The basic formula for your monthly payment
Your monthly mortgage payment is calculated using a fixed formula that accounts for the loan amount, interest rate, and number of months you have to repay. 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 you borrowed), r is your monthly interest rate (annual rate divided by 12), and n is the total number of payments (years multiplied by 12).
In practice, you do not need to do this by hand. Your lender will calculate it for you, and you can use an online mortgage calculator to see the result before you apply. But understanding what goes into the number helps you see why a higher interest rate or longer loan term raises your payment.
Key Takeaways
- Your monthly payment depends on three things: how much you borrowed, your interest rate, and how many years you have to pay it back.
- A higher interest rate or longer loan term increases your monthly payment; a larger down payment decreases it.
- Interest is calculated monthly on the remaining balance, so early payments go mostly toward interest and later payments go mostly toward principal.
- You can find your total interest paid by multiplying your monthly payment by the number of payments and subtracting the original loan amount.
- An amortization schedule shows exactly how much of each payment goes to interest versus principal.
How the three factors change your payment
The loan amount (principal) is the price of the home minus your down payment. A $300,000 home with a $60,000 down payment means you borrow $240,000. Borrowing more raises your monthly payment proportionally—borrow twice as much, pay roughly twice as much per month.
The interest rate is set by your lender based on market conditions, your credit score, and the type of loan. A 30-year mortgage at 6% interest costs less per month than the same loan at 7%, but you pay more total interest over the life of the loan. A 15-year mortgage at the same rate costs more per month but saves you thousands in interest because you pay it off faster.
The loan term (15, 20, or 30 years are most common) spreads your payments over time. A 15-year mortgage has higher monthly payments but lower total interest. A 30-year mortgage has lower monthly payments but you pay interest for twice as long, so total interest is much higher.
Understanding how interest is calculated each month
Your lender does not charge you all the interest upfront. Instead, they calculate interest monthly on the balance you still owe. In month one, you owe the full principal, so the interest charge is highest. As you pay down the balance, the interest charge shrinks.
Here is a simplified example: a $200,000 loan at 6% annual interest (0.5% monthly) with a 30-year term has a monthly payment of about $1,199. In month one, the interest portion is $1,000 (0.5% of $200,000), leaving $199 to reduce the principal. In month two, you owe $199,801, so the interest is $999, leaving $200 for principal. This pattern continues for 360 months.
Early in the loan, most of your payment covers interest. After 15 years on a 30-year mortgage, you have paid off only about one-third of the principal but paid nearly two-thirds of the total interest. This is why paying extra toward principal early in the loan saves the most interest.
Calculating total interest paid over the life of the loan
Total interest is simple to find: multiply your monthly payment by the number of payments, then subtract the original loan amount.
Example: a $200,000 loan at 6% over 30 years has a monthly payment of $1,199. Over 360 months, you pay $1,199 × 360 = $431,640. Subtract the original $200,000, and your total interest is $231,640.
The same $200,000 at 6% over 15 years has a monthly payment of about $1,432. Over 180 months, you pay $1,432 × 180 = $257,760. Total interest is $57,760—a savings of $174,000 compared to the 30-year loan, though your monthly payment is $233 higher.
This calculation shows why the loan term has such a large effect on total interest. You pay interest for twice as long on a 30-year loan, so even though the monthly payment is lower, you pay far more in total.
What an amortization schedule shows you
An amortization schedule is a month-by-month breakdown of your payment, showing how much goes to interest and how much goes to principal each month, plus your remaining balance. Your lender provides this when you close on the loan, and you can generate one using an online calculator.
The schedule reveals the pattern described above: early payments are mostly interest, later payments are mostly principal. It also shows your exact balance at any point, which matters if you want to pay off the loan early or refinance.
Some lenders charge points (also called discount points) to lower your interest rate. One point costs 1% of the loan amount and typically lowers your rate by 0.25%. An amortization schedule helps you decide whether paying points upfront makes sense—if you plan to stay in the home long enough to recoup the cost through lower monthly payments, points can save money.
How property taxes and insurance affect your total payment
Your actual monthly housing payment often includes more than just principal and interest. Many lenders require you to pay property taxes and homeowners insurance as part of your mortgage payment, held in an escrow account.
Property taxes vary by location and are assessed on the home's value. Insurance protects the lender's investment if the home is damaged. Both are added to your principal-and-interest payment to create your total monthly mortgage payment (often called PITI: principal, interest, taxes, insurance).
If you put down less than 20%, you will also pay mortgage insurance (PMI), which protects the lender if you default. PMI is calculated as a percentage of the loan amount and added to your monthly payment. Once your balance drops to 80% of the home's original value, you can request that PMI be removed.
Using online calculators versus doing it yourself
Online mortgage calculators let you enter the loan amount, interest rate, and term to see your monthly payment instantly. Most also show total interest paid and let you adjust the inputs to compare scenarios (a lower rate, a larger down payment, a shorter term).
These calculators are accurate for the basic payment calculation. However, they may not account for property taxes, insurance, or PMI unless you enter those figures separately. Your lender's official quote will include all costs and is the number to use when deciding whether you can afford the home.
If you want to understand the math without a calculator, the formula works, but it requires a scientific calculator or spreadsheet. For most people, a calculator is faster and less error-prone. The value is in understanding what each input does to the result, not in doing the arithmetic by hand.
Frequently Asked Questions
Why does my first payment have so much interest?
Interest is calculated on the full balance you owe at the start of each month. In month one, you owe the entire loan amount, so the interest charge is at its highest. As you pay down the principal, the interest charge shrinks. This is normal and expected for all mortgages.
Can I pay extra toward principal to reduce interest?
Yes. Any payment above your required monthly amount goes directly to principal (confirm this with your lender first). Paying extra early in the loan saves the most interest because you reduce the balance on which future interest is calculated. Even small extra payments add up over time.
What happens to my payment if interest rates drop?
Your current payment does not change unless you refinance. Refinancing means taking out a new loan at the lower rate to pay off the old one. You pay closing costs to refinance, so it only makes sense if the lower rate saves enough interest to cover those costs before you sell or pay off the home.
How does a down payment affect my monthly payment?
A larger down payment reduces the loan amount, which lowers your monthly payment. A 20% down payment also eliminates the need for mortgage insurance (PMI). For example, a $300,000 home with a $60,000 down payment (20%) requires a $240,000 loan; a $30,000 down payment (10%) requires a $270,000 loan and includes PMI.
Is a 15-year or 30-year mortgage better?
A 15-year mortgage costs less in total interest but requires a higher monthly payment. A 30-year mortgage has a lower monthly payment but you pay much more interest over time. The choice depends on your budget and how long you plan to stay in the home. Use a calculator to compare both scenarios with your numbers.