A fixed-rate mortgage payment is calculated using a standard formula that factors in your loan amount, interest rate, and loan term. The formula gives you the principal and interest portion of your payment, but your actual monthly bill also includes property taxes, homeowners insurance, and sometimes mortgage insurance. Here’s how to work through each piece.
The Standard Payment Formula
The formula for a fixed-rate mortgage payment looks like this:
M = P × [ r(1 + r)ⁿ / ((1 + r)ⁿ − 1) ]
- M = your monthly payment (principal and interest only)
- P = the principal, meaning the amount you’re borrowing
- r = your monthly interest rate (annual rate divided by 12)
- n = the total number of monthly payments over the life of the loan
Two conversions trip people up. First, you need to convert your annual interest rate to a monthly one. If your rate is 6%, divide by 12 to get 0.005. Second, convert your loan term into months. A 30-year mortgage has 360 payments (30 × 12), and a 15-year mortgage has 180.
A Worked Example
Say you’re borrowing $300,000 at a 6.5% annual interest rate on a 30-year fixed mortgage. Start by converting your variables:
- P = $300,000
- r = 0.065 / 12 = 0.005417
- n = 30 × 12 = 360
Plug those into the formula. First, calculate (1 + r)ⁿ: that’s (1.005417)³⁶⁰, which equals roughly 6.9916. Then the numerator is r × 6.9916, or 0.005417 × 6.9916 = 0.03788. The denominator is 6.9916 − 1 = 5.9916. Divide the numerator by the denominator: 0.03788 / 5.9916 = 0.006322. Multiply by the principal: $300,000 × 0.006322 = approximately $1,896.70 per month.
That $1,896.70 covers only the principal and interest. Your actual monthly obligation will be higher once taxes and insurance are added.
What PITI Means for Your Real Payment
Lenders and housing professionals refer to “PITI,” which stands for principal, interest, taxes, and insurance. These are the four core components of a monthly mortgage payment. The formula above handles the first two. Here’s how the other two work.
Property taxes are based on a percentage of your home’s assessed value. Rates vary widely by location. The national median effective property tax rate is 0.89%, though it can range from under 0.3% in some areas to nearly 2% in others. To estimate your monthly property tax, multiply your home’s value by the local tax rate, then divide by 12. On a $375,000 home in an area with a 1% rate, that’s $3,750 per year, or about $313 per month.
Homeowners insurance covers damage to your property and can range from a few hundred dollars to several thousand per year, depending on the home’s size, age, location, and the coverage you choose. Divide your annual premium by 12 to get the monthly figure. If your annual premium is $1,800, that adds $150 per month.
Many lenders collect taxes and insurance through an escrow account. Instead of you paying these bills directly, the lender bundles them into your monthly payment and makes the payments on your behalf when they come due.
When Private Mortgage Insurance Applies
If your down payment is less than 20% of the home’s purchase price, your lender will typically require private mortgage insurance, or PMI. This protects the lender (not you) in case you default. PMI costs generally range from 0.46% to 1.50% of the original loan amount per year, according to the Urban Institute’s Housing Finance Policy Center.
On that $300,000 loan, PMI at 0.75% would cost $2,250 per year, or about $188 per month. That gets added to your PITI payment. Once you’ve built up 20% equity in the home, you can typically request that PMI be removed.
Putting the Full Payment Together
Using the numbers from the examples above, here’s what a complete monthly payment might look like on a $300,000 loan for a $375,000 home with less than 20% down:
- Principal and interest: $1,897
- Property taxes: $313
- Homeowners insurance: $150
- PMI: $188
- Total: approximately $2,548
That’s a significant jump from the $1,897 you’d get using the formula alone. When budgeting for a home purchase, always account for the full PITI payment plus any PMI, not just the principal and interest figure.
How Loan Term Affects the Payment
Shorter loan terms produce higher monthly payments but dramatically lower total interest costs. Using the same $300,000 loan at 6.5%, switching from a 30-year term to a 15-year term (n = 180) changes the principal and interest payment from about $1,897 to roughly $2,613. That’s $716 more per month, but over the life of the loan, you’d pay around $170,000 less in total interest because you’re borrowing for half as long and the balance shrinks faster.
The formula works the same way regardless of term. Just change n to reflect the number of months.
Adjustable-Rate Mortgage Payments
If you have an adjustable-rate mortgage (ARM), the calculation during the initial fixed period works exactly like a fixed-rate loan. The difference comes when the rate adjusts. At that point, your new interest rate is determined by adding two numbers together: an index (a benchmark rate your lender ties the loan to, like the Secured Overnight Financing Rate) and a margin (a fixed percentage set in your loan agreement).
For example, if the index is 4.25% and your margin is 2.75%, your new rate would be 7%. You’d then recalculate your payment using the formula with the new rate, the remaining loan balance as P, and the remaining months as n. Most ARMs include rate caps that limit how much the rate can increase at each adjustment and over the life of the loan, which puts a ceiling on how high your payment can go.
Quick Ways to Run the Numbers
You don’t need to calculate this by hand every time. Online mortgage calculators from sites like Bankrate, NerdWallet, and the Consumer Financial Protection Bureau let you plug in a purchase price, down payment, interest rate, and loan term to see a full payment breakdown including taxes and insurance estimates. Spreadsheet software also has built-in functions. In Excel or Google Sheets, the PMT function handles the formula for you: =PMT(monthly rate, number of payments, loan amount). The result will be negative (representing money going out), so wrap it in ABS() if you want a positive number.
Running the calculation yourself at least once is still worthwhile. It helps you understand exactly how the rate, term, and loan size each push the payment up or down, which puts you in a stronger position when comparing loan offers.