How to Calculate Your Monthly Mortgage Payment
The Basic Formula: Principal, Interest Rate, and Loan Term
Your monthly mortgage payment is calculated using three numbers: the amount you borrowed (called the principal), the interest rate your lender charges, and how many months you have to pay it back (the loan term). The formula banks use is called the amortization formula, and it produces a single monthly payment that covers both principal and interest.
The formula looks like this: M = P [ r(1 + r)^n ] / [ (1 + r)^n – 1 ], where M is your monthly payment, P is the principal, r is your monthly interest rate (annual rate divided by 12), and n is the total number of payments. You do not need to memorize this—a calculator or mortgage payment table does the work—but understanding what each piece means helps you see why changing one number changes your payment.
For example, if you borrow $300,000 at 6.5% annual interest over 30 years, your monthly payment (before taxes, insurance, and fees) would be roughly $1,896. If you borrowed the same amount at 5.5% instead, it would drop to about $1,703. If you kept the 6.5% rate but shortened the loan to 15 years, your payment would jump to about $2,896 per month because you are paying back the same debt in half the time.
Key Takeaways
- Your monthly payment depends on three things: how much you borrowed, your interest rate, and how many years you have to repay it.
- A higher interest rate or shorter loan term raises your monthly payment; a lower rate or longer term lowers it.
- The formula produces only the principal and interest portion—your actual monthly bill also includes property taxes, homeowners insurance, and possibly mortgage insurance.
- You can use an online mortgage calculator, a spreadsheet, or ask your lender to show you the exact payment before you sign.
- Small changes in interest rate have large effects over time: a 1% difference on a $300,000 loan costs roughly $200 more per month.
Why Interest Rate Matters More Than You Might Think
The interest rate is the most powerful lever in the formula. A 1% change in rate does not sound like much, but it compounds over 360 payments (a 30-year loan). On a $300,000 mortgage, moving from 6% to 7% adds about $200 to your monthly payment and costs you roughly $72,000 more over the life of the loan.
This is why shopping for rates across multiple lenders before you commit matters. Different banks offer different rates based on your credit score, down payment size, loan type, and current market conditions. A lender might quote you 6.2%, another 6.5%, and a third 5.9%—all for the same loan amount and term. Over 30 years, that 0.6% difference between the highest and lowest quote could mean $150 to $200 per month in your pocket.
How Loan Term Changes Your Payment
Loan term is the second major variable. Most mortgages are either 15-year or 30-year loans, but 10-year, 20-year, and other terms exist. A shorter term means higher monthly payments but far less interest paid overall. A longer term spreads the debt across more months, lowering your payment but increasing total interest.
Using a $300,000 loan at 6.5% as an example: a 30-year term costs about $1,896 per month and roughly $382,000 in total interest. A 15-year term costs about $2,896 per month but only about $220,000 in total interest. You pay $1,000 more per month with the 15-year loan, but you save $162,000 in interest and own the home free and clear 15 years sooner. The choice depends on your budget and whether you can afford the higher payment.
Principal: What You Actually Borrow
The principal is the amount you borrow from the lender—the sale price minus your down payment. If a house costs $400,000 and you put down $100,000, your principal is $300,000. The larger your down payment, the smaller your principal, and the lower your monthly payment.
Down payment size also affects your interest rate. Lenders typically offer better rates to borrowers who put down 20% or more, because a larger down payment means less risk for the bank. A 10% down payment might come with a rate 0.25% to 0.5% higher than a 20% down payment on the same loan. That rate bump, combined with the larger principal, makes a real difference in your monthly cost.
What the Formula Does Not Include
The amortization formula calculates only principal and interest. Your actual monthly mortgage bill—called a PITI payment (Principal, Interest, Taxes, Insurance)—also includes property taxes, homeowners insurance, and possibly private mortgage insurance (PMI) if your down payment was less than 20%.
Property taxes vary widely by location and are paid to your city or county. Homeowners insurance protects the lender's investment and is required by all mortgage lenders. PMI is an extra monthly fee that protects the lender if you default; it typically costs 0.5% to 1% of your loan amount per year and disappears once you have paid down the principal to 80% of the home's value. Your lender will bundle these into your monthly payment, so your actual bill is always higher than what the formula alone produces.
Using a Calculator Versus Doing It by Hand
You can calculate your payment three ways: using the formula by hand (tedious and error-prone), using a spreadsheet with the PMT function, or using an online mortgage calculator. Most people use a calculator because it is fast and accurate.
If you use a spreadsheet, the PMT function works like this: =PMT(rate, nper, pv), where rate is your monthly interest rate (annual rate ÷ 12), nper is the number of payments (years × 12), and pv is the loan amount as a negative number. For a $300,000 loan at 6.5% over 30 years, you would enter =PMT(0.065/12, 360, -300000) and get $1,896.
Online calculators ask you to enter the loan amount, interest rate, and term, then show you the monthly payment instantly. Many also let you adjust numbers to see how changes affect your payment, which is useful for comparing different scenarios before you apply for a mortgage.
How Amortization Schedules Show Principal Versus Interest Over Time
Your monthly payment stays the same for the life of the loan, but the split between principal and interest changes every month. Early in the loan, most of your payment goes to interest; late in the loan, most goes to principal. This breakdown is shown in an amortization schedule, a month-by-month table your lender provides.
On a $300,000 loan at 6.5% over 30 years, your first payment of $1,896 might include $1,625 in interest and only $271 in principal. By payment 180 (halfway through), you might pay $900 in interest and $996 in principal. By the final payment, nearly all $1,896 goes to principal because so little debt remains. This is why paying extra toward principal early in the loan saves so much interest—you are attacking the balance when interest charges are highest.
Frequently Asked Questions
Does the formula change if I have an adjustable-rate mortgage?
No, the formula stays the same, but the interest rate changes. With an adjustable-rate mortgage (ARM), your rate is fixed for a set period (often 3, 5, 7, or 10 years), then adjusts annually based on market conditions. Your payment stays the same during the fixed period, then recalculates using the new rate when the adjustment happens. Your lender will tell you the exact adjustment dates and how the new rate will be set.
What if I want to pay off my mortgage early?
The formula does not change, but your timeline does. If you make extra payments toward principal, you reduce the balance faster and pay less total interest. Some lenders allow you to make extra payments without penalty; others charge a prepayment penalty if you pay off the loan early. Check your loan documents or ask your lender before sending extra money.
How do I know if my lender calculated my payment correctly?
Use an online calculator or spreadsheet to verify. Enter your loan amount, interest rate, and term, and compare the result to what your lender quoted. They should match (within a few dollars). If they do not, ask your lender to explain the difference—it may be that taxes, insurance, or PMI are included in their quote but not in the calculator.
Can I change my loan term after I sign?
Not without refinancing, which means taking out a new loan to pay off the old one. Refinancing involves new fees and a new application, but it can lower your payment if rates have dropped or raise it if rates have risen. You would only refinance if the savings outweigh the costs, which your lender can calculate for you.
Why do different lenders give me different payment quotes for the same loan?
The formula is the same, but the interest rate varies by lender, your credit score, down payment size, and loan type. A lender might also quote you different rates for a 15-year versus 30-year loan, or charge different fees that affect your total cost. Always ask for a Loan Estimate from each lender—it shows the interest rate, monthly payment, and all fees so you can compare apples to apples.