How Your Monthly Mortgage Payment Is Calculated
The formula behind your monthly payment
Your monthly mortgage payment is calculated using a standard formula that takes three pieces of information: the loan amount you borrowed, the interest rate, and the number of months you have to repay it. The Consumer Financial Protection Bureau (CFPB) publishes this formula so borrowers can verify their payments are correct and understand what they're paying for each month.
The formula is: M = P [ r(1 + r)^n ] / [ (1 + r)^n – 1 ], where M is your monthly payment, P is the principal (the amount borrowed), r is the monthly interest rate (annual rate divided by 12), and n is the total number of payments. This produces the same payment amount every month for a fixed-rate mortgage, which is why your payment stays the same from month one through the final payment.
You do not need to do this math yourself—your lender calculates it and shows you the result before you sign. But understanding how it works helps you see why a higher interest rate or longer loan term increases your monthly payment, and why paying extra principal early in the loan saves you thousands in interest over time.
Key Takeaways
- The CFPB formula uses your loan amount, interest rate, and loan term to produce a fixed monthly payment that stays the same for the life of a fixed-rate mortgage.
- Your lender must show you the calculated payment before closing, and you can verify it using the same formula or an online calculator.
- A higher interest rate or longer loan term increases your monthly payment; a shorter term or lower rate decreases it.
- The first payments go mostly toward interest; later payments go mostly toward principal, which is why paying extra early saves significant interest.
- The formula assumes you make one payment per month; biweekly or accelerated payment plans change how interest accrues and how fast you build equity.
What each part of the formula means
The principal (P) is the amount of money you borrowed. If you put down 20 percent on a $300,000 house, your principal is $240,000. This number does not change during the life of the loan—only the portion of your payment that goes toward it changes.
The monthly interest rate (r) is your annual interest rate divided by 12. If your rate is 6.5 percent per year, your monthly rate is 0.065 ÷ 12, or about 0.00542. Lenders use this smaller number because interest compounds monthly, not annually. A higher rate here makes your monthly payment larger.
The number of payments (n) is how many months you have to repay the loan. A 30-year mortgage has 360 payments; a 15-year mortgage has 180. A longer loan term spreads the principal over more payments, which lowers your monthly payment but increases the total interest you pay over the life of the loan.
The exponent in the formula—(1 + r)^n—accounts for how interest compounds over time. This is why the formula produces a payment that covers both principal and interest in a way that keeps your payment the same every month, even though the split between principal and interest changes.
Why your payment stays the same but the split changes
Every month, your lender calculates the interest owed on the remaining balance. Early in the loan, the balance is high, so most of your payment goes to interest. As you pay down the principal, the balance shrinks, so less of each payment goes to interest and more goes to principal. By the end of the loan, almost all of your payment is principal.
This is shown in an amortization schedule, a month-by-month breakdown that your lender must provide at closing. On a $240,000 loan at 6.5 percent over 30 years, your payment is roughly $1,520. In month one, about $1,300 goes to interest and $220 to principal. In month 360 (the final payment), almost all $1,520 goes to principal because the balance is nearly zero.
This front-loaded interest is why paying extra principal early in the loan has such a large effect. An extra $100 per month in the first year saves you tens of thousands in interest over 30 years, because that $100 reduces the balance on which future interest is calculated. The same $100 extra in year 25 saves much less, because there are fewer years left for compounding to work.
How to verify your lender's calculation
Your lender is required to show you the payment amount and the amortization schedule before you close. You can verify the math using the CFPB formula, a mortgage calculator (many are free online), or by asking your lender to walk you through the numbers.
To check it yourself: plug in your principal, annual interest rate, and loan term into a calculator. The result should match what your lender quoted. If it does not, ask your lender to explain the difference—it may be because taxes, insurance, or HOA fees are included in the quoted payment, or because the lender is using a slightly different rate or term than you thought.
The CFPB website includes resources and calculators that use this same formula. Your state's attorney general's office or housing authority may also have tools to help you verify your numbers before you sign.
What changes your monthly payment
The three inputs to the formula—principal, rate, and term—are the only things that affect your base mortgage payment. If any of these changes, your payment changes.
