How to Calculate Your Mortgage Payment: Formula Guide (2026)
Your monthly mortgage payment is determined by three numbers: the loan amount, the interest rate, and the loan term. The standard amortization formula M = P x [r(1+r)^n] / [(1+r)^n - 1] converts these inputs into a fixed payment that fully repays the loan over time. On a $280,000 loan at 6.5% for 30 years, the formula produces $1,770 per month in principal and interest.
At UseCalcPro, we've processed over 2 million mortgage calculations since launching. The most common mistake we see: people focus on the monthly number without understanding why it is what it is. That understanding is the difference between confidently negotiating with a lender and blindly signing paperwork.
This guide walks through the math behind every mortgage payment, so you know exactly where each dollar goes.
Try It Yourself
How the Amortization Formula Works
Every fixed-rate mortgage in the United States uses the same amortization formula. Banks, credit unions, and online lenders all calculate payments identically. Here's the formula broken down:
M = P x [r(1+r)^n] / [(1+r)^n - 1]
Where:
- M = your monthly principal and interest payment
- P = the principal (loan amount after down payment)
- r = the monthly interest rate (annual rate divided by 12, expressed as a decimal)
- n = the total number of monthly payments (years x 12)
Why this formula exists
A mortgage is an amortizing loan, meaning each payment covers both interest on the remaining balance and a portion of the principal. The formula ensures the balance reaches exactly zero on the final payment. Without amortization, you'd pay interest forever and never reduce the principal.
Step-by-step calculation
Let's work through a real example: a $350,000 home with 20% down ($70,000) at 6.5% interest for 30 years.
Step 1: Calculate the loan amount.
$350,000 - $70,000 = $280,000 (this is P)
Step 2: Convert the annual interest rate to a monthly decimal.
6.5% / 12 / 100 = 0.005417 (this is r)
Step 3: Calculate the total number of payments.
30 years x 12 months = 360 payments (this is n)
Step 4: Plug into the formula.
(1 + 0.005417)^360 = 6.9913
M = $280,000 x [0.005417 x 6.9913] / [6.9913 - 1]
M = $280,000 x [0.03787] / [5.9913]
M = $280,000 x 0.006321
M = $1,770 per month
That $1,770 covers only principal and interest. Your actual monthly obligation is higher once you add taxes, insurance, and potentially PMI.
The PITI Breakdown: What's Really in Your Payment
Lenders and real estate agents use the acronym PITI: Principal, Interest, Taxes, and Insurance. Here's what each component means and how it affects your total cost.
Principal
This is the portion that reduces your loan balance. In the early years, very little of your payment goes toward principal. On a $280,000 loan at 6.5%, only $247 of your first $1,770 payment goes to principal. By year 25, that flips to $1,327 toward principal.
Interest
Interest is the cost of borrowing money. It's calculated monthly on your remaining balance. In month one: $280,000 x 0.005417 = $1,517 in interest. As your balance decreases, the interest portion shrinks and the principal portion grows. This shift is called amortization.
Property taxes
Property taxes vary enormously by location. The national average is roughly 1.1% of home value annually. On a $350,000 home, that's $3,850/year or about $321/month. Some states like New Jersey average 2.2%, while Hawaii averages 0.3%.
Your lender typically collects property taxes monthly as part of your mortgage payment and holds them in an escrow account, paying the tax bill on your behalf.
Homeowner's insurance
Standard homeowner's insurance runs $1,200 to $3,000 per year depending on location, coverage, and home value. This is also typically escrowed. On a $350,000 home, expect roughly $150 to $250 per month.
PMI (if applicable)
If your down payment is less than 20%, lenders require Private Mortgage Insurance. PMI protects the lender (not you) if you default. Rates range from 0.5% to 1% of the loan amount annually.
On a $315,000 loan (10% down on $350,000): PMI costs roughly $131 to $263 per month. That's money that provides you zero benefit and goes away once you build 20% equity.
