditions, the Federal Reserve's actions, and your own credit score and down payment size. A lender may offer you a lower rate if you have a higher credit score or put down more money.

You can lock in an interest rate when you apply for a mortgage, which protects you if rates rise before closing. The lock period is usually 30 to 60 days, though you can pay for a longer lock if you need more time. Once your rate is locked, your payment calculation is set, and the lender cannot change the rate unless you agree to it.

Using an online calculator versus working with a lender

An online mortgage calculator gives you a quick estimate based on the numbers you enter. It is useful for comparing different loan amounts, interest rates, or terms to see how each changes your payment. However, a calculator cannot account for all the costs and details of your specific situation — it does not know your exact property taxes, your insurance quote, or whether you will have PMI.

A lender can give you a more precise estimate because they have access to your credit score, can pull your property tax information, and can calculate your exact PMI cost. When you get a loan estimate from a lender (which they are required to provide within three business days of your application), it includes all these details and shows your true monthly payment, not just a rough estimate.

Start with an online calculator to explore your options and see how different scenarios change your payment. Once you have narrowed down what you are looking for, get a loan estimate from a lender to see the real numbers for your specific situation. The loan estimate is the document that matters for your actual decision.

Frequently Asked Questions

Does my monthly payment include property taxes and insurance?

Only if your lender requires an escrow account, which is typical when you put down less than 20 percent. If you put down 20 percent or more, you may be able to pay taxes and insurance separately. Check your loan estimate to see whether escrow is required and what portion of your payment goes to taxes and insurance.

What happens to my payment if interest rates drop after I lock in my rate?

Your payment stays the same — you are locked into the rate you agreed to. However, you may be able to refinance later if rates drop significantly and it makes financial sense to do so. Refinancing involves applying for a new loan to pay off the old one, and it comes with closing costs, so it only makes sense if the savings are large enough to cover those costs.

Can I pay extra toward principal without changing my monthly payment?

Yes. Your monthly payment is the minimum you owe, but you can pay more anytime. Any amount over your regular payment goes directly toward principal and reduces the interest you pay over the life of the loan. Specify that the extra money should go to principal, not to future payments, so you actually shorten the loan term.

How much does PMI cost per month?

PMI typically costs between 0.5 and 1.5 percent of your loan amount per year, depending on your down payment and credit score. On a $240,000 loan, that is roughly $100 to $300 per month. Your lender will tell you the exact PMI cost when you get your loan estimate.

What if I want to know my payment before I have an interest rate?

Use the current average interest rate for your area and loan type as a placeholder. Mortgage rates change daily, so check a financial website or call a few lenders to see what rates are being offered. Your actual rate will depend on your credit score, down payment, and the specific lender, but using a current average gives you a realistic ballpark number.