The basic formula: principal, interest rate, and loan term
Your monthly mortgage payment depends on three numbers: how much you borrowed (the principal), the interest rate your lender charges, and how many months you have to repay it (the loan term). A larger loan or higher interest rate raises your payment. A longer term spreads the cost over more months, lowering each payment — but you pay more interest overall.
The actual calculation uses a formula that accounts for how interest compounds monthly. You do not need to do this by hand. Your lender will show you the payment before you sign, and you can calculate it yourself using an online mortgage calculator or a spreadsheet.
Key Takeaways
- Your monthly payment is determined by the loan amount, interest rate, and number of years to repay, and you can calculate it using a free online calculator or spreadsheet formula.
- The standard mortgage calculator shows principal and interest only; property taxes, homeowners insurance, and HOA fees are separate and vary by location and property.
- A lower interest rate or longer loan term reduces your monthly payment, but a longer term means paying significantly more interest over the life of the loan.
- Your lender is required to provide a Loan Estimate within three business days of your application, which shows your exact payment and all costs before you commit.
Using an online calculator to find your payment
The fastest way to see what your payment would be is to use a free mortgage calculator. You enter the loan amount, interest rate, and loan term (usually 15, 20, or 30 years), and the calculator shows your monthly principal and interest payment in seconds.
Most calculators also let you add property taxes, homeowners insurance, and HOA fees if you know them. These are not part of your base mortgage payment but are often rolled into your total monthly housing cost. Property tax rates vary by county and city, so you will need to look up the rate for the specific property or area you are considering.
Understanding the Loan Estimate from your lender
Once you apply for a mortgage, your lender must send you a Loan Estimate within three business days. This document shows your exact monthly payment for principal and interest, plus estimates for property taxes, insurance, and any other costs. It is the most accurate picture of what you will actually pay each month.
The Loan Estimate also breaks down all upfront costs — origination fees, appraisal, title search, and closing costs. Read it carefully to see where your money goes. If numbers look wrong or higher than you expected, ask your lender to explain them before you move forward.
How interest rate changes affect your payment
Even a small change in interest rate shifts your monthly payment noticeably. On a $300,000 loan over 30 years, the difference between a 6% rate and a 7% rate is roughly $200 per month. Over 30 years, that adds up to more than $70,000 in extra interest paid.
If you are shopping for a mortgage, compare interest rates from multiple lenders. A lower rate saves you money every month for the entire life of the loan. Some lenders offer the option to buy down your rate by paying points upfront — each point costs 1% of the loan amount and typically lowers your rate by 0.25%. Whether this makes sense depends on how long you plan to stay in the home.
How loan term affects your payment and total interest
A 15-year mortgage has a higher monthly payment than a 30-year mortgage on the same loan amount, but you pay off the debt faster and pay far less interest overall. On a $300,000 loan at 6.5%, a 30-year term costs about $1,896 per month and totals roughly $682,000 in interest. A 15-year term costs about $2,596 per month but totals only about $167,000 in interest.
Choose a 15-year term if you can afford the higher payment and want to build equity faster. Choose 30 years if you need the lower monthly payment or want to keep cash available for other goals. Some borrowers do a 30-year mortgage but pay extra toward principal each month, which gives them flexibility if their finances tighten.
What is not included in your mortgage payment
Your base mortgage payment covers only principal and interest. It does not include property taxes, homeowners insurance, or HOA fees. Many lenders bundle these into a single monthly payment called PITI (Principal, Interest, Taxes, Insurance), but they are technically separate.
Property taxes vary widely by location — from less than 0.5% of home value per year in some states to over 2% in others. Homeowners insurance depends on the home's value, location, and your coverage choices. If you put down less than 20%, your lender will also require mortgage insurance (PMI), which protects the lender if you default. All of these costs should appear on your Loan Estimate.
Using a spreadsheet if you want to see the math
If you prefer to build your own calculation, most spreadsheet programs (Excel, Google Sheets) have a built-in mortgage payment function. In Excel, the formula is =PMT(rate, nper, pv), where rate is your monthly interest rate (annual rate divided by 12), nper is the total number of months, and pv is the loan amount as a negative number.
For example, a $300,000 loan at 6.5% annual interest over 30 years would be =PMT(0.065/12, 360, -300000). The result is your monthly principal and interest payment. This method is useful if you want to test different scenarios quickly or understand exactly how the payment is calculated.
Frequently Asked Questions
Does my mortgage payment change every month?
No, on a fixed-rate mortgage your principal and interest payment stays the same for the entire loan term. Property taxes and insurance may increase over time, which would raise your total monthly payment, but the mortgage portion itself does not change.
What is the difference between a fixed rate and an adjustable rate?
A fixed-rate mortgage has the same interest rate for the entire loan. An adjustable-rate mortgage (ARM) has a lower rate for the first few years, then adjusts periodically based on market conditions. Your payment can rise significantly after the initial period. Fixed rates are more predictable; ARMs can save money short-term but carry risk if rates spike.
Can I pay off my mortgage early without a penalty?
Most mortgages allow you to pay extra toward principal without penalty. Paying extra reduces the total interest you pay and shortens the loan term. Check your loan documents or ask your lender to confirm there is no prepayment penalty, though these are rare on mortgages.
How much of my payment goes to principal versus interest?
Early in the loan, most of your payment goes to interest. As you pay down the principal, more of each payment goes toward principal. Your lender's amortization schedule shows exactly how much of each payment goes to each. You can also find amortization calculators online that break this down month by month.
What if I want to lower my monthly payment?
You can refinance to a longer term, lock in a lower interest rate if rates have dropped, or buy down your rate by paying points upfront. Refinancing has closing costs, so calculate whether the monthly savings justify the upfront expense. Generally, you need to stay in the home long enough for the savings to exceed the costs.