Your monthly mortgage payment is calculated using a standard formula that combines your loan amount, interest rate, and loan term. While most people use an online calculator, understanding the math helps you see exactly where your money goes each month and how changes to your loan terms affect what you pay.
The Core Formula for Principal and Interest
The foundation of every mortgage payment is the principal and interest (P&I) portion. Here’s the formula:
M = P × [ r(1 + r)ⁿ / ((1 + r)ⁿ − 1) ]
- M = your monthly payment
- P = the principal (the amount you’re borrowing)
- r = your monthly interest rate (annual rate divided by 12)
- n = the total number of monthly payments over the life of the loan
Two conversions matter before you plug anything in. First, convert your annual interest rate to a monthly rate by dividing by 12. A 7% annual rate becomes 0.005833 per month (0.07 ÷ 12). Second, convert your loan term into months. A 30-year mortgage is 360 payments. A 15-year mortgage is 180.
A Step-by-Step Example
Say you’re borrowing $300,000 at a 7% interest rate on a 30-year fixed mortgage. Here’s how to work through it:
Start with your inputs. P = $300,000. The monthly rate r = 0.07 ÷ 12 = 0.005833. The number of payments n = 30 × 12 = 360.
Next, calculate (1 + r)ⁿ. That’s (1.005833)³⁶⁰, which equals approximately 8.1165. Now plug everything into the formula: M = 300,000 × [0.005833 × 8.1165 / (8.1165 − 1)]. The numerator is 0.04735. The denominator is 7.1165. Divide those and you get 0.006653. Multiply by $300,000, and your monthly principal and interest payment comes to roughly $1,996.
That same loan at a 15-year term would produce a higher monthly payment (around $2,696) but dramatically less total interest over the life of the loan. Playing with these three variables, loan amount, rate, and term, is the fastest way to understand how much house you can afford.
What Your Payment Actually Includes
The formula above gives you only the principal and interest piece. Your actual monthly payment sent to your mortgage servicer almost always includes more than that. Lenders typically bundle four components, often called PITI:
- Principal: the portion that reduces your loan balance
- Interest: the cost the lender charges for the loan
- Taxes: your annual property tax bill, divided into monthly installments
- Insurance: your homeowners insurance premium, also split monthly
Your lender collects the tax and insurance portions into an escrow account, then pays those bills on your behalf when they come due. To estimate these costs, take your annual property tax bill and your annual insurance premium and divide each by 12. The average homeowners insurance cost in the U.S. runs about $2,490 a year for $400,000 in dwelling coverage, which adds roughly $208 per month. Property tax rates vary widely but commonly fall between 0.5% and 2.5% of your home’s assessed value per year.
Using the $300,000 loan example above on a $340,000 home, if your property taxes are 1.2% ($4,080 per year) and your insurance is $2,490 per year, your full monthly payment would look like this: $1,996 (P&I) + $340 (taxes) + $208 (insurance) = roughly $2,544.
When Private Mortgage Insurance Applies
If your down payment is less than 20% of the home’s purchase price, your lender will almost certainly require private mortgage insurance, known as PMI. This protects the lender if you default. PMI typically costs between 0.46% and 1.50% of your original loan amount per year, with the exact rate depending on your credit score, your debt-to-income ratio, and how much you put down. A larger down payment and a higher credit score both push PMI costs lower.
On a $300,000 loan, PMI at 0.75% would cost $2,250 per year, or about $188 per month added to your payment. Once your loan balance drops to 80% of the home’s original value, you can request that PMI be removed. It automatically cancels when the balance hits 78%.
HOA Dues Are Usually Separate
If you’re buying in a community with a homeowners association, those monthly dues typically are not part of your mortgage payment. According to the Consumer Financial Protection Bureau, HOA fees are usually paid directly to the association, not through your mortgage servicer. HOA dues can range from a few hundred dollars a month to more than a thousand, so factor them into your budget even though they won’t appear on your mortgage statement. In rare cases a servicer may agree to include HOA payments in your escrow, but that’s the exception.
How Each Payment Splits Between Interest and Principal
Your monthly P&I amount stays the same for the life of a fixed-rate loan, but the split between interest and principal shifts every single month. This process is called amortization. Early in the loan, the vast majority of each payment goes toward interest. Over time, as your balance shrinks, more of each payment chips away at principal.
The math is straightforward. Each month, your lender multiplies your outstanding balance by your monthly interest rate. That’s the interest portion. Whatever remains from your fixed payment goes toward reducing the principal. Using our $300,000 example at 7%, your first month’s interest charge is $300,000 × 0.005833 = $1,750. Out of your $1,996 payment, only $246 actually reduces your loan balance. By month 200, your remaining balance has dropped enough that roughly half of each payment goes to principal. In the final months, nearly the entire payment is principal.
This is why making even small extra payments toward principal early in the loan can save you tens of thousands of dollars in interest over its lifetime. Every dollar that reduces principal today means less interest accruing next month and every month after.
Quick Way to Estimate Without the Formula
If you don’t want to work through the full formula, a rough rule of thumb can get you in the ballpark. For every $100,000 borrowed on a 30-year fixed mortgage, here’s approximately what the P&I payment looks like at common rates:
- 6% rate: about $600 per month per $100,000
- 7% rate: about $665 per month per $100,000
- 8% rate: about $734 per month per $100,000
So a $350,000 loan at 7% would be roughly 3.5 × $665 = $2,328 per month in principal and interest alone. Then add your estimated taxes, insurance, and PMI if applicable to get your true monthly cost.