Most people shopping for a house look at one number: the principal and interest payment a lender quotes them. Then the first bill arrives and it's $400 higher than expected. The gap isn't a mistake — it's the taxes and insurance nobody mentioned. If you want to know what a house actually costs each month, you need to calculate the full payment yourself before you fall in love with a listing.
The good news is the math isn't as scary as the formula looks. Let's break it into the four pieces that make up every mortgage payment, then run a real 2026 example.
The Four Parts of a Mortgage Payment: PITI
Lenders use the acronym PITI — Principal, Interest, Taxes, and Insurance. Your monthly payment is the sum of these four.
- Principal is the chunk that pays down what you borrowed. Early on this is small; it grows every year.
- Interest is the lender's fee for the loan, based on your rate. Early on this is the biggest piece.
- Taxes are property taxes your county charges, usually 0.5%–2% of the home's value per year.
- Insurance is homeowners insurance, plus PMI (private mortgage insurance) if you put down less than 20%.
The first two — principal and interest, or "P&I" — are what most calculators and ads show you. The last two are why your real bill is higher.
The Principal and Interest Formula, in Plain English
Here's the standard amortization formula for the monthly P&I:
M = P × [ r(1 + r)^n ] / [ (1 + r)^n − 1 ]
Where:
- M = monthly principal + interest payment
- P = the loan amount (price minus your down payment)
- r = your monthly interest rate (annual rate ÷ 12)
- n = total number of payments (years × 12)
The one trick people miss: the rate has to be monthly, not annual. A 6.5% annual rate becomes 0.065 ÷ 12 = 0.005417 per month. And a 30-year loan is 360 payments, not 30.
You don't have to do this by hand — any mortgage calculator will, and so will a spreadsheet's PMT function. But knowing what's under the hood means you can sanity-check any number a lender gives you.
A Real 2026 Example
As of June 2026, the average 30-year fixed mortgage rate sits around 6.5%. Let's buy a $350,000 home with 20% down.
- Down payment: $70,000
- Loan amount (P): $280,000
- Rate: 6.5% → monthly rate 0.005417
- Term: 30 years → 360 payments
Run those through the formula and the principal + interest comes to about $1,770/month.
Now add the parts the ad skipped:
- Property tax at roughly 1.1% of $350,000 = about $321/month
- Homeowners insurance at $1,800/year = $150/month
Total PITI: roughly $2,241/month — about 27% higher than the $1,770 P&I figure alone. That $471 gap is exactly the kind of surprise that breaks a tight budget.
If you'd put down less than 20%, you'd also owe PMI. In 2026, PMI runs about $30–$70 per month for every $100,000 borrowed, so on a $280,000 loan you might add $100–$180/month until you build 20% equity. (One bright spot: PMI became tax-deductible again starting in tax year 2026, treated like mortgage interest.)
How Rate and Term Change Everything
Two levers move your payment the most, and it's worth seeing how sensitive the number is.
The rate. On that same $280,000 loan, dropping from 6.5% to 5.5% cuts the P&I from about $1,770 to roughly $1,590 — $180/month, or more than $64,000 over the life of the loan. This is why a single percentage point matters so much, and why people obsess over rates.
The term. A 15-year loan on the same $280,000 at a slightly lower rate might run about $2,400/month — painful monthly, but you'd pay tens of thousands less in total interest because you're borrowing for half as long. A 30-year loan is easier on the monthly budget but far more expensive over time. There's no universally right answer; it depends on whether your priority is cash flow now or total cost later.
If you want a deeper feel for how interest compounds against you over decades, our breakdown of the math behind compounding shows the same forces that grow your investments working in reverse on a loan.
Don't Forget What's Not in PITI
Even PITI isn't the whole story of homeownership. Budget separately for:
- HOA fees, if your home has them — often $200–$500/month
- Maintenance, a rough rule being 1% of the home's value per year ($3,500/year on a $350,000 home)
- Utilities, which are usually higher than in a rental
A common mistake is stretching to the maximum monthly payment a lender approves, leaving nothing for these. A safer approach is keeping your total housing cost — PITI plus the extras — under about 28% of your gross monthly income, and making sure your emergency fund survives the move intact.
What To Do Next
Before you tour a single house, pick a realistic price, plug it into a mortgage calculator with your local property tax rate and an insurance estimate, and look at the full PITI number — not the teaser P&I figure. Then ask yourself if that payment fits comfortably, with room for maintenance and a cash cushion.
Run the math first and the house-hunting gets a lot calmer. You'll walk into every listing already knowing what it costs, instead of finding out the hard way on the first statement.