A lower interest rate reduces your payment. If rates drop and you refinance, your new payment is calculated using the new rate, which is why refinancing can lower your monthly cost. A shorter loan term also reduces the total interest you pay, but increases your monthly payment because you are paying off the principal faster. A larger principal (borrowing more money) increases your payment proportionally.
Taxes, homeowners insurance, and HOA fees are separate from the mortgage payment itself, though lenders often bundle them into a single "PITI" payment (principal, interest, taxes, insurance). These do not follow the amortization formula—they are added on top and may change year to year. Your lender must show you these separately so you know what is going to the mortgage and what is going elsewhere.
Fixed-rate versus adjustable-rate mortgages
The CFPB formula applies to fixed-rate mortgages, where the interest rate and payment stay the same for the entire loan. This is the most common type and the easiest to budget for, because you know your payment will not change.
An adjustable-rate mortgage (ARM) starts with a lower rate for a set period (often 3, 5, 7, or 10 years), then adjusts annually or semi-annually based on market conditions. During the fixed period, the formula works the same way. When the rate adjusts, the lender recalculates your payment using the new rate, the remaining principal, and the remaining term. This can increase your payment significantly, which is why ARMs carry more risk than fixed-rate loans.
If you have an ARM, your lender must tell you the rate cap (the highest your rate can go) and the adjustment schedule before you close. Understanding the formula helps you estimate what your payment might be after an adjustment, so you can plan ahead.
How extra payments affect the formula
The standard formula assumes you make one payment per month for the full term. If you pay extra principal, you reduce the balance faster, which shortens the loan and saves interest—but it does not change the formula itself. Instead, it changes the inputs: your remaining principal shrinks, so your next payment (if you recalculate) would be lower, or your loan ends sooner.
Some borrowers make biweekly payments (half the monthly payment every two weeks) instead of one monthly payment. This results in 26 half-payments per year, which is equivalent to 13 full monthly payments. Over time, this extra payment per year shortens the loan and saves interest. However, not all lenders allow biweekly payments, and some charge a fee to set them up. Ask your lender whether this option is available and whether there are costs.
If you refinance—taking out a new loan to pay off the old one—the formula recalculates using the new rate, the remaining principal, and a new term. This is a separate transaction, and your new payment is based on the new loan, not the original one.
Where to find the CFPB's resources
The CFPB publishes guides and calculators on its website (consumerfinance.gov) that explain how mortgages work and let you see how changes to rate, term, or principal affect your payment. These tools are free and do not require you to enter personal information.
Your lender is also required to provide you with a Loan Estimate within three business days of your application. This document shows the calculated payment, the amortization schedule, and all fees. You can use this to compare offers from different lenders and to verify that the numbers match the formula.
If you believe your payment is calculated incorrectly, contact your lender first. If they cannot explain it or you believe there is an error, you can file a complaint with the CFPB through its website.
Frequently Asked Questions
Can I use the CFPB formula to calculate my own mortgage payment?
Yes. If you know your loan amount, annual interest rate, and loan term in months, you can plug those numbers into the formula or use any online mortgage calculator that uses the same formula. The result should match what your lender quoted. Most people find a calculator easier than doing the math by hand.
Why does my payment include taxes and insurance if the formula only uses rate, principal, and term?
The formula calculates only the mortgage payment itself (principal and interest). Taxes, insurance, and HOA fees are separate costs that lenders often collect from you each month and hold in an escrow account, then pay on your behalf. Your lender must show these separately on your Loan Estimate so you know what is going where.
If I pay extra principal, does my monthly payment go down automatically?
No. Your required monthly payment stays the same unless you refinance or your loan adjusts (if you have an ARM). Extra principal payments reduce your balance, which means you pay off the loan faster and pay less total interest, but your regular payment does not change. Some lenders let you apply extra payments directly to principal; ask yours how to do this.
What happens to the formula if I refinance?
Refinancing creates a new loan with a new calculation. The formula uses your new loan amount (usually the remaining balance), your new interest rate, and a new term. Your new payment is based on these new numbers, not your original loan. This is why refinancing can lower your payment even if rates have not changed—if you shorten the term, you pay it off faster.
Does the formula work the same way for 15-year and 30-year mortgages?
Yes, the formula is the same. The only difference is the input: a 15-year mortgage has 180 payments instead of 360. This shorter term means a higher monthly payment but much less total interest paid over the life of the loan. You can use the formula to compare what your payment would be at different terms.