Quick Answer
Quick Answer: A monthly mortgage payment is calculated using the formula M = P[r(1+r)n] / [(1+r)n - 1]. For a $300,000 loan at 6.5% for 30 years, the monthly principal and interest payment is $1,896.20. Add property taxes, insurance, and PMI for your total monthly housing cost (PITI). This page shows the complete math behind every number our calculator produces.
Run Your Own Mortgage CalculationThe Mortgage Payment Formula in Plain English
When you take out a fixed-rate mortgage, your lender needs to determine a single monthly payment amount that accomplishes two things simultaneously over the life of the loan:
- Pays the interest that accrues on your outstanding balance each month
- Reduces the principal (the amount you originally borrowed) so that it reaches zero by the final payment
The challenge is that these two goals compete with each other. Early in the loan, your balance is high, so most of your payment covers interest. As the balance decreases, more of each payment goes toward principal. The formula produces a constant monthly amount that perfectly balances these competing demands over the entire loan term.
This formula is formally known as the annuity payment formula in finance. It has been used by lenders since the standardization of amortizing loans in the early 20th century. The Consumer Financial Protection Bureau (CFPB)(opens in new tab) provides oversight to ensure lenders apply this calculation transparently.
The Mathematical Formula
Here is the exact formula used by our Mortgage Calculator and by virtually every fixed-rate mortgage lender in the United States:
M = P [ r(1 + r)n ] / [ (1 + r)n - 1 ]
Each variable in this formula has a specific financial meaning. The table below defines every term with a concrete example based on a typical mortgage scenario.
Variable Definitions
| Variable | Meaning | How to Calculate | Example ($300K, 6.5%, 30 yr) |
|---|---|---|---|
| M | Monthly principal & interest payment | Output of the formula | $1,896.20 |
| P | Principal (loan amount) | Home price minus down payment | $300,000 |
| r | Monthly interest rate | Annual rate / 12 | 0.065 / 12 = 0.00541667 |
| n | Total number of monthly payments | Loan term in years x 12 | 30 x 12 = 360 |
| (1+r)n | Compound growth factor | Raise (1 + monthly rate) to the power of n | 1.00541667360 = 6.99180 |
This formula calculates only the principal and interest portion of your mortgage payment. Your actual monthly housing cost also includes property taxes, homeowner's insurance, and possibly PMI. These components are covered in the PITI section below.
Worked Example: $300,000 at 6.5% for 30 Years
This section walks through every arithmetic step of the mortgage payment calculation. You can follow along with a standard calculator to verify each number.
Step 1: Convert the Annual Rate to a Monthly Rate
Mortgage rates are quoted as annual percentages, but payments are monthly. Divide the annual rate by 12 to get the monthly rate.
- Annual interest rate = 6.5% = 0.065
- r = 0.065 / 12 = 0.00541667
Step 2: Calculate the Total Number of Payments
Multiply the loan term in years by 12 months per year.
- Loan term = 30 years
- n = 30 x 12 = 360 monthly payments
Step 3: Calculate the Compound Growth Factor (1+r)n
This exponentiation is the most computationally intensive part. It represents how much a single dollar would grow if compounded monthly at the given rate for the entire loan term.
- (1 + r) = 1 + 0.00541667 = 1.00541667
- (1.00541667)360 = 6.99180
Step 4: Calculate the Numerator
Multiply the principal by the monthly rate by the compound growth factor.
- Numerator = P x r x (1+r)n
- = $300,000 x 0.00541667 x 6.99180
- = $11,358.47
Step 5: Calculate the Denominator
Subtract 1 from the compound growth factor.
- Denominator = (1+r)n - 1
- = 6.99180 - 1
- = 5.99180
Step 6: Divide to Get the Monthly Payment
- M = Numerator / Denominator
- = $11,358.47 / 5.99180
- M = $1,896.20 per month
Total Cost of the Loan
Multiply the monthly payment by the total number of payments to find the total amount you will pay over the life of the loan:
- Total paid = $1,896.20 x 360 = $682,633
- Total interest = $682,633 - $300,000 = $382,633
On a $300,000 mortgage at 6.5% for 30 years, you pay $382,633 in interest over the life of the loan -- more than the original amount borrowed. This is why understanding the math matters when comparing mortgage offers.
Verify This Calculation With Our Mortgage CalculatorHow Loan Term Affects Payment and Total Interest
The same formula works for any fixed-rate loan term. Shorter terms produce higher monthly payments but dramatically lower total interest costs. Here is a side-by-side comparison using verified calculator outputs.
| Scenario | Monthly P&I | Total Paid | Total Interest |
|---|---|---|---|
| $300,000 at 6.5%, 30 yr | $1,896.20 | $682,633 | $382,633 |
| $300,000 at 6.5%, 15 yr | $2,613.32 | $470,398 | $170,398 |
| $250,000 at 7.0%, 30 yr | $1,663.26 | $598,772 | $348,772 |
Principal and interest only. Property taxes, insurance, and PMI are additional. Values calculated using the standard annuity formula and verified against our Mortgage Calculator.
