The formula and what each number means
Your monthly mortgage payment depends on three things: the loan amount, the interest rate, and how many months you have to pay it back. The standard formula is:
M = P × [r(1 + r)^n] / [(1 + r)^n − 1]
In this formula, M is your monthly payment, P is the principal (the amount you borrowed), r is your monthly interest rate (the annual rate divided by 12), and n is the total number of payments (years multiplied by 12). The formula accounts for the fact that you pay interest on the remaining balance each month, not on the original loan amount all at once.
The monthly interest rate is not the same as the annual rate. If your loan has a 6% annual interest rate, divide 6 by 100 to get 0.06, then divide that by 12 to get 0.005 as your monthly rate. This small monthly rate compounds over time, which is why the formula uses exponents.
Key Takeaways
- Your monthly payment is calculated from the loan amount, monthly interest rate, and total number of payments using a standard formula that accounts for compound interest.
- A higher interest rate increases your monthly payment and the total amount you pay over the life of the loan.
- You can calculate your payment by hand using the formula, or use a mortgage calculator, a spreadsheet, or your lender's tools to avoid arithmetic errors.
- The payment stays the same each month for a fixed-rate mortgage, but adjustable-rate mortgages change when the interest rate resets.
- Your actual monthly payment may be higher than the calculated amount because it often includes property taxes, insurance, and HOA fees.
Working through a real example
Say you borrow $300,000 at 6.5% annual interest over 30 years. First, convert the annual rate to a monthly rate: 6.5 ÷ 100 = 0.065, then 0.065 ÷ 12 = 0.00542 (rounded). The number of payments is 30 years × 12 months = 360.
Plugging into the formula: M = 300,000 × [0.00542(1.00542)^360] / [(1.00542)^360 − 1]. The term (1.00542)^360 equals approximately 6.898. So M = 300,000 × [0.00542 × 6.898] / [6.898 − 1] = 300,000 × [0.03738] / [5.898] = 300,000 × 0.00634 = $1,902 per month.
This is the principal and interest payment only. Your actual monthly bill will be higher if your lender collects property taxes, homeowners insurance, or mortgage insurance as part of an escrow account.
How interest rate changes affect your payment
A small change in the interest rate creates a large change in your total cost. Using the same $300,000 loan over 30 years, a 5.5% rate gives a monthly payment of about $1,703, while a 7.5% rate gives about $2,098. That is a difference of $395 per month, or $142,200 over the life of the loan.
The higher the interest rate, the more of each payment goes toward interest rather than principal in the early years. With a 6.5% rate, your first payment includes about $1,625 in interest and $277 in principal. With a 7.5% rate, the first payment includes about $1,875 in interest and only $223 in principal.
This is why the interest rate you lock in matters so much. Even a 0.5% difference compounds over 360 payments and affects how much equity you build in the first years of the loan.
Using calculators and spreadsheets instead of doing it by hand
The formula is correct but tedious to calculate without a tool. Most people use a mortgage calculator (available free on lender websites, real estate sites, and financial calculators), a spreadsheet, or a financial calculator app.
In a spreadsheet like Excel or Google Sheets, use the PMT function: =PMT(rate, nper, pv). The rate is your monthly interest rate (annual rate ÷ 12), nper is the number of payments, and pv is the loan amount as a negative number. For the example above, you would type =PMT(0.00542, 360, -300000) and the spreadsheet returns $1,902.
A mortgage calculator on your lender's website or a third-party site like Bankrate or NerdWallet lets you enter the loan amount, rate, and term, then shows the monthly payment instantly. These tools often also show how much total interest you will pay and let you compare different rates or loan terms side by side.
Fixed-rate versus adjustable-rate mortgages
A fixed-rate mortgage locks in the same interest rate for the entire loan term—15 years, 30 years, or whatever you choose. Your monthly payment never changes. You calculate it once using the formula above, and that payment stays the same from month one to the final payment.
An adjustable-rate mortgage (ARM) starts with a lower introductory rate for a set period (often 3, 5, 7, or 10 years), then the rate adjusts periodically based on market conditions. When the rate changes, your monthly payment recalculates. If rates go up, your payment goes up; if rates go down, your payment goes down. The formula is the same, but you recalculate it each time the rate resets.
ARMs are riskier because you cannot predict your payment after the introductory period ends. If you plan to stay in the home for the full loan term, a fixed rate removes this uncertainty. If you plan to sell or refinance before the rate adjusts, an ARM may offer a lower initial payment.
What happens to your payment breakdown over time
Early in the loan, most of your payment goes to interest. Late in the loan, most goes to principal. This is called amortization. On a $300,000 loan at 6.5% over 30 years, your first payment of $1,902 includes $1,625 in interest and $277 in principal. By payment 180 (halfway through), you pay about $950 in interest and $952 in principal. By the final payment, almost all $1,902 goes to principal because very little balance remains.
This is why paying extra toward principal early in the loan saves you the most interest. An extra $100 per month in year one reduces the total interest you pay by thousands of dollars, because that $100 reduces the balance on which future interest is calculated.
Factors that change your actual monthly bill
The formula calculates principal and interest only. Your actual monthly payment to your lender may include other costs. Property taxes vary by location and are collected by your lender in an escrow account, then paid to your local government. Homeowners insurance is required by lenders and also collected in escrow. Private mortgage insurance (PMI) is added if you put down less than 20% and is removed once you reach 20% equity.
Some loans also include HOA fees if the property is in a homeowners association. These are not part of the mortgage payment formula but are a real monthly cost of homeownership. Ask your lender for a loan estimate, which breaks down all these costs separately so you know what your total monthly payment will actually be.
Frequently Asked Questions
What is the difference between APR and interest rate?
The interest rate is the cost of borrowing the principal. The APR (annual percentage rate) includes the interest rate plus other costs like origination fees, closing costs, and insurance, expressed as an annual rate. For calculating your monthly payment, use the interest rate, not the APR. The APR is useful for comparing loans because it shows the true yearly cost.
Can I calculate my payment if I don't know my exact interest rate yet?
Yes. Use the current market rate for your loan type as an estimate. If you are shopping for a 30-year fixed mortgage, check what rates lenders are offering today and use one of those numbers. Your actual payment will change once you lock in your rate, but this gives you a realistic picture of what to expect.
What if I want to pay off the loan faster than 30 years?
Use a shorter term in the formula. A 15-year mortgage on the same $300,000 at 6.5% costs about $2,596 per month instead of $1,902. You pay more each month but pay off the loan in half the time and pay far less total interest. You can also make extra payments toward principal on a 30-year loan without changing the term.
Does my credit score affect the interest rate I calculate with?
Yes. Lenders offer different rates to different borrowers based on credit score, down payment, loan type, and other factors. A higher credit score usually gets a lower rate. Before you calculate, get a rate quote from your lender or use their rate tool, which factors in your credit profile. This gives you the rate you will actually receive.
What if the interest rate changes after I start paying?
On a fixed-rate mortgage, it does not change—your rate and payment are locked in. On an ARM, the rate resets on a schedule (for example, every 5 years), and your payment recalculates using the new rate. Your lender will notify you before the reset and tell you your new payment amount.