Total monthly cost example
| Component | Monthly Amount |
|---|---|
| Principal & Interest | $1,770 |
| Property Tax (1.1%) | $321 |
| Homeowner's Insurance | $175 |
| Total (20% down, no PMI) | $2,266 |
| PMI (if 10% down) | +$175 |
| Total (10% down, with PMI) | $2,441 |
The difference between the "advertised" $1,770 and the real cost of $2,266 (or $2,441 with PMI) is $496 to $671 per month. This gap catches many first-time buyers off guard.
How Amortization Shifts Over Time
The most misunderstood aspect of a mortgage is how the principal-to-interest ratio changes over the life of the loan.
Early years: interest dominates
In the first year of a $280,000 loan at 6.5%, you'll pay $18,108 in interest and only $3,132 in principal. That means 85% of your payments go to interest. This feels like you're making no progress, but it's mathematically normal.
Middle years: the crossover
Around year 17 of a 30-year mortgage, the principal and interest portions become roughly equal. This is the "crossover point." Before this point, you're paying more interest than principal. After it, more of each payment reduces your balance.
Final years: principal dominates
In the last 5 years, roughly 80% of each payment goes to principal. Your balance drops quickly. This is why people who sell after 5 years barely reduce their loan balance, while those who hold for 25+ years have significant equity.
Why extra payments matter most early on
If you pay an extra $200/month starting in year one on our $280,000 example, you'd save $82,700 in interest and pay off the loan 5 years and 8 months early. The same $200/month starting in year 15 saves only $24,200. Early extra payments have a multiplier effect because they reduce the balance that compounds interest for decades.
Common Mistakes When Estimating Mortgage Costs
Mistake 1: Ignoring the escrow portion
Many people see "$1,770/month" and budget exactly that. The real cost with escrow (taxes + insurance) is $2,200 to $2,500+. Always calculate PITI, not just P&I.
Mistake 2: Forgetting about PMI's long-term cost
PMI on a $315,000 loan at 0.7% costs $2,205 per year. If it takes 7 years to reach 20% equity, you'll spend $15,435 on insurance that protects the bank, not you. A larger down payment or a lender-paid PMI option may save money over time.
Mistake 3: Comparing only monthly payments between loan terms
A 30-year mortgage at 6.5% on $280,000 costs $1,770/month. A 15-year at 6.0% costs $2,363/month. The 30-year "wins" on monthly payment by $593. But the total interest differs dramatically: $357,200 (30-year) vs. $145,340 (15-year). That's $211,860 in extra interest for lower monthly payments.
Mistake 4: Not accounting for rate changes on ARMs
Adjustable-rate mortgages (ARMs) start with a lower rate, often 1-2% below fixed rates. A 5/1 ARM at 5.5% on $280,000 costs $1,590/month initially. But after 5 years, if the rate adjusts to 7.5%, the payment jumps to $1,886. If rates spike to 8.5%, it becomes $2,054. Budget for the worst case, not the teaser rate.
When You Need This Calculation
Before house hunting
Knowing your budget before you start looking prevents falling in love with a home you can't afford. Run the numbers at several price points to understand your comfortable range. The 28/36 rule suggests housing costs shouldn't exceed 28% of gross income.
When comparing lender offers
Two lenders might offer the same rate but different terms. One charges 0.5 points upfront for a 0.25% lower rate. Running both scenarios through the formula shows whether the upfront cost saves money over your expected ownership period.
During refinancing decisions
If current rates are lower than your existing rate, the formula helps you calculate the new payment and determine your break-even point. Closing costs on a refinance typically run $3,000 to $6,000. Divide that by your monthly savings to find how many months until the refinance pays for itself.
When considering extra payments
Even $100/month extra toward principal makes a significant difference. On our $280,000 example, $100/month extra saves $49,200 in interest and shaves 4 years off the loan. Use the mortgage calculator to model different extra payment scenarios.
The 28/36 Rule: How Much Can You Actually Afford?
Financial advisors and lenders use the 28/36 rule as a baseline:
- 28% rule: Your total monthly housing cost (PITI + HOA) shouldn't exceed 28% of your gross monthly income.
