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

Your monthly mortgage payment depends on three numbers: how much you borrow (the principal), the interest rate your lender charges, and how many months you have to repay it. The standard way to find your payment is the amortization formula, which spreads your 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 (loan amount), r is your monthly interest rate (annual rate divided by 12), and n is the total number of payments (years multiplied by 12). If you borrowed $300,000 at 6.5% annual interest over 30 years, you would divide 6.5% by 12 to get your monthly rate (0.00542), then plug in the numbers.

You do not need to do this math by hand. A mortgage calculator—available free from most lenders' websites, from Bankrate, from the Consumer Financial Protection Bureau, or from your bank—will do the calculation instantly once you enter the principal, rate, and term.

Key Takeaways

  • Your monthly payment is determined by three inputs: the loan amount, the interest rate, and the number of years to repay, which you can plug into any free online calculator.
  • The amortization formula spreads your principal and interest into equal monthly payments, but you do not need to calculate it yourself—lenders and free tools do this for you.
  • Early in the loan, most of your payment goes toward interest; later, more goes toward principal, which is why an amortization schedule shows you the breakdown month by month.
  • Property taxes, homeowners insurance, and mortgage insurance (if your down payment is less than 20%) are separate from the principal-and-interest calculation and must be added to find your true monthly housing cost.

Where to find a free mortgage calculator

The easiest route is to use a calculator provided by your lender or a major financial website. Bankrate's mortgage calculator, the CFPB's mortgage payment calculator, and calculators from Chase, Wells Fargo, and other large banks all work the same way: you enter the loan amount, interest rate, and loan term (usually 15, 20, or 30 years), and the tool shows your monthly principal-and-interest payment instantly.

Some calculators also let you add property taxes, homeowners insurance, and mortgage insurance (PMI) to see your full monthly housing payment. This is useful because your actual monthly bill to the lender will include these costs if they are escrowed—meaning the lender collects them along with your mortgage payment and pays them on your behalf.

Understanding the amortization schedule

An amortization schedule is a month-by-month breakdown of your payment. It shows how much of each payment goes to interest and how much goes to principal, and what your remaining balance is after each payment. Most mortgage calculators can generate this schedule for you.

Early in the loan, the vast majority of your payment covers interest. On a $300,000 loan at 6.5% over 30 years, your first payment is about $1,896. Of that, roughly $1,625 goes to interest and only $271 goes to principal. By payment 300 (near the end of the loan), the split flips: interest drops to about $50 and principal rises to about $1,846. This is why paying extra toward principal early in the loan saves you the most interest overall.

How interest rate changes affect your payment

A small change in interest rate creates a large change in your monthly payment. On a $300,000 loan over 30 years, a rate of 5.5% gives a monthly payment of about $1,703. At 6.5%, it jumps to about $1,896. At 7.5%, it rises to about $2,098. The difference between 5.5% and 7.5% is $395 per month, or $4,740 per year.

This is why shopping for the best rate matters. Even a 0.25% difference in rate can save you tens of thousands of dollars over the life of the loan. When you receive loan estimates from lenders, the interest rate is always listed, and you can use that rate in a calculator to see the exact monthly payment you would owe.

The difference between a 15-year and 30-year loan

Shortening your loan term raises your monthly payment but cuts the total interest you pay. On a $300,000 loan at 6.5%, a 30-year mortgage costs about $1,896 per month and you pay roughly $382,000 in total interest. A 15-year mortgage on the same loan at the same rate costs about $2,596 per month but you pay only about $167,000 in total interest—a savings of $215,000.

The trade-off is cash flow: the 15-year payment is $700 higher each month. Some borrowers choose the 30-year term to keep monthly payments lower and have more money available for other goals, even though they pay more interest overall. Others choose 15 years to build equity faster and own the home sooner. A calculator lets you compare both scenarios side by side.

What is not included in the basic calculation

The principal-and-interest payment is only part of your total monthly housing cost. You must also account for property taxes, homeowners insurance, and mortgage insurance (PMI), if your down payment is less than 20% of the home's purchase price.

Property taxes vary widely by location and are assessed by your county or municipality—there is no single number to use. Homeowners insurance also varies by location, home value, and the insurer you choose. Mortgage insurance (PMI) is typically 0.5% to 1.5% of the loan amount per year, divided into monthly payments, and it stays on your loan until you have paid down the balance to 80% of the home's original value or you refinance.

Many lenders will provide an estimate of these costs when you request a loan estimate, so you can see the full monthly payment before you commit. Some calculators let you enter estimated tax and insurance amounts to show you the complete picture.

How to use a calculator to compare loan offers

When you receive loan estimates from multiple lenders, each will show a different interest rate and possibly different fees. To compare them fairly, use the same calculator for each offer and enter the exact loan amount, rate, and term from each estimate. This shows you the true monthly payment difference between lenders.

Keep in mind that a lower monthly payment is not always the best deal if it comes with higher fees or a longer term. A calculator helps you see the full picture: the monthly payment, the total interest paid over the life of the loan, and the total cost of borrowing. Some lenders offer a lower rate but charge higher origination fees; others charge lower fees but a higher rate. Running the numbers through a calculator lets you decide which trade-off works for your situation.

Frequently Asked Questions

What if my interest rate is variable or adjustable?

An adjustable-rate mortgage (ARM) has an interest rate that changes after an initial fixed period, usually 3, 5, 7, or 10 years. A calculator can show your payment during the fixed period, but your payment will change when the rate adjusts. To estimate the impact, ask your lender what the rate could be after the adjustment period ends, then run a new calculation with that higher rate to see what your payment might become.

How do I know what interest rate to use in the calculator?

Your lender will tell you the interest rate when you receive a loan estimate. If you are shopping around and do not have an estimate yet, you can check current average rates on Bankrate, LendingTree, or the Federal Reserve's website to get a ballpark figure for your area and credit profile. The actual rate you receive depends on your credit score, down payment, loan type, and current market conditions.

Does the calculator include property taxes and insurance?

Most basic calculators show only principal and interest. Some advanced calculators let you enter estimated property tax and insurance amounts to show your full monthly payment. If your calculator does not have this feature, you can add these costs separately once you know the principal-and-interest amount. Ask your lender or a tax assessor for estimates of these costs in your area.

What happens to my payment if I make extra payments toward principal?

Extra payments reduce your loan balance and shorten the life of the loan, but they do not change your required monthly payment unless you refinance. If you pay extra, you will pay off the loan faster and pay less total interest. A calculator can show you how much interest you save if you add a specific extra amount each month.

Can I use a calculator to figure out how much house I can afford?

Yes. Work backward: decide what monthly payment you can afford, then use a calculator to find what loan amount that payment supports at current interest rates and your preferred loan term. Keep in mind that lenders typically want your total monthly housing costs (mortgage, taxes, insurance, and PMI) to be no more than 28% of your gross monthly income, so factor that into your decision.