The basic formula: principal, interest rate, and loan term

Your monthly mortgage payment is determined by three numbers: the amount you borrowed (the principal), the interest rate your lender charges, and how many months you have to repay it (the loan term). The lender uses a standard formula to divide the total interest across all your payments so that each month's payment stays the same.

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 your monthly interest rate (annual rate divided by 12), and n is the total number of payments. You do not need to do this math yourself — lenders and online calculators do it for you — but understanding what goes into the number helps you see why a lower rate or longer term changes your payment.

A $300,000 loan at 6.5% interest over 30 years produces a different payment than the same loan at 5.5% or over 20 years. The difference is not small. This is why shopping for a lower rate and deciding between a 15-year and 30-year term are two of the most important decisions you make when getting a mortgage.

Key Takeaways

  • Your monthly payment depends on three things: how much you borrowed, your interest rate, and your loan term in years.
  • A mortgage calculator (from your lender, Bankrate, or the Consumer Financial Protection Bureau) will show you the exact payment for any combination of these three numbers.
  • Your actual monthly payment may be higher than the principal-and-interest number because it often includes property taxes, homeowners insurance, and mortgage insurance (called PITI).
  • The interest rate you receive depends on your credit score, down payment size, loan term, and current market rates — shopping with multiple lenders can save you thousands over the life of the loan.
  • Changing your loan term from 30 years to 15 years raises your monthly payment but cuts the total interest you pay nearly in half.

Using a mortgage calculator to see your payment

The fastest way to find your monthly payment is to use a mortgage calculator. Enter the loan amount, interest rate, and loan term (usually 15, 20, or 30 years), and the calculator shows you the payment immediately. Most calculators also let you add property taxes, homeowners insurance, and mortgage insurance to see your full monthly cost.

Your lender will provide a calculator on their website. The Consumer Financial Protection Bureau (CFPB) also offers a free calculator at consumerfinance.gov. Bankrate, NerdWallet, and LendingTree have calculators as well. All of them use the same formula, so the payment will be the same across all of them for the same inputs.

When you use a calculator, start with the numbers from your loan estimate — the document your lender sends you within three business days of your application. The loan estimate shows your principal, interest rate, and loan term. If you are shopping for a mortgage and do not yet have a rate, use the current market rate for your area and credit profile as a starting point, but understand that your actual rate may be higher or lower.

What happens when you change the interest rate or loan term

Small changes to your interest rate create large changes to your payment and total cost. A $300,000 loan at 6% over 30 years costs about $1,799 per month in principal and interest. The same loan at 5% costs about $1,610 per month — a difference of $189 per month, or $68,040 over 30 years. That is why a 0.5% difference in your rate matters enough to shop around.

Shortening your loan term also raises your monthly payment but saves you money on interest. A $300,000 loan at 6% over 15 years costs about $2,331 per month, compared to $1,799 for 30 years. Your payment goes up by $532 per month, but you pay off the loan in half the time and pay roughly $200,000 less in total interest. The trade-off is real: you must be able to afford the higher payment.

A 20-year term sits between these two. It is less common than 15 or 30, but some lenders offer it. Use a calculator to compare all three and see which fits your budget and long-term plans.

The difference between principal-and-interest and your actual payment

The number a calculator shows you for "monthly payment" is usually just principal and interest. Your actual payment to your lender is often higher because it includes other costs bundled into one payment, called PITI: Principal, Interest, Taxes, and Insurance.

Property taxes vary by location and are set by your county or municipality. Homeowners insurance protects your house and is required by your lender. If you put down less than 20% of the home's price, your lender also requires private mortgage insurance (PMI), which protects the lender if you stop paying. All three of these are added to your principal-and-interest payment and sent to an escrow account that your lender manages. The lender then pays your taxes, insurance, and PMI on your behalf.

To see your full monthly payment, use a calculator that includes these costs. Enter your property tax rate (your lender or a real estate agent can tell you the rate for the area), your estimated homeowners insurance cost (get quotes from insurers), and whether you will pay PMI. The total is what you actually owe each month.

How your interest rate is set

Your interest rate is not the same for everyone. It depends on your credit score, the size of your down payment, the loan term you choose, and the current market rate for mortgages. A borrower with a 750 credit score and 20% down will receive a lower rate than someone with a 650 score and 5% down, even if they apply on the same day.

Market rates change daily based on economic conditions and Federal Reserve policy. When rates are rising, locking in your rate early matters. When rates are falling, you may have the option to refinance later. Your lender will tell you the rate they are offering and how long it is locked in (usually 30, 45, or 60 days).

Shopping with multiple lenders is the most direct way to lower your rate. Each lender will send you a loan estimate with their rate and fees. Comparing three to five lenders can reveal rate differences of 0.25% to 0.75%, which translates to tens of thousands of dollars in savings over the life of the loan. Hard inquiries from mortgage lenders within a 45-day window typically count as a single inquiry on your credit report, so shopping around does not harm your score.

What changes your payment after you lock in your rate

Once your lender locks your interest rate, your principal-and-interest payment is set and will not change for the life of the loan (assuming a fixed-rate mortgage). However, your total monthly payment can still change if your property taxes or homeowners insurance costs rise, because these are recalculated annually and added to your escrow account.

If you have PMI, your payment will drop once you reach 20% equity in your home. You can request PMI removal at that point, though some lenders remove it automatically. Refinancing to a lower rate is also an option if rates fall significantly, but refinancing means paying new closing costs and restarting the loan term, so it only makes sense if you plan to stay in the home long enough to recoup those costs.

Your payment will not change if you make extra payments toward principal. Extra payments reduce the amount you owe and shorten your loan term, but your required monthly payment stays the same unless you formally refinance or modify your loan.

Frequently Asked Questions

What is the difference between a fixed-rate and adjustable-rate mortgage?

A fixed-rate mortgage keeps the same interest rate for the entire loan term, so your principal-and-interest payment never changes. An adjustable-rate mortgage (ARM) has a lower rate for an initial period (often 3, 5, 7, or 10 years), then adjusts annually based on market rates. After the initial period, your payment can rise significantly. Fixed-rate mortgages are more common because the payment is predictable.

Can I pay off my mortgage early without a penalty?

Most mortgages have no prepayment penalty, so you can pay extra toward principal at any time without owing a fee. Paying extra reduces the total interest you pay and shortens your loan term. However, your required monthly payment stays the same unless you refinance or formally modify your loan with your lender.

How do I know if my interest rate is competitive?

Compare your rate offer to rates from at least two other lenders. Current rates are published daily by Bankrate, Freddie Mac, and the Mortgage Bankers Association. Your credit score, down payment, and loan term affect your rate, so compare offers for the same loan details. A rate that is 0.5% higher than competitors will cost you tens of thousands over 30 years.

What if I want to change my loan term after I have locked in my rate?

Changing your loan term after locking your rate requires refinancing, which means applying for a new loan and paying closing costs again. Refinancing makes sense only if rates have dropped enough to offset the closing costs, or if you want to shorten your term and can afford the higher payment. Talk to your lender about whether refinancing is worth the cost in your situation.

Does making a larger down payment lower my monthly payment?

Yes. A larger down payment reduces the principal you borrow, which lowers your monthly payment. A 20% down payment also eliminates the need for PMI, which further reduces your payment. However, putting down more cash upfront means less money available for other uses, so weigh the monthly savings against your other financial needs.