Choosing the 15-year term on a $300,000 loan at 6.5% increases the monthly payment by $717.12 but saves $212,235 in total interest. The monthly payment is 37.8% higher, but total interest is 55.5% lower.
In practice, 15-year mortgages typically carry lower interest rates than 30-year loans. According to Freddie Mac's Primary Mortgage Market Survey (PMMS)(opens in new tab), the spread between 15-year and 30-year rates has historically averaged 0.5 to 0.75 percentage points. This makes the 15-year option even more cost-effective than the same-rate comparison above suggests.
Amortization: How Each Payment Is Split
Amortization is the process of paying off a loan through a series of fixed payments that cover both interest and principal. Although your monthly payment stays constant at $1,896.20 for all 30 years, the split between interest and principal changes with every single payment.
The Monthly Amortization Calculation
Each month, the lender performs two calculations:
- Interest charge: Remaining balance x monthly rate (r)
- Principal reduction: Monthly payment (M) minus the interest charge
The remaining balance then decreases by the principal reduction amount, and the process repeats the following month.
Month-by-Month: How the First Payment Works
- Month 1 interest: $300,000 x 0.00541667 = $1,625.00
- Month 1 principal: $1,896.20 - $1,625.00 = $271.20
- Remaining balance: $300,000 - $271.20 = $299,728.80
In month 1, 85.7% of your payment goes to interest and only 14.3% reduces your loan balance. This is not a design flaw -- it is a mathematical consequence of how compound interest works on a large balance.
How the Split Changes Over Time
As your balance decreases, less interest accrues each month, so more of your fixed payment goes toward principal. The table below shows this progression at key milestones.
| Payment | Interest Portion | Principal Portion | Remaining Balance | % to Interest |
|---|---|---|---|---|
| Month 1 | $1,625.00 | $271.20 | $299,728.80 | 85.7% |
| Month 60 (Year 5) | $1,523.20 | $373.01 | $280,832.93 | 80.3% |
| Month 120 (Year 10) | $1,380.41 | $515.80 | $254,328.38 | 72.8% |
| Month 180 (Year 15) | $1,182.95 | $713.25 | $217,677.42 | 62.4% |
| Month 240 (Year 20) | $909.90 | $986.30 | $166,995.85 | 48.0% |
| Month 300 (Year 25) | $532.33 | $1,363.87 | $96,912.49 | 28.1% |
| Month 360 (Year 30) | $10.22 | $1,885.99 | $0.00 | 0.5% |
Based on a $300,000 loan at 6.5% fixed for 30 years. Values computed using the standard amortization algorithm. Use our Mortgage Calculator for a complete month-by-month schedule.
The Crossover Point
The crossover point is the month when more of your payment goes to principal than to interest. For a $300,000 loan at 6.5% over 30 years, this crossover occurs at month 233 (approximately year 20). Before that point, the majority of each payment services interest. After it, the majority reduces your balance.
This crossover point moves earlier if you have a lower interest rate, and later if your rate is higher. It is also why extra payments early in the loan term have such a large impact -- they reduce the principal balance when interest charges are at their highest.
After 5 years of payments on a $300,000 mortgage at 6.5%, you have paid $113,772 but only reduced your balance to $280,833 -- meaning you have built just $19,167 in equity through principal payments. The remaining $94,605 went to interest. Understanding this timeline helps you set realistic expectations for equity growth.
How PMI Is Calculated
Private mortgage insurance (PMI) is typically required when your down payment is less than 20% of the home's purchase price. PMI protects the lender -- not you -- in case of default, and it adds a monthly cost to your mortgage payment.
PMI Rate Ranges
According to the CFPB(opens in new tab), PMI rates generally range from 0.5% to 1.5% of the original loan amount per year. The exact rate depends on:
- Credit score: Higher scores qualify for lower PMI rates
- Down payment size: Larger down payments (closer to 20%) reduce the rate
- Loan-to-value (LTV) ratio: Lower LTV means lower risk to the insurer
- Loan type: Fixed-rate loans typically have lower PMI than adjustable-rate
PMI Calculation Formula
The monthly PMI payment is calculated as:
Monthly PMI = (Loan Amount x Annual PMI Rate) / 12
PMI Cost Examples: $300,000 Loan
| Annual PMI Rate | Annual PMI Cost | Monthly PMI Cost | Typical Borrower Profile |
|---|---|---|---|
| 0.5% | $1,500 | $125 | Excellent credit (760+), 15-19% down |
| 0.8% | $2,400 | $200 | Good credit (700-759), 10-14% down |
| 1.0% | $3,000 | $250 | Average credit (660-699), 5-9% down |
| 1.5% | $4,500 | $375 | Lower credit (620-659), 3-5% down |
PMI rates are approximate and vary by insurer. Based on industry data from Urban Institute(opens in new tab) and CFPB guidance. See our Understanding PMI guide for strategies to avoid or remove PMI.
When PMI Ends
Under the Homeowners Protection Act (HPA)(opens in new tab), you can request PMI cancellation when your loan balance reaches 80% of the original home value. The lender must automatically terminate PMI when the balance reaches 78%. On a $375,000 home with a $300,000 loan (80% LTV), PMI must automatically terminate when the balance falls to $292,500 (78% of $375,000).
