The basic formula: principal, interest rate, and loan term
Your monthly mortgage payment is determined by three numbers: the amount you borrow (the principal), the interest rate your lender charges, and how many years you have to repay it. A larger loan or higher interest rate raises your payment. A longer loan term spreads the cost over more months, which lowers each payment but increases the total interest you pay over the life of the loan.
The calculation itself is done by lenders using a standard amortization formula, but you do not need to do the math by hand. Mortgage calculators—available free from most banks, the Consumer Financial Protection Bureau, and financial websites—will show you the payment in seconds once you enter those three numbers.
The payment amount also includes property taxes and homeowners insurance if you put down less than 20 percent. Your lender will collect these costs monthly and hold them in an escrow account, then pay the bills on your behalf. This combined payment is called PITI: principal, interest, taxes, and insurance.
Key Takeaways
- Your monthly payment depends on the loan amount, interest rate, and loan term—a mortgage calculator will show you the exact figure once you enter these three numbers.
- If you are putting down less than 20 percent, your payment will also include property taxes, homeowners insurance, and possibly mortgage insurance (PMI), all collected by your lender each month.
- The interest rate you receive depends on your credit score, down payment size, loan type, and current market rates, and it can vary between lenders.
- Your actual payment may be higher than the calculator shows if you live in a high-tax area, have a new home that needs insurance, or are required to carry mortgage insurance.
- Refinancing later can lower your payment if interest rates drop or your credit score improves, but it involves closing costs and a new loan term.
How interest rates affect your payment
A difference of even 0.5 percent in interest rate can change your monthly payment by $100 or more on a typical loan. The rate you receive depends on several factors: your credit score, the size of your down payment, the type of loan (fixed-rate or adjustable-rate), the lender you choose, and the current market rate for mortgages.
You can see current rates from multiple lenders by getting quotes, which most lenders provide for free without affecting your credit score. Comparing rates across at least three lenders is standard practice and takes a few hours. The difference between the highest and lowest rate you receive can easily save or cost you tens of thousands of dollars over the life of the loan.
Fixed-rate mortgages lock in your interest rate for the entire loan term—usually 15, 20, or 30 years. Adjustable-rate mortgages (ARMs) start with a lower rate that is fixed for a set period (often 3, 5, 7, or 10 years), then adjust periodically based on market conditions. ARMs carry the risk that your payment will rise sharply when the fixed period ends.
What happens when you put down less than 20 percent
If your down payment is smaller than 20 percent of the home's purchase price, your lender will require you to carry private mortgage insurance (PMI). This insurance protects the lender if you stop paying, and the cost is added to your monthly payment. PMI typically runs between 0.5 and 1.5 percent of the loan amount per year, divided into 12 monthly payments.
On a $300,000 loan, PMI might add $125 to $375 per month to your payment. You can remove PMI once you have paid down the loan to 80 percent of the home's original purchase price, though you usually have to request it—your lender will not remove it automatically. Some loans allow you to remove PMI sooner if your home has appreciated in value.
Your payment will also include property taxes and homeowners insurance, which vary widely by location and home value. A home in a high-tax county or in an area with expensive insurance can add $300 to $800 or more per month to your payment compared to the same loan in a lower-cost area.
Using a mortgage calculator to estimate your payment
A mortgage calculator asks for four pieces of information: the loan amount, the interest rate, the loan term in years, and sometimes your down payment size and location (to estimate taxes and insurance). Enter these numbers and the calculator shows your monthly payment broken down by principal, interest, taxes, insurance, and PMI if applicable.
Most calculators also show an amortization schedule, which lists how much of each payment goes toward principal versus interest over time. Early in the loan, most of your payment covers interest. As years pass, more of each payment reduces the principal. This schedule helps you understand why paying extra toward principal early in the loan saves significant interest later.
Free calculators are available from Bankrate, NerdWallet, the Consumer Financial Protection Bureau, and most major lenders' websites. They all use the same underlying math, so the results should be nearly identical. If you are comparing loan offers from different lenders, use the same calculator for each to keep the comparison fair.
How down payment size changes your payment
A larger down payment lowers your monthly payment in two ways: it reduces the loan amount (so you borrow less), and it may eliminate PMI if you put down 20 percent or more. On a $400,000 home, the difference between a 10 percent down payment ($40,000) and a 20 percent down payment ($80,000) is a $40,000 smaller loan, which lowers your monthly payment by roughly $200 to $250 depending on the interest rate and loan term.
