The basic formula for your monthly payment
Your monthly mortgage payment comes from a fixed formula that divides what you owe into equal chunks spread across your loan term. The formula is: M = P [ r(1 + r)^n ] / [ (1 + r)^n – 1 ], where M is your monthly payment, P is the principal (the amount borrowed), r is your monthly interest rate (annual rate divided by 12), and n is the total number of payments (years times 12).
You do not need to memorize or manually calculate this. A mortgage calculator—available free from your lender, Bankrate, or the Consumer Financial Protection Bureau—plugs in your numbers and returns your payment in seconds. But understanding what the formula does helps you see why your payment stays the same every month even though the split between principal and interest changes.
The payment covers both principal (the money you borrowed) and interest (what the lender charges for lending it). Early in the loan, most of your payment goes to interest. By the end, most goes to principal. Amortization is the schedule that shows you exactly how much of each payment goes where.
Key Takeaways
- Your monthly payment is calculated using a fixed formula that spreads your loan balance evenly across your entire loan term, so the payment never changes for a fixed-rate mortgage.
- An amortization schedule breaks down each payment into principal and interest, showing you that early payments are mostly interest and later payments are mostly principal.
- The three numbers that determine your payment are loan amount, interest rate, and loan term—changing any one of them changes your monthly cost.
- You can request an amortization schedule from your lender, download a template, or use an online calculator to see the full breakdown for your specific loan.
What an amortization schedule actually shows
An amortization schedule is a table with one row for each payment. It lists the payment number, the payment amount, how much goes to principal, how much goes to interest, and your remaining balance after that payment. The schedule runs from payment 1 all the way to your final payment.
For example, on a $300,000 loan at 6.5% interest over 30 years, your monthly payment is roughly $1,896. On payment 1, about $1,625 goes to interest and $271 goes to principal, leaving a balance of $299,729. On payment 360 (the last one), almost all $1,896 goes to principal because you have paid down the balance so far that interest is nearly zero.
This schedule matters because it shows you how slowly you build equity early on. Many borrowers are surprised to learn that after five years of payments on a 30-year loan, they have paid down only about 10% of the principal. The schedule also helps you understand what happens if you make extra payments—those go straight to principal and shorten your loan term.
The three numbers that change your payment
Only three inputs matter: the loan amount (principal), the interest rate, and the loan term. Change any one and your payment changes.
Loan amount is straightforward—borrow more, pay more each month. A $300,000 loan costs more per month than a $250,000 loan at the same rate and term.
Interest rate has an outsized effect. A half-percentage-point difference in rate can mean $100 or more per month on a $300,000 loan. Rates vary by lender, credit score, down payment size, and loan type (conventional, FHA, VA, USDA). Your rate also depends on whether you lock it in at application or later.
Loan term is how many years you have to repay. A 15-year mortgage has a higher monthly payment than a 30-year mortgage on the same amount and rate, because you are squeezing the payoff into half the time. But you pay far less interest overall because the loan ends sooner.
| Loan Amount | Interest Rate | Term | Monthly Payment (approx.) |
|---|---|---|---|
| $300,000 | 6.5% | 30 years | $1,896 |
| $300,000 | 6.5% | 15 years | $2,717 |
| $300,000 | 7.0% | 30 years | $1,996 |
| $250,000 | 6.5% | 30 years | $1,580 |
How to get your amortization schedule
Your lender must provide an amortization schedule as part of your loan documents. It typically appears in the Closing Disclosure, a document you receive at least three business days before closing. Ask your loan officer to send it earlier if you want to review it before closing day.
If you want to build one yourself, download a free template from Excel or Google Sheets. Search "amortization schedule template" and you will find dozens. Plug in your loan amount, annual interest rate, and number of payments, and the template calculates the rest. This is useful if you want to model different scenarios—what if you paid an extra $200 per month, or refinanced at a lower rate.
