The basic formula and what each number means
Your monthly mortgage payment comes from a formula that divides what you owe across the life of the loan, accounting for interest. The formula is:
M = P [ r(1 + r)^n ] / [ (1 + r)^n – 1 ]
In this formula: M is your monthly payment. P is the principal—the amount you borrowed. r is your monthly interest rate (your annual rate divided by 12). n is the total number of payments (years multiplied by 12).
The formula works because it spreads your debt and the interest you owe over equal monthly chunks. Early payments cover more interest than principal. Later payments cover more principal than interest. By the end, you own the home outright.
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 amortization formula divides your total debt and interest into equal monthly payments that stay the same for the life of the loan.
- You can calculate your payment by hand using the formula, or use an online calculator, a spreadsheet, or your lender's tools.
- An amortization schedule shows you exactly how much of each payment goes to principal versus interest over time.
- Property taxes, insurance, and HOA fees are not part of the amortization calculation—your lender adds those separately to your total monthly housing cost.
Working through a real example step by step
Say you borrow $300,000 at 6.5% annual interest over 30 years. First, convert the annual rate to a monthly rate: 6.5% ÷ 12 = 0.542% per month, or 0.00542 as a decimal. Next, count your total payments: 30 years × 12 months = 360 payments.
Now plug into the formula: M = 300,000 [ 0.00542(1.00542)^360 ] / [ (1.00542)^360 – 1 ]. The exponent (1.00542)^360 equals about 6.898. So M = 300,000 [ 0.00542 × 6.898 ] / [ 6.898 – 1 ]. That simplifies to M = 300,000 [ 0.03735 ] / [ 5.898 ], which equals M = $1,897 per month.
This is your principal and interest payment only. Your actual monthly bill will be higher once your lender adds property taxes, homeowners insurance, and mortgage insurance (if you put down less than 20%).
Why the payment stays the same but the split between principal and interest changes
Your $1,897 payment never changes over 30 years—that is the point of amortization. But what that payment covers shifts dramatically. In month one, nearly all of it goes to interest because you owe the full $300,000. By month 360, almost all of it goes to principal because you owe very little.
Here is why: interest is calculated on your remaining balance each month. In month one, your balance is $300,000, so one month of interest at 6.5% annually is about $1,625. Your $1,897 payment covers that interest plus $272 toward principal. Your new balance is $299,728.
In month 180 (halfway through), your balance is roughly $179,000. One month of interest is now about $765. Your $1,897 payment covers that plus $1,132 toward principal. In month 359, your balance is tiny—maybe $1,800—so interest is only $8, and nearly the entire payment reduces what you owe.
Using a spreadsheet or calculator instead of doing it by hand
The formula works, but most people use tools. In Excel or Google Sheets, use the PMT function: =PMT(rate, nper, pv). For the example above, that is =PMT(0.00542, 360, -300000). The negative sign tells the spreadsheet you are borrowing money. The result is $1,897.
Online mortgage calculators ask for loan amount, interest rate, and loan term, then show your monthly payment when ready. Many also show a full amortization schedule—a month-by-month breakdown of how much principal and interest you pay each month.
Your lender provides an amortization schedule with your loan documents. If you do not have one, ask for it. It shows exactly what you owe after each payment and how much interest you have paid to date. This matters if you refinance or pay off the loan early, because you need to know your remaining balance.
What changes your monthly payment and what does not
Only three things affect your principal-and-interest payment: the loan amount, the interest rate, and the loan term. Borrow more, and your payment rises. Get a lower rate, and your payment falls. Stretch the loan over more years, and your payment shrinks (but you pay more interest overall).
Property taxes, homeowners insurance, and HOA fees do not change your amortization calculation. Your lender collects these separately and adds them to your bill. If your property taxes rise, your total monthly payment rises, but your principal-and-interest portion stays exactly the same.
Mortgage insurance (PMI) also sits outside the amortization formula. If you put down less than 20%, your lender requires PMI until you reach 20% equity. This is a separate monthly charge that your lender adds to your bill. Once you hit 20% equity, you can ask your lender to remove it.
How to read an amortization schedule
An amortization schedule is a table with columns for payment number, payment amount, principal paid, interest paid, and remaining balance. Payment 1 shows your starting balance ($300,000 in the example). After you make that payment, the remaining balance drops to $299,728.
The schedule lets you answer real questions: How much interest will I pay over the life of the loan? (In the example, about $382,000—more than the original loan.) How much will I owe after 10 years? (About $237,000.) What if I pay an extra $100 per month—how much faster is the loan paid off? (You can recalculate with a higher payment to see.)
Many lenders let you read your amortization schedule from your online account. If yours does not, ask for it by phone or email. You can also generate one using a spreadsheet or an online amortization calculator by entering your loan details.
The difference between a 15-year and 30-year mortgage
A 15-year mortgage has 180 payments instead of 360. Using the same $300,000 at 6.5%, a 15-year loan costs about $2,596 per month—$699 more than the 30-year version. But you pay only about $167,000 in total interest instead of $382,000. You own the home free and clear 15 years sooner.
The trade-off is straightforward: higher monthly payment, lower total interest. Lower monthly payment, higher total interest. Neither is wrong—it depends on your budget and how long you plan to stay in the home. If you can afford the higher payment and want to build equity faster, 15 years makes sense. If you need the lower payment to may have access to for the loan or to have breathing room in your budget, 30 years is the right choice.
Some people choose a 20-year or 25-year term as a middle ground. The amortization formula works the same way—just change the number of years and recalculate.
Frequently Asked Questions
Can I use the amortization formula to figure out what interest rate I am paying?
Not easily by hand—the formula does not rearrange neatly to solve for the rate. But if you know your payment, loan amount, and term, an online calculator or spreadsheet can work backward to show you the rate. This is useful if you are comparing loan offers and the lender quoted a payment but not a clear interest rate.
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 $237,000 at a lower rate over 20 years, the formula calculates a new monthly payment based on those numbers. Your old schedule stops being relevant.
Does paying extra principal each month change my amortization schedule?
Yes. If you pay an extra $100 toward principal each month, your balance drops faster, and you pay off the loan years earlier. Your lender may provide a new amortization schedule showing this, or you can recalculate using a higher monthly payment to see the effect. Always tell your lender that extra money is for principal, not next month's payment.
Why do I pay so much more interest in the early years?
Interest is calculated on your remaining balance. Early on, your balance is highest, so interest is highest. As you pay down the principal, the balance shrinks, and interest shrinks with it. This is why the split between principal and interest shifts so dramatically over the life of the loan.
Is the amortization formula the same for all mortgages?
Yes, for fixed-rate mortgages. Adjustable-rate mortgages (ARMs) use the same formula for each period, but the interest rate changes on a set schedule, so your payment changes too. Interest-only loans and balloon mortgages use different structures entirely. Always confirm your loan type with your lender so you understand how your payment is calculated.