The basic 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. Lenders use a standard formula to turn those three numbers into a monthly payment amount. You do not need to memorize the formula—a calculator or spreadsheet can do it—but understanding what goes into it helps you see why your payment is what it is.
The formula accounts for the fact that you pay interest every month on whatever balance remains. Early payments go mostly toward interest; later payments go mostly toward principal. The formula spreads that uneven split across all your payments so each month's payment is the same amount.
If you know your loan amount, interest rate, and loan term in months, you can calculate the payment yourself using a mortgage calculator, a spreadsheet formula, or by hand. Most people use a calculator because the hand math is tedious and error-prone.
Key Takeaways
- Monthly payment depends on loan amount, interest rate, and loan term—change any one and the payment changes.
- Free online mortgage calculators let you enter those three numbers and see the monthly payment when ready.
- Your actual monthly payment may be higher than the principal-and-interest number because it often includes property taxes, homeowners insurance, and mortgage insurance.
- The interest rate you receive depends on your credit score, down payment size, loan type, and current market rates.
- Paying extra toward principal each month shortens the loan term and reduces total interest paid over the life of the loan.
Using an online calculator
The fastest way to see your payment is to use a free mortgage calculator. You enter the loan amount (the price of the home minus your down payment), the interest rate, and the loan term in years. The calculator shows you the monthly principal-and-interest payment in seconds.
Most calculators also let you add property taxes, homeowners insurance, and mortgage insurance if you want to see the full monthly housing cost. These amounts vary by location and your situation, so the calculator may ask for estimates or let you enter actual numbers from your lender or insurance quotes.
Reputable calculators include those from Bankrate, NerdWallet, and the Consumer Financial Protection Bureau. Your lender's website usually has one too. They all use the same underlying math, so the payment number should be the same across them.
The spreadsheet formula if you want to build your own
If you use Excel, Google Sheets, or another spreadsheet program, you can build a payment calculator yourself using the PMT function. The formula is: =PMT(rate, nper, pv). Here is what each part means:
Rate is your monthly interest rate. If your annual rate is 6.5%, divide by 12 to get 0.065/12, or about 0.00542 per month. Nper is the total number of monthly payments. A 30-year loan is 360 months; a 15-year loan is 180 months. Pv is the loan amount as a negative number (so if you borrowed $300,000, you enter -300000).
Example: A $300,000 loan at 6.5% over 30 years would be =PMT(0.065/12, 360, -300000). The result is about $1,896 per month in principal and interest.
Why your actual payment may be higher than the calculator shows
The calculator usually shows only principal and interest. Your actual monthly payment to your lender often includes four things, sometimes called PITI: Principal, Interest, Taxes, and Insurance.
Property taxes vary by county and city. Homeowners insurance is required by your lender and covers damage to the structure. Mortgage insurance (PMI) is required if you put down less than 20%. All three are added to your principal-and-interest payment and sent to your lender, who pays the taxes and insurance on your behalf.
If you put down 20% or more, you avoid mortgage insurance, which can save $100 to $300 per month depending on the loan size. Your lender will tell you the exact amounts for taxes and insurance once you are under contract on a specific home.
How interest rate affects your payment
A small change in interest rate creates a large change in your monthly payment. On a $300,000 loan over 30 years, the difference between 5.5% and 6.5% is about $170 per month. Over 30 years, that is more than $61,000 in extra payments.
Your interest rate depends on your credit score, the size of your down payment, the type of loan (conventional, FHA, VA, USDA), and the current market rate for mortgages. You can see current rates from multiple lenders by getting quotes, and you can improve your rate by raising your credit score or putting down a larger down payment before you explore.
Rates change daily based on market conditions. If you see a rate you like, ask your lender about locking it in while you shop for a home. A rate lock usually lasts 30 to 60 days.
What happens when you pay extra toward principal
If you pay more than your monthly payment requires, the extra goes toward principal (not interest). Paying extra shortens your loan term and reduces the total interest you pay over the life of the loan.
For example, on a $300,000 loan at 6.5% over 30 years, the monthly payment is about $1,896. If you pay $2,000 per month instead, the extra $104 goes straight to principal. Over time, this compounds: you pay off the loan faster, which means fewer months of interest charges.
Some lenders charge a prepayment penalty if you pay off the loan early, though this is rare on mortgages. Check your loan documents to see if yours does. If there is no penalty, paying extra is always a way to save money on interest.
Comparing loan terms: 15-year versus 30-year
A 15-year mortgage has a higher monthly payment but costs far less in total interest. A 30-year mortgage has a lower monthly payment but you pay interest for twice as long.
On a $300,000 loan at 6.5%, a 30-year payment is about $1,896 per month and total interest is roughly $382,000. A 15-year payment on the same loan is about $2,596 per month, but total interest is only about $167,000. You save over $215,000 in interest by choosing the shorter term, but your monthly payment is $700 higher.
The right choice depends on your budget and goals. If you can afford the higher payment and want to build equity faster, a 15-year loan makes sense. If you need the lower monthly payment to stay within your budget, a 30-year loan is the right choice even though you pay more interest overall.
Frequently Asked Questions
Does the calculator include property taxes and insurance?
Most basic calculators show only principal and interest. You can usually add property taxes, homeowners insurance, and mortgage insurance as separate fields to see the full monthly cost. If the calculator does not have those fields, you can add those amounts to the principal-and-interest number yourself.
What if I want to pay off my mortgage early?
You can pay extra toward principal any time without penalty on most mortgages. Contact your lender to confirm there is no prepayment penalty, then specify that extra payments go toward principal, not the next month's payment. Paying even $50 or $100 extra per month shortens the loan term and saves interest.
How do I know what interest rate I will actually get?
You get a rate quote from your lender based on your credit score, down payment, loan type, and current market rates. Rates vary between lenders, so get quotes from at least two or three. Once you are under contract on a home, you can lock in a rate for 30 to 60 days while you finish the purchase process.
Can I change my payment amount after I close?
You cannot change the required monthly payment without refinancing the loan. You can pay extra toward principal any month you want, but your lender will still expect the regular payment. If you want a lower payment, you would need to refinance into a longer-term loan, which costs money and resets your interest clock.
What is the difference between a fixed rate and an adjustable rate?
A fixed-rate mortgage keeps the same interest rate and payment for the entire loan term. An adjustable-rate mortgage (ARM) has a lower rate for the first few years, then adjusts up or down based on market conditions. ARMs are riskier because your payment can increase significantly after the initial period. Most people choose fixed-rate mortgages for predictability.