Online calculators from Bankrate, NerdWallet, or your lender's website also generate amortization schedules. Enter your numbers and download or print the full table. These tools are free and do not require you to enter personal information.
Why your payment stays the same but the split changes
The reason your monthly payment is identical every month (on a fixed-rate loan) is that it was calculated to pay off the entire loan in exactly the time you agreed to. The formula ensures that 360 equal payments of $1,896, for example, will retire a $300,000 loan at 6.5% interest in 30 years—no more, no less.
But the composition of each payment shifts because interest is calculated on the remaining balance. Early on, your balance is high, so interest is high and principal is low. As you pay down the balance, interest shrinks and principal grows. By the end, your balance is nearly zero, so interest is nearly zero and almost the entire payment is principal.
This is why making extra principal payments early in the loan saves you the most money. An extra $100 toward principal in year 1 stops accruing interest for the remaining 29 years. An extra $100 in year 29 stops accruing interest for only a few months.
What changes if you have an adjustable-rate mortgage
An adjustable-rate mortgage (ARM) works differently. Your payment is fixed for an initial period—often 3, 5, 7, or 10 years—then adjusts annually or semi-annually based on market interest rates. When your rate adjusts upward, your payment increases. When it adjusts downward, your payment decreases.
During the fixed period, you calculate your payment the same way as a fixed-rate loan. Your lender provides an amortization schedule for that period. Once the rate adjusts, a new amortization schedule applies to the remaining balance and new term. This is why ARMs are riskier—you cannot predict your payment after the fixed period ends, and a sharp rate increase can make your payment unaffordable.
If you have an ARM, ask your lender for the rate adjustment caps (the maximum your rate can rise per adjustment and over the life of the loan) and the index your rate is tied to. This helps you estimate worst-case scenarios.
How extra payments affect your amortization
When you make an extra payment toward principal, you reduce your remaining balance, which means less interest accrues in future months. Your amortization schedule shortens—you reach payoff sooner and pay less total interest.
For example, on that $300,000 loan at 6.5% over 30 years, adding $200 per month to your payment shortens the loan to about 24 years and saves roughly $80,000 in interest. The earlier you make extra payments, the more you save, because you are reducing the balance that accrues interest for the longest time.
Some lenders allow you to make extra payments without penalty. Others charge a prepayment penalty—a fee for paying off the loan early. Check your loan documents or ask your lender whether extra payments are allowed and whether they trigger any fees.
Frequently Asked Questions
Can I calculate my payment without knowing my exact interest rate?
You can estimate using current market rates, but your actual payment depends on the rate your lender locks in. Rates vary by lender, credit score, down payment, and loan type. Get a loan estimate from your lender to see the rate they are offering you specifically.
What if I want to pay off my mortgage in 20 years instead of 30?
Plug 20 years (240 payments) into the amortization formula or calculator instead of 30 years (360 payments). Your monthly payment will be higher, but you will pay significantly less total interest. You can also keep a 30-year loan and make extra principal payments to reach payoff in 20 years.
Does my property tax or homeowners insurance show up in the amortization schedule?
No. An amortization schedule shows only principal and interest. Property tax, homeowners insurance, and mortgage insurance (if required) are separate costs that may be bundled into your total monthly payment by your lender, but they are not part of the amortization calculation.
What happens to my amortization schedule if I refinance?
Refinancing creates a new loan with a new amortization schedule. Your remaining balance becomes the new principal, and you start over with a new interest rate and term. If you refinance a $250,000 remaining balance at a lower rate over 25 years, your new schedule begins at payment 1 of that 25-year loan.
Can I see how much interest I will pay over the life of my loan?
Yes. Add up the interest column in your amortization schedule, or subtract the original loan amount from the sum of all your payments. On a $300,000 loan at 6.5% over 30 years, you will pay roughly $383,000 total, meaning about $83,000 goes to interest. This is why a 15-year loan saves so much—you pay only about $49,000 in interest.