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How to Calculate Your Monthly Mortgage Payment
Buying a home is one of the largest financial decisions you will ever make. While online calculators are great for quick estimates, understanding the underlying math empowers you to see exactly how your interest rate, down payment, and loan term impact your monthly budget.
A standard mortgage payment consists of four primary components, commonly referred to as PITI: Principal, Interest, Taxes, and Insurance. The mathematical formula below specifically calculates the Principal and Interest—the core of your loan repayment.
The Amortization Formula
To determine your exact monthly principal and interest payment, lenders use the standard loan amortization formula. Here is the equation:
What the variables mean:
- M = Total Monthly Payment (Principal & Interest)
- P = Principal Loan Amount (Purchase price minus your down payment)
- r = Monthly Interest Rate (Your annual interest rate divided by 12. For example, a 6% annual rate becomes 0.06 / 12 = 0.005)
- n = Total Number of Payments (Your loan term in years multiplied by 12. A 30-year loan equals 360 payments)
Step-by-Step Calculation Example
Let’s look at a practical example. Imagine you are purchasing a home and taking out a $300,000 loan on a 30-year fixed-rate mortgage with a 6.0% annual interest rate.
- Step 1: Find P (Principal) — $300,000
- Step 2: Find r (Monthly Rate) — 6.0% annually is 0.06. Divide by 12 to get 0.005.
- Step 3: Find n (Total Payments) — 30 years × 12 months = 360 payments.
Plug these numbers into the equation:
When you run the math, the numerator becomes approximately 0.03011, and the denominator becomes 5.02257. Divide the two, multiply by $300,000, and your resulting monthly principal and interest payment is $1,798.65.
Don’t Forget the Extras
Remember that the formula above only covers the debt you owe the bank. To calculate your true out-of-pocket housing expense, you must also add your monthly property tax property allocations, homeowner’s insurance premiums, and any applicable HOA fees or Private Mortgage Insurance (PMI).