Most people know their mortgage payment as a number that shows up in their bank account every month. Fewer know how that number is actually calculated — or why a small change in interest rate or down payment can shift it by hundreds of dollars. Here's the full picture, from the core formula to the hidden line items that catch buyers off guard.
The mortgage payment formula
The monthly principal and interest payment on a fixed-rate mortgage is determined by one formula:
M = P × [r(1+r)²³] ÷ [(1+r)²³ − 1]
Each variable has a plain-English meaning:
- M — the monthly payment you're solving for
- P — the principal, meaning the loan amount (home price minus your down payment)
- r — the monthly interest rate, which is your annual rate divided by 12. A 6.5% annual rate becomes 0.065 ÷ 12 = 0.005417 per month.
- n — the total number of monthly payments. A 30-year mortgage has 360; a 15-year has 180.
The formula produces a fixed payment that covers the interest owed each month and chips away at the principal, so the loan reaches exactly zero at payment number n. In the early years, most of each payment goes to interest. By the final years, almost all of it reduces the principal — this gradual shift is called amortization.
As a concrete example: a $280,000 loan at 6.5% for 30 years gives r = 0.005417 and n = 360. Plugging those into the formula produces a monthly principal-and-interest payment of roughly $1,770.
PITI: the four pieces of a real mortgage payment
The formula above gives you principal and interest (P&I) only. Your actual monthly obligation is almost always higher because lenders collect two additional items through an escrow account:
- Principal (P) — the portion of your payment that reduces what you owe on the loan.
- Interest (I) — the cost of borrowing, paid to the lender. This is front-loaded: in month one of a 30-year mortgage, roughly 80–85% of your payment is interest.
- Taxes (T) — your annual property tax bill divided by 12. On a $350,000 home in a county with a 1.2% tax rate, that's $350 per month added to your payment.
- Insurance (I) — homeowner's insurance, typically $100–$200 per month depending on the home and location.
There is a fifth item that applies to many buyers: PMI, or private mortgage insurance. If your down payment is less than 20% of the home's purchase price, most conventional lenders require PMI to protect themselves in case you default. PMI typically costs 0.5–1.5% of the loan amount per year — on a $300,000 loan, that is $125–$375 per month. The good news: once you build 20% equity (through payments or appreciation), you can request PMI removal.
How your down payment changes everything
The down payment is the single biggest lever a buyer controls before signing. It affects three things simultaneously: the loan amount, the monthly payment, and whether you pay PMI. The table below shows a $350,000 home at 6.5% for 30 years with three different down payments. Property tax and insurance are assumed at $350 and $150 per month respectively; PMI is estimated at 0.85% of the loan per year.
| Down payment | Loan amount | P&I payment | PMI | Total PITI+PMI |
|---|---|---|---|---|
| 5% ($17,500) | $332,500 | $2,102 | $235/mo | $2,837/mo |
| 10% ($35,000) | $315,000 | $1,991 | $223/mo | $2,714/mo |
| 20% ($70,000) | $280,000 | $1,770 | None | $2,270/mo |
Going from 5% to 20% down cuts the monthly payment by roughly $567 — about $6,800 per year. More than half of that gap comes from eliminating PMI alone, not from the smaller loan. This is why “save to 20%” is standard advice: it's not just a smaller balance, it's also the point where you stop paying for insurance that protects the lender, not you.
15-year vs. 30-year: the great trade-off
Choosing a loan term is choosing between a lower monthly payment (30 years) and dramatically lower total interest paid (15 years). On a $300,000 loan at 6.5%, the numbers look like this:
| Term | Monthly P&I | Total interest paid | Total paid (principal + interest) |
|---|---|---|---|
| 30-year at 6.5% | $1,896 | $382,560 | $682,560 |
| 15-year at 6.5% | $2,614 | $170,520 | $470,520 |
The 15-year mortgage costs $718 more per month but saves over $212,000 in interest over the life of the loan. In addition, 15-year rates are typically 0.5–0.75% lower than 30-year rates, which widens the total-cost gap further.
The 30-year is not irrational, though. The lower required payment gives you flexibility: you can always pay extra toward principal when cash is available, while keeping your contractual obligation low during tight months. Many financial planners argue that if your investment returns exceed your mortgage rate, putting extra cash into a brokerage account instead of mortgage principal can build more wealth over time.
How interest rate changes your total cost
A 1-percentage-point change in rate feels small on paper but compounds dramatically over 30 years. The table below shows a $300,000, 30-year mortgage at three common rate scenarios:
| Interest rate | Monthly P&I | Total interest paid |
|---|---|---|
| 5.5% | $1,703 | $313,080 |
| 6.5% | $1,896 | $382,560 |
| 7.5% | $2,098 | $455,280 |
Moving from 5.5% to 7.5% costs an additional $395 per month and over $142,000 in extra interest over the life of the loan. This is why buyers are advised to shop multiple lenders: even a 0.25% rate difference on a $300,000 mortgage saves roughly $16,000 in total interest. Every eighth of a point matters.
Credit score is the most direct lever most buyers have over their rate. Borrowers with scores above 760 routinely qualify for rates 0.5–1% lower than those with scores in the 620–680 range. On a $300,000 loan, that half-point difference alone is worth more than $30,000 in interest over 30 years.
Run the numbers
The formula and tables above give you the concepts, but your actual numbers — your purchase price, your down payment, your local tax rate, your credit score — are what determine your real monthly payment. Use our Mortgage Calculator to plug in your specifics and see an instant breakdown of principal, interest, taxes, insurance, and PMI, along with a full amortization schedule showing exactly how much of each payment goes where, month by month.