The formula that tells you what you'll pay each month

A mortgage payment is calculated using four pieces of information: the loan amount (called the principal), the interest rate, the loan term in years, and how often you pay. The standard formula multiplies the principal by a factor that accounts for interest and time, then divides by the total number of payments you'll make.

The math itself is straightforward once you have the numbers. Most people use a calculator or spreadsheet rather than doing it by hand, because the formula involves exponents. But understanding what each number means—and where to find it—matters more than the calculation itself.

Your monthly payment stays the same for the life of a fixed-rate mortgage. That payment covers principal (the money you borrowed), interest (what the lender charges for lending it), property taxes, homeowners insurance, and sometimes mortgage insurance, depending on your down payment. The payment you see quoted is usually just principal and interest; taxes and insurance are added on top.

Key Takeaways

  • A mortgage payment depends on the loan amount, interest rate, and how many years you have to repay it—a higher rate or shorter timeline raises your monthly cost.
  • The standard formula uses exponents and is easiest to calculate with a spreadsheet, online calculator, or your lender's quote.
  • Your quoted payment typically includes only principal and interest; property taxes, homeowners insurance, and mortgage insurance are separate line items.
  • Changing any one of the four inputs—principal, rate, term, or payment frequency—changes your monthly payment in a predictable way.

Where the four numbers come from

Principal is the amount you borrow. If you buy a house for $300,000 and put down $60,000, your principal is $240,000. This is the number lenders use to calculate your payment.

Interest rate is what the lender charges you annually, expressed as a percentage. A 6.5% rate means the lender charges 6.5% of the remaining balance each year. This rate is locked in when you close on a fixed-rate mortgage and does not change for the life of the loan. Adjustable-rate mortgages (ARMs) have rates that change after an initial period, which changes your payment later.

Loan term is how many years you have to repay the loan. The most common terms are 15 years and 30 years. A 15-year mortgage has higher monthly payments but you pay less interest overall. A 30-year mortgage spreads payments over twice as long, so each payment is smaller but you pay more interest because you owe the money for twice as long.

Payment frequency is how often you pay. Most mortgages require monthly payments, but some allow biweekly payments (every two weeks). Biweekly payments are slightly smaller but add up to more per year, which shortens the loan and saves interest.

The calculation step by step

The formula for a fixed-rate mortgage payment 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 (years × 12).

Here is a concrete example. You borrow $240,000 at 6.5% annual interest over 30 years. Your monthly interest rate is 6.5% ÷ 12 = 0.541667% or 0.00541667 as a decimal. Your total number of payments is 30 × 12 = 360. Plugging these into the formula gives you a monthly payment of approximately $1,520 for principal and interest alone.

You do not need to memorize or calculate this by hand. Every mortgage lender provides an amortization schedule—a document that shows your exact payment, how much goes to principal each month, how much goes to interest, and your remaining balance. You can also use an online mortgage calculator, a spreadsheet with the formula built in, or ask your lender directly.

How changes to each number affect your payment

Increasing the principal raises your payment proportionally. Borrow $300,000 instead of $240,000 and your payment rises by 25%.

Increasing the interest rate raises your payment, but not proportionally—a 1% increase in rate raises your payment by roughly 10% to 12%, depending on the term. This is why shopping for a lower rate saves so much money over time.

Extending the loan term lowers your monthly payment but increases total interest paid. A 30-year mortgage at 6.5% costs about $1,520 per month; a 15-year mortgage at the same rate costs about $2,000 per month. Over the life of the loan, you pay roughly $187,000 in interest on the 30-year loan and roughly $120,000 on the 15-year loan.

Switching to biweekly payments does not change the calculation itself, but it changes how much you pay per year. Twenty-six biweekly payments per year equal 13 monthly payments, so you make one extra payment annually. This shortens your loan and reduces total interest.

What gets added to your principal-and-interest payment

Your lender quotes a payment for principal and interest only. When you actually make your mortgage payment, you typically also pay property taxes, homeowners insurance, and possibly mortgage insurance. These are rolled into a single payment called PITI (Principal, Interest, Taxes, Insurance).

Property taxes vary by location and are set by your county or municipality. Homeowners insurance protects the lender's investment and is required by all lenders. Mortgage insurance (PMI on conventional loans, or MIP on FHA loans) is required if you put down less than 20%. All three of these costs are separate from the principal-and-interest calculation but are part of what you actually owe each month.

Your lender estimates these costs and adds them to your principal-and-interest payment. The total is what appears on your mortgage statement. If property taxes or insurance rates change, your payment may adjust once a year.

Using a spreadsheet or calculator instead of the formula

Most people use a tool rather than calculating by hand. A spreadsheet like Excel or Google Sheets has a built-in PMT function that does the calculation for you. You enter the monthly interest rate, the number of payments, and the loan amount, and the function returns your payment.

Online mortgage calculators are faster if you just need a quick estimate. You enter the loan amount, interest rate, and term, and the calculator shows your monthly payment when ready. Many lender websites have their own calculators, and they are free to use.

Your lender will provide an official Loan Estimate within three business days of your process. This document shows your exact payment, all costs, and the terms of your loan. This is the number to use for budgeting, not an estimate from a calculator.

Why your actual payment might differ from the calculation

The formula assumes a fixed interest rate and regular monthly payments. If you have an adjustable-rate mortgage, your rate changes on a set schedule, which changes your payment. If you make extra payments toward principal, you reduce the balance and shorten the loan, which lowers your total interest.

Property taxes and insurance also change over time. If your county raises property tax rates or your insurance premium increases, your monthly payment rises even though your principal-and-interest portion stays the same. Conversely, if you pay down your principal enough to reach 20% equity, your mortgage insurance drops off and your payment falls.

Some lenders allow you to pay biweekly instead of monthly, which changes the payment schedule and total interest. Others offer the option to round up your payment, which accelerates payoff. These choices are available at closing or sometimes after, depending on your lender's policies.

Frequently Asked Questions

Can I calculate my mortgage payment if I don't know my interest rate yet?

You can estimate using current market rates, but your actual payment depends on the rate you lock in. Rates change daily and vary based on your credit score, down payment, loan type, and term. Once you have a loan offer, the Loan Estimate shows your exact rate and payment.

Does a lower down payment change how the payment is calculated?

A lower down payment increases the principal you borrow, which raises your payment. It also triggers mortgage insurance, which is added on top. A $300,000 house with 10% down means you borrow $270,000 plus pay mortgage insurance; with 20% down you borrow $240,000 with no insurance.

What's the difference between a 15-year and 30-year mortgage payment?

A 15-year mortgage has a higher monthly payment but you pay off the loan twice as fast and pay roughly 40% less interest overall. A 30-year mortgage has a lower monthly payment but you pay interest for twice as long. The choice depends on whether you prioritize lower monthly costs or paying less interest.

If I pay extra toward principal, does my monthly payment go down?

Your required monthly payment stays the same, but extra principal payments shorten the loan and reduce total interest. Some lenders allow you to explore extra payments directly to principal; others require you to make a separate payment. Check your loan documents or ask your lender about their policy.

How do property taxes and insurance affect the calculation?

They do not affect the principal-and-interest calculation, but they are added to your monthly payment. Your lender estimates these costs at closing and includes them in your total monthly payment. If taxes or insurance rates change, your payment adjusts, usually once per year.