What Is a Mortgage Payment?
A mortgage payment is the monthly amount you send to your lender to repay a home loan. It covers two things: principal (the actual loan balance going down) and interest (the lender's cut for lending you the money). Most payments also bundle in property taxes and insurance through escrow — but for the math, we're talking principal + interest only.
The monthly mortgage formula looks complicated. It's not, once you see how it works.
The Formula
M = P[r(1+r)^n] / [(1+r)^n - 1]
Where:
- M = monthly payment
- P = loan principal (total amount borrowed)
- r = monthly interest rate (annual rate ÷ 12)
- n = total number of payments (loan term in years × 12)
Real-World Example
Say you're buying a $400,000 home, putting 20% down, and borrowing $320,000 at 6.5% for 30 years.
Monthly rate: 6.5% ÷ 12 = 0.5417% Total payments: 30 × 12 = 360
Plug it in: M = 320,000 × [0.005417 × (1.005417)^360] / [(1.005417)^360 - 1]
M ≈ $2,023/month
Over 30 years, you'll pay about $728,280 total. On a $320,000 loan. That means $408,280 goes purely to interest. That number tends to stop people cold.
Here's how term length affects total cost:
| Loan Term | Monthly Payment | Total Interest Paid | |-----------|----------------|---------------------| | 15 years | $2,791 | $182,370 | | 20 years | $2,390 | $253,600 | | 25 years | $2,155 | $326,500 | | 30 years | $2,023 | $408,280 |
Shorter loan, higher payment — but you save over $225,000 in interest going 15-year vs. 30-year. Worth knowing before you pick a term.
How Interest Rate Moves Your Payment
Even a 1% shift in rate has serious impact over 30 years. On $320,000:
| Interest Rate | Monthly Payment | Total Interest | |--------------|----------------|----------------| | 5.5% | $1,817 | $333,960 | | 6.0% | $1,919 | $370,840 | | 6.5% | $2,023 | $408,280 | | 7.0% | $2,129 | $446,440 | | 7.5% | $2,237 | $485,320 |
Going from 6.5% to 5.5% saves $75,000+ in interest over the life of the loan. This is why shopping around and negotiating with lenders actually matters — half a point on a $300k+ loan is real money.
Down Payment and PMI
Put down less than 20% and you'll usually pay private mortgage insurance (PMI) on top of your payment. PMI typically runs 0.5–1.5% of the loan annually. On a $320,000 loan that's $1,600–$4,800/year, or $133–$400/month.
It cancels once you hit 20% equity — but you're paying it until then, and unlike interest, it's not building any equity for you.
Three Things That Actually Move the Needle
1. Make Extra Principal Payments
An extra $100/month toward principal on a 30-year mortgage can cut your payoff time by 4–6 years and save tens of thousands in interest. Each extra payment shrinks the balance that future interest is calculated on — the savings compound.
2. Refinance When Rates Drop
If you locked in at 7% and rates fall to 5.5%, refinancing a $300,000 balance drops your payment by roughly $300/month. Closing costs usually run $2,000–$5,000, so you'd break even in 7–17 months. If you're staying in the home, it's worth doing the math.
3. Buy Mortgage Points to Lower Your Rate
Each point costs 1% of the loan amount and typically shaves 0.25% off your rate. On $320,000, one point runs $3,200 and reduces your monthly payment by about $53. Break-even: ~5 years. If you're planning to stay long-term, buying points often makes sense.
PITI: Your Real Monthly Housing Cost
The monthly payment you calculate with the formula covers principal and interest only. Your actual monthly housing cost — what lenders use to assess affordability — is PITI: Principal, Interest, Taxes, and Insurance.
On that $320,000 loan at 6.5%:
| Component | Monthly Cost | Notes | |-----------|-------------|-------| | Principal + Interest | $2,023 | From the formula | | Property taxes | ~$400 | Estimate: 1.2% of $400k home ÷ 12 | | Homeowners insurance | ~$150 | Varies by location and coverage | | PMI (if under 20% down) | $133–$400 | Only until you hit 20% equity | | Total PITI | $2,573–$2,973 | Depending on down payment |
Lenders use the 28% rule: your total PITI shouldn't exceed 28% of your gross monthly income. On a $130,000 household income ($10,833/month), the max PITI by that rule is $3,033/month. Our $2,573 estimate fits. Barely.
The 28% threshold matters because lenders check it before approving your loan. But it's also worth checking for yourself — a mortgage that's "technically approved" can still stretch your budget uncomfortably if your other expenses are high. PITI gives you a more realistic number to stress-test than principal + interest alone. If you're buying a home, our Home Buying Financial Guide walks you through the full cost picture and long-term financial planning around mortgages.
One more thing: property tax estimates vary enormously by location. A 1.2% effective rate is reasonable nationally, but you'll find rates from 0.3% in parts of Hawaii to over 2% in New Jersey and Illinois. Check your specific county before planning around any estimate.
Try It Yourself
Running "what if" scenarios by hand gets old fast. Our Mortgage Payment Calculator lets you compare loan amounts, interest rates, and terms instantly — adjust one variable and see the ripple effect.
If you want to see exactly how each payment splits between principal and interest over the life of the loan, our Loan Amortization Calculator generates the complete month-by-month schedule. For a deeper dive into how amortization works and strategies to pay off faster, check out our Loan Amortization Guide.