Putting down 20 percent also saves you the cost of PMI, which on that same loan could add another $200 to $400 per month. The total savings from a larger down payment can be $400 to $650 per month or more. However, a larger down payment means less cash available for closing costs, home repairs, and emergencies, so the right down payment size depends on your overall financial situation.
Loan term and how it shapes your payment
A 15-year mortgage has a higher monthly payment than a 30-year mortgage on the same loan amount and interest rate, because you are repaying the debt in half the time. On a $300,000 loan at 6.5 percent interest, a 30-year term costs roughly $1,900 per month, while a 15-year term costs roughly $2,900 per month—about $1,000 more each month.
However, the 15-year loan saves you roughly $200,000 in total interest over the life of the loan compared to the 30-year loan. The 30-year loan is more affordable month-to-month but costs significantly more in total. The 20-year loan sits between these two options. Your choice depends on whether you prioritize a lower monthly payment or paying off the home faster and saving on interest.
Some borrowers choose a 30-year loan for the lower payment, then pay extra toward principal when they can afford it. This approach gives you flexibility: you can make the minimum payment in tight months and pay extra when your income is higher. Making even one extra principal payment per year can shorten your loan term by several years.
What your actual payment might include beyond the calculator
A basic mortgage calculator shows principal, interest, taxes, insurance, and PMI. Your actual payment statement may also include homeowners association (HOA) fees if you live in a planned community, and possibly flood insurance if your home is in a flood zone. These costs are not always included in online calculators, so ask your lender for a complete estimate before you commit to a loan.
Your lender is required to provide a Loan Estimate within three business days of your application. This document shows your projected monthly payment, all closing costs, and the interest rate you have been offered. Compare the Loan Estimate to the calculator results to see what the calculator missed. The Loan Estimate is the most accurate picture of what you will actually pay each month.
Property taxes and insurance can also change after you buy the home. If your home is reassessed for tax purposes or your insurance company raises rates, your monthly payment will increase. Lenders typically review and adjust your escrow account annually to account for these changes.
Refinancing if rates drop or your situation changes
If interest rates fall after you take out your mortgage, you can refinance—pay off your current loan with a new loan at a lower rate. This lowers your monthly payment and can save tens of thousands in interest over the remaining loan term. However, refinancing involves closing costs (typically 2 to 5 percent of the loan amount), so you need to stay in the home long enough for the monthly savings to cover those costs.
You can also refinance to change your loan term. Some borrowers refinance from a 30-year loan to a 15-year loan once their income increases, or from a 15-year loan to a 30-year loan if their financial situation tightens. Each refinance is a new loan with new closing costs, so calculate whether the change is worth the upfront expense.
Refinancing also requires a credit check and a new appraisal, and it resets the clock on your loan term. If you refinance a 30-year loan after 10 years into a new 30-year loan, you will not pay off the home until 40 years after you originally borrowed the money—unless you shorten the term.
Frequently Asked Questions
How much does a $300,000 mortgage cost per month?
On a $300,000 loan at 6.5 percent interest over 30 years, the principal and interest payment is roughly $1,900 per month. Add property taxes, homeowners insurance, and PMI (if your down payment was less than 20 percent), and your total payment could be $2,300 to $2,700 per month depending on your location and down payment size. Use a calculator with your actual interest rate and location to see your specific payment.
Does my credit score affect my mortgage payment?
Your credit score does not change the payment formula, but it determines the interest rate you receive. Borrowers with higher credit scores receive lower interest rates, which lowers the monthly payment. A 50-point difference in credit score can mean 0.25 to 0.5 percent difference in interest rate, which translates to $75 to $150 per month on a typical loan.
Can I pay off my mortgage faster without refinancing?
Yes. You can make extra payments toward principal without refinancing. Even one extra principal payment per year can shorten your loan term by several years and save tens of thousands in interest. Ask your lender whether there are any penalties for early repayment before you start making extra payments.
What if I want to know my payment before I find a house?
Use a mortgage calculator with an estimated interest rate based on current market rates and your expected credit score. Most lenders publish current rates on their websites. Enter a loan amount you think you can afford, and the calculator will show you the payment. This gives you a ballpark figure to use when house hunting, though your actual rate may be different once you apply.
How often can my payment change if I have an adjustable-rate mortgage?
Adjustable-rate mortgages have a fixed rate for an initial period (3, 5, 7, or 10 years), then adjust annually or every few years depending on the loan terms. When the rate adjusts, your monthly payment changes. Some ARMs have rate caps that limit how much the rate can rise per adjustment and over the life of the loan, but your payment can still increase significantly.