The basic formula: principal, interest rate, and loan term
Your mortgage payment is determined by three numbers: the amount you borrowed (the principal), the interest rate your lender charges, and how many years you have to repay it (the loan term). The lender uses a standard formula to spread that principal and interest across equal monthly payments over the life of the loan.
The formula is: M = P [ r(1 + r)^n ] / [ (1 + r)^n – 1 ], where M is your monthly payment, P is the principal, r is the monthly interest rate (annual rate divided by 12), and n is the total number of payments. You do not need to do this by hand—a mortgage calculator handles it—but understanding what goes into the number helps you see why changing one variable changes your payment.
For example, a $300,000 loan at 6.5% over 30 years produces a different payment than the same loan over 15 years, even though the interest rate is identical. The shorter term compresses the payments into fewer months, so each one is larger. A higher interest rate on the same principal and term also raises the payment. These relationships are direct and predictable.
Key Takeaways
- Your monthly payment depends on three fixed inputs: how much you borrowed, the interest rate, and the number of years to repay it.
- A mortgage calculator or your lender's loan estimate will show you the exact payment before you sign anything.
- The payment shown on your loan estimate covers only principal and interest; property taxes, insurance, and HOA fees are separate and often added to create your total monthly housing cost.
- Changing the loan term or putting down a larger down payment both lower your monthly payment, but they affect how much interest you pay over the life of the loan.
- Your actual payment may increase over time if your property taxes or insurance premiums rise, even though the principal and interest portion stays the same.
What a mortgage calculator shows you
A mortgage calculator takes the three inputs—loan amount, interest rate, and term—and outputs your monthly payment for principal and interest. Most lenders provide one on their website, and many are free and public (Bankrate, NerdWallet, and the Consumer Financial Protection Bureau all offer them). You enter the numbers and get an instant result.
The calculator shows you only the payment toward principal and interest. It does not include property taxes, homeowners insurance, or HOA fees, which vary by location and property and are added separately. If you want to see your total monthly housing cost, you need to add those separately or use a calculator that includes them.
Run the calculator with different scenarios: a 15-year term versus 30 years, a 5% down payment versus 20%, a 6% interest rate versus 7%. Watching the payment change as you adjust each number teaches you which lever has the biggest effect on affordability.
The loan estimate: what your lender will show you
When you apply for a mortgage, the lender is required by federal law to give you a Loan Estimate within three business days. This document shows your monthly payment for principal and interest, plus estimates for property taxes, homeowners insurance, and any mortgage insurance (PMI) if your down payment is less than 20%. It also lists all fees the lender will charge.
The Loan Estimate is not a final bill—the actual property taxes and insurance may differ slightly when you close—but it is the most accurate picture you will get before signing. Compare Loan Estimates from multiple lenders side by side. The payment for principal and interest should be nearly identical if the loan amount, rate, and term are the same, but fees and insurance estimates can vary.
The Loan Estimate also shows you the Annual Percentage Rate (APR), which includes the interest rate plus certain fees spread across the loan term. The APR is higher than the interest rate and is the better number to use when comparing loans from different lenders.
How down payment size affects your payment
A larger down payment lowers the amount you borrow, which lowers your monthly payment. If a house costs $400,000 and you put down 20% ($80,000), you borrow $320,000. If you put down only 5% ($20,000), you borrow $380,000. The difference in monthly payment is substantial.
A down payment below 20% also triggers Private Mortgage Insurance (PMI), an extra monthly fee that protects the lender if you default. PMI typically costs 0.5% to 1% of the loan amount per year, divided into monthly payments. This fee is added to your monthly bill until you have paid down the loan enough to reach 20% equity in the home. Once you reach that point, you can request to have PMI removed.
Putting down more money upfront reduces both your loan amount and eliminates PMI, making the monthly payment significantly lower. However, it also means less cash in your pocket for other needs. The trade-off between a larger down payment and keeping cash reserves is a personal decision based on your financial situation.