- 36% rule: Your total monthly debt (housing + car payments + student loans + credit cards) shouldn't exceed 36% of gross monthly income.
Affordability at different income levels
| Gross Annual Income | Monthly Gross | 28% Housing Max | Approximate Home Price |
|---|---|---|---|
| $60,000 | $5,000 | $1,400 | $200,000 - $220,000 |
| $80,000 | $6,667 | $1,867 | $270,000 - $300,000 |
| $100,000 | $8,333 | $2,333 | $340,000 - $380,000 |
| $120,000 | $10,000 | $2,800 | $420,000 - $460,000 |
These ranges assume 20% down, 6.5% interest, and average property taxes and insurance. Your local market may differ significantly.
15-Year vs. 30-Year: A Full Comparison
This is the single most impactful decision after choosing your home. Here's the math on a $280,000 loan:
| Factor | 15-Year (6.0%) | 30-Year (6.5%) |
|---|---|---|
| Monthly P&I | $2,363 | $1,770 |
| Total Interest | $145,340 | $357,200 |
| Total Cost | $425,340 | $637,200 |
| Interest Savings | $211,860 | -- |
| Monthly Difference | +$593 | -- |
The 15-year saves $211,860 in interest but requires $593 more per month. If that $593 would strain your budget, the 30-year with occasional extra payments is the safer choice. If you can comfortably afford it, the 15-year builds equity twice as fast.
Frequently Asked Questions
How does the interest rate affect monthly payment?
Every 0.5% increase in interest rate raises your monthly payment by roughly $80-$90 per $100,000 borrowed on a 30-year term. On a $280,000 loan, going from 6.0% to 7.0% increases the monthly payment by approximately $185 (from $1,679 to $1,864). Over 30 years, that 1% difference costs an additional $66,600 in total interest.
What happens if I make biweekly payments instead of monthly?
Biweekly payments mean you make 26 half-payments per year, which equals 13 full payments instead of 12. That one extra payment per year on a $280,000 loan at 6.5% saves approximately $62,000 in interest and pays off the mortgage about 4.5 years early. It's one of the simplest acceleration strategies available.
How do points affect the total cost of a mortgage?
One discount point costs 1% of the loan amount and typically reduces your interest rate by 0.25%. On a $280,000 loan, one point costs $2,800 upfront but saves roughly $47 per month. The break-even point is about 60 months (5 years). If you plan to stay longer than 5 years, buying points usually saves money. If you might move sooner, skip the points.
Is it better to put more money down or invest the difference?
If you put 20% down on a $350,000 home instead of 10%, you invest an extra $35,000 into your home. That eliminates PMI (saving roughly $175/month) and reduces your payment by $221/month. If you invested that $35,000 at an average 8% return instead, you'd earn roughly $2,800/year. The PMI savings alone ($2,100/year) plus the lower payment make the larger down payment favorable in most scenarios, especially since mortgage interest is a guaranteed cost while investment returns are not.
How does property tax reassessment affect my payment?
Property taxes are reassessed periodically (annually in most states). If your home value increases, your property tax bill increases, which raises your escrow payment. A $350,000 home that appreciates to $400,000 at a 1.1% tax rate sees annual taxes rise from $3,850 to $4,400, adding $46/month to your payment. Your lender adjusts the escrow amount annually.
What is the difference between pre-qualification and pre-approval?
Pre-qualification is an informal estimate based on self-reported financial information. Pre-approval involves a credit check, income verification, and asset documentation, resulting in a conditional commitment from the lender for a specific loan amount. Pre-approval carries significantly more weight when making an offer because the seller knows your financing is substantiated.
Related Resources
- Mortgage Refinance Calculator - Calculate potential savings from refinancing
- Bi-Weekly Mortgage Calculator - See how biweekly payments accelerate payoff
- Complete Guide to Mortgage Payment Components - Deep dive into PITI breakdown
- First-Time Homebuyer Guide - Step-by-step guidance for new buyers
- Loan Calculator - Calculate payments for any type of loan
This article is provided for informational and educational purposes only. Content should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on the information in this article.
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