Sarah and her husband found the perfect house listed at $350,000. The real estate agent told them to “just run the numbers,” but neither of them had any idea how mortgage math actually works. If you’ve ever stared at a listing price wondering what your actual monthly payment would be, you’re not alone.
Understanding mortgage calculations doesn’t require a finance degree — just a few key concepts and a good calculator.
What Makes Up a Mortgage Payment
Your monthly mortgage payment has four components, known as PITI: Principal, Interest, Taxes, and Insurance.
Principal is the portion that actually pays down your loan balance. In the early years of a mortgage, this is surprisingly small — most of your payment goes toward interest.
Interest is what the bank charges you for borrowing money. On a $300,000 loan at 7%, you’d pay roughly $21,000 in interest during just the first year.
Property taxes vary by location but typically run between 0.5% and 2.5% of your home’s value annually. A $350,000 home in Texas might have $8,750 in annual property taxes, while the same home in Hawaii might only owe $1,225.
Homeowners insurance protects your property and usually costs $1,000 to $3,000 per year depending on coverage and location.
The Mortgage Payment Formula
The core formula for calculating monthly principal and interest is:
M = P × [r(1+r)^n] / [(1+r)^n – 1]
Where M is your monthly payment, P is the loan principal, r is the monthly interest rate (annual rate divided by 12), and n is the total number of payments.
For a $280,000 loan (after a 20% down payment on $350,000) at 7% interest over 30 years: the monthly principal and interest payment comes to approximately $1,863.
Add roughly $400 for taxes and $150 for insurance, and the total monthly payment is around $2,413.
How Down Payment Affects Your Payment
The down payment directly reduces your loan amount and can eliminate private mortgage insurance (PMI). Here’s how different down payments affect a $350,000 home at 7% interest:
A 5% down payment ($17,500) means borrowing $332,500, resulting in roughly $2,212 monthly for principal and interest, plus PMI of about $140/month.
A 10% down payment ($35,000) reduces the loan to $315,000, dropping the payment to about $2,096 plus PMI of around $105/month.
A 20% down payment ($70,000) brings the loan to $280,000 and a payment of $1,863 — with no PMI required.
15-Year vs. 30-Year Comparison
A shorter loan term means higher monthly payments but dramatically less total interest. On a $280,000 loan at 7%:
The 30-year option gives you a $1,863 monthly payment but you’ll pay $390,680 in total interest over the life of the loan.
The 15-year option bumps the payment to $2,517 monthly, but total interest drops to $173,060 — saving you over $217,000.
Smart Strategies for Lower Payments
Improve your credit score before applying. Going from a 670 to a 740 score could save you 0.5% on your rate, which on a $280,000 loan saves roughly $100/month.
Shop multiple lenders. Rates can vary by 0.25% to 0.75% between lenders on the same day. Always get at least three quotes.
Consider buying points. Paying 1% of your loan amount upfront can reduce your rate by about 0.25%. This makes sense if you plan to stay in the home for 5+ years.
Look into first-time buyer programs. Many states offer down payment assistance or reduced-rate mortgages for qualified buyers.
Try It Yourself
Rather than doing all this math by hand, use our free Loan Calculator to instantly see your monthly payment, total interest, and full amortization schedule. Just enter your loan amount, interest rate, and term to get a complete breakdown.