Interest rate changes and adjustable-rate mortgages
Most mortgages are fixed-rate: the interest rate stays the same for the entire loan term, so your principal and interest payment never changes. This makes budgeting predictable. A 30-year fixed mortgage at 6% will have the same payment in year 1 and year 30.
An adjustable-rate mortgage (ARM) starts with a lower interest rate for a set period (often 3, 5, 7, or 10 years), then adjusts periodically based on market conditions. When the rate adjusts upward, your monthly payment increases. ARMs are riskier because you cannot predict your payment after the initial period ends. They make sense only if you plan to sell or refinance before the rate adjusts, or if you are confident you can absorb a higher payment.
If you are comparing a fixed-rate offer to an ARM, use the fixed rate to calculate your payment, not the introductory ARM rate. The introductory rate is temporary and will rise.
What happens to your payment over time
Your principal and interest payment stays the same for the life of a fixed-rate mortgage. However, your total monthly housing payment may increase because property taxes and insurance premiums rise over time. Your lender collects these in an escrow account (a holding account) and pays them on your behalf, so when taxes or insurance go up, your monthly payment goes up too.
Early in the loan, most of your payment goes toward interest; later, more goes toward principal. This is by design. A $300,000 loan at 6.5% over 30 years has a payment of roughly $1,896 per month. In month 1, about $1,625 goes to interest and $271 to principal. By month 360 (the last payment), almost all of it goes to principal. This shift does not change your payment amount, but it changes where your money goes.
If you refinance—taking out a new loan to pay off the old one—you reset this timeline. A refinance can lower your payment if interest rates have dropped, but it also extends the time you spend paying interest unless you shorten the loan term.
Tools and resources for calculating your payment
You do not need to memorize the formula or do the math yourself. Use a mortgage calculator provided by a lender, a financial website, or the Consumer Financial Protection Bureau. Enter your loan amount, interest rate, and term, and the calculator gives you the monthly payment instantly.
Your lender will also provide a Loan Estimate, which is the official document showing your payment and all associated costs. This is the number to trust when making a decision, not an estimate from a third-party calculator. The Loan Estimate is binding on the lender (with limited exceptions for property taxes and insurance) and is what you will see again at closing.
If you are shopping for a mortgage, request Loan Estimates from at least three lenders. Compare the interest rate, the APR, the monthly payment, and the total fees. A lower interest rate does not always mean a lower total cost if one lender charges significantly higher fees.
Frequently Asked Questions
Does the mortgage payment include property taxes and insurance?
The payment shown on a mortgage calculator includes only principal and interest. Your actual monthly bill usually includes property taxes, homeowners insurance, and PMI (if applicable), which are added on top. Your lender's Loan Estimate shows all of these together, so you see your true total monthly cost.
What if I want to pay off my mortgage early?
You can make extra payments toward principal at any time without penalty on most mortgages. Paying extra reduces the total interest you pay and shortens the loan term. However, your regular monthly payment amount does not change unless you refinance. Some lenders allow you to set up a biweekly payment plan, which results in one extra payment per year and speeds up payoff.
How much does a 1% change in interest rate affect my payment?
A 1% increase in interest rate raises your monthly payment by roughly 10% to 12%, depending on the loan amount and term. On a $300,000 loan over 30 years, the difference between 5% and 6% is about $180 per month. This is why shopping for the best rate matters—even a 0.25% difference adds up over 30 years.
Can I change my loan term after I sign the mortgage?
You cannot change the term of your existing mortgage without refinancing, which means taking out a new loan. Refinancing has closing costs and a new application process, so it only makes sense if the interest rate has dropped enough to offset those costs, or if you want to shorten the term and can afford the higher payment.
What is the difference between the interest rate and the APR?
The interest rate is what you pay to borrow the money. The APR includes the interest rate plus certain lender fees, expressed as an annual percentage. The APR is always higher than the interest rate and is the better number to use when comparing loans from different lenders, because it accounts for differences in fees.