The Basic Formula for Monthly Mortgage Payment
Your monthly mortgage payment comes from a formula that takes three pieces of information: the loan amount you borrowed, the interest rate, and how many months you have to pay it back. Banks use this same formula, and you can work through it yourself with a calculator.
The formula is: M = P [ r(1 + r)^n ] / [ (1 + r)^n – 1 ]. Here, M is your monthly payment, P is the principal (the amount borrowed), r is the monthly interest rate (your annual rate divided by 12), and n is the total number of payments (years times 12).
If that looks intimidating, it is — which is why most people use a mortgage calculator or ask their lender to show them the math. But understanding what each number means helps you see why your payment is what it is, and why a different interest rate or loan term changes it so much.
Key Takeaways
- Your monthly payment depends on three things: how much you borrowed, your interest rate, and how many years you have to repay it.
- The monthly interest rate is your annual rate divided by 12, and this number is used multiple times in the payment formula.
- A mortgage calculator (free online or on your lender's website) will give you the exact payment faster and more reliably than doing the math by hand.
- Changing the loan term from 30 years to 15 years raises your monthly payment but cuts the total interest you pay over the life of the loan.
- Your lender must show you the payment breakdown in your Loan Estimate, which you receive within three business days of submitting an application.
Breaking Down the Three Numbers You Need
The principal (P) is straightforward: it is the amount of money you are borrowing. If you are buying a $300,000 house and putting down $60,000, your principal is $240,000. If you are refinancing, the principal is the balance you still owe, not the original loan amount.
The interest rate (r in the formula) is where most confusion happens. Your lender quotes an annual rate — say, 6.5 percent. To use it in the payment formula, you divide by 12 to get the monthly rate: 6.5 ÷ 12 = 0.542 percent per month. In the formula, you write this as a decimal: 0.00542, not 0.542.
The loan term (n) is the number of months you have to repay. A 30-year mortgage is 360 months. A 15-year mortgage is 180 months. Some loans run 20 years (240 months) or other lengths. The term you choose affects how much you pay each month and how much total interest you pay by the end.
Why the Formula Works the Way It Does
The formula accounts for the fact that interest compounds — you pay interest on the interest. Early in the loan, most of your payment goes toward interest. Later, more of it goes toward paying down the principal. The formula spreads the total interest across all your payments in a way that keeps your monthly payment the same every month.
If you borrowed $240,000 at 6.5 percent over 30 years, your first payment includes about $1,300 in interest and only $150 toward principal. By payment 300, it flips: most of that payment is principal, with only a small amount going to interest. The formula ensures this happens smoothly.
This is why paying extra toward principal early in the loan saves you so much money — you are reducing the balance that future interest is calculated on. A single extra $100 payment in year one might save you $10,000 in total interest over 30 years.
Using a Mortgage Calculator Instead
Most people do not work through the formula by hand. Your lender's website usually has a free calculator. Bankrate, NerdWallet, and other financial sites offer them too. You enter the loan amount, interest rate, and term, and the calculator gives you the monthly payment in seconds.
The calculator also shows you an amortization schedule — a month-by-month breakdown of how much of each payment goes to interest versus principal. This is useful because it shows you exactly when you will have paid off half the loan, and how much interest you will have paid by then.
If you are shopping for a mortgage, use a calculator to compare scenarios. See what happens if you choose a 15-year term instead of 30 years. See how a 0.5 percent difference in interest rate changes your payment. These comparisons help you understand what you are actually paying for.
What Your Lender Shows You in Writing
When you apply for a mortgage, federal law requires your lender to send you a Loan Estimate within three business days. This document shows your estimated monthly payment, broken down into principal and interest. It also lists property taxes, homeowners insurance, and any mortgage insurance (PMI) you will have to pay.
Your actual monthly payment may be higher than the principal-and-interest number because of these other costs. If you are putting down less than 20 percent, you will pay PMI until you reach 20 percent equity. Property taxes and insurance vary by location and your home value, so the lender estimates them based on the area.
The Loan Estimate is the official document to use when comparing offers from different lenders. It is designed to be consistent across all lenders, so you can see the real differences in rates and fees without confusion.
How Changing the Loan Term Affects Your Payment
A shorter loan term means a higher monthly payment but much less total interest. A longer term means a lower monthly payment but more total interest paid over time.
For example, a $240,000 loan at 6.5 percent costs roughly $1,520 per month over 30 years, or about $2,050 per month over 15 years. The 15-year payment is $530 higher each month, but you pay off the loan in half the time and pay roughly $150,000 less in total interest.
Some people choose a 30-year term because they need the lower monthly payment to fit their budget. Others choose 15 years because they want to build equity faster and pay less interest. There is no single right choice — it depends on your income, other debts, and how long you plan to stay in the home.
How Interest Rate Changes Impact Your Payment
Even a small change in interest rate makes a big difference in your monthly payment and total cost. A 0.5 percent difference might not sound like much, but it changes your payment by $100 to $150 per month on a typical loan.
On a $240,000 loan over 30 years, the difference between 6.0 percent and 6.5 percent is about $144 per month. Over 30 years, that adds up to more than $51,000 in extra interest. This is why shopping around with multiple lenders matters — even a small rate difference costs you real money.
Your interest rate depends on your credit score, down payment size, loan term, and current market rates. Lenders also offer different rates for different loan types (fixed-rate versus adjustable-rate, for example). Always ask your lender to show you how different rates change your payment.
Frequently Asked Questions
Can I calculate my mortgage payment if I do not know my exact interest rate yet?
Yes. Use the current market rate for your area as an estimate. Your lender can tell you what rates are available based on your credit score and down payment. Once you have a rate locked in, recalculate to see your actual payment. Rates change daily, so an estimate from a week ago may not be accurate.
Does my monthly payment include property taxes and insurance?
The formula only calculates principal and interest. Property taxes, homeowners insurance, and mortgage insurance (if required) are separate costs added on top. Your lender will show all of these together in your Loan Estimate so you see the full monthly cost.
What happens to my payment if I refinance?
Refinancing means taking out a new loan to pay off the old one. Your new payment is calculated using the new loan amount (which may be less if you have paid down the principal), the new interest rate, and the new term you choose. You can use the same formula or calculator with these new numbers.
Why does my actual payment differ from what the calculator showed?
Calculators estimate property taxes and insurance based on averages for your area. Your actual taxes and insurance may be higher or lower. Also, if you are paying PMI, that amount varies by your down payment size and credit score. Your Loan Estimate will show the exact numbers your lender is using.
If I make extra payments toward principal, does my monthly payment go down?
No. Your required monthly payment stays the same. Extra payments reduce the total interest you pay and shorten the loan term, but they do not lower the monthly amount you owe. You make extra payments on top of your regular payment, not instead of it.