Property Tax and Insurance: The Full PITI Payment
Lenders and housing affordability guidelines typically evaluate your ability to pay based on the full PITI payment -- not just principal and interest. PITI stands for Principal, Interest, Taxes, and Insurance. Here is how each component is estimated.
Property Tax Estimation
Property taxes vary significantly by location. According to the U.S. Census Bureau's American Housing Survey(opens in new tab), the national median effective property tax rate is approximately 1.1% of the home's assessed value per year. However, rates range from under 0.3% in some states to over 2% in others.
- Home value: $375,000
- Assumed tax rate: 1.0% (conservative estimate)
- Annual property tax: $375,000 x 0.01 = $3,750
- Monthly property tax: $3,750 / 12 = $312.50
Homeowner's Insurance Estimation
Homeowner's insurance premiums depend on coverage amount, location, deductible, and risk factors. A common industry estimate is $1,200 to $2,000 per year for a standard policy on a home in the $300,000 to $400,000 range.
- Assumed annual premium: $1,500
- Monthly insurance: $1,500 / 12 = $125.00
Full PITI Calculation
Combining all four components for our example ($300,000 loan on a $375,000 home with a 20% down payment, so no PMI):
| Component | Monthly Cost | Annual Cost | % of Total |
|---|---|---|---|
| Principal & Interest (P&I) | $1,896.20 | $22,754 | 81.3% |
| Property Taxes | $312.50 | $3,750 | 13.4% |
| Homeowner's Insurance | $125.00 | $1,500 | 5.4% |
| Total PITI | $2,333.70 | $28,004 | 100% |
Based on $300,000 loan at 6.5% for 30 years with 20% down payment on a $375,000 home. Tax rate of 1.0% and annual insurance of $1,500. No PMI with 20% down. See PITI Explained for a detailed breakdown.
PITI With PMI (Less Than 20% Down)
If you make a smaller down payment, PMI adds a fifth component. Using the same $375,000 home but with only 5% down ($18,750), the loan amount would be $356,250:
| Component | Monthly Cost |
|---|---|
| Principal & Interest (6.5%, 30 yr on $356,250) | $2,251.74 |
| Property Taxes (1.0% of $375,000) | $312.50 |
| Homeowner's Insurance | $125.00 |
| PMI (1.0% of $356,250) | $296.88 |
| Total Monthly Payment | $2,986.12 |
The difference between a 20% and 5% down payment on the same home is $652.42 per month -- a 28.0% increase in your total housing cost, driven by the larger loan amount plus the addition of PMI.
Calculate Your Full PITI PaymentData Sources and Methodology Notes
Our Mortgage Calculator uses the standard annuity payment formula documented above, which is the industry-standard method for computing fixed-rate mortgage payments. Here are the specific data sources and assumptions that inform our calculations and educational content.
Interest Rate Data
Freddie Mac's Primary Mortgage Market Survey (PMMS)(opens in new tab) is the benchmark source for U.S. mortgage rate data. Published weekly since 1971, it surveys lenders nationwide for the prevailing 30-year and 15-year fixed rates. The 6.5% rate used in our primary example reflects rates in the range observed during early 2026.
Regulatory Framework
- The Truth in Lending Act (TILA) / Regulation Z(opens in new tab) requires lenders to disclose the APR, finance charge, and total of payments using standardized calculations
- The Real Estate Settlement Procedures Act (RESPA) / Regulation X(opens in new tab) governs how escrow accounts for taxes and insurance are calculated and managed
- The Homeowners Protection Act of 1998(opens in new tab) establishes rules for PMI cancellation and automatic termination
Property Tax Data
Property tax estimates reference the U.S. Census Bureau American Housing Survey(opens in new tab) and Tax Foundation state-level property tax data(opens in new tab). Actual rates vary significantly by jurisdiction.
Calculator Assumptions and Limitations
- All calculations assume a fixed interest rate for the entire loan term. Adjustable-rate mortgages (ARMs) recalculate the payment at each rate adjustment using the remaining balance and remaining months
- Property tax and insurance estimates are approximations. Your actual costs depend on your specific location, property type, and insurer
- The calculator does not account for closing costs, origination fees, or discount points, which affect the total cost of the loan but not the monthly P&I payment
- PMI rates used are industry averages and may differ from quotes you receive from specific insurers
- Rounding: Our calculator carries full decimal precision through all intermediate calculations and rounds only the final monthly payment to the nearest cent
Frequently Asked Questions
How is a monthly mortgage payment calculated?
A monthly mortgage payment is calculated using the annuity formula: M = P[r(1+r)n] / [(1+r)n - 1]. M is the 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 monthly payments (loan term in years multiplied by 12). For example, a $300,000 loan at 6.5% for 30 years produces a monthly principal and interest payment of $1,896.20.
What is the difference between P&I and PITI?
P&I (principal and interest) is the base mortgage payment calculated by the standard formula. PITI adds property taxes, homeowner's insurance, and (if applicable) private mortgage insurance (PMI) to the P&I amount. For a $300,000 loan at 6.5% for 30 years on a $375,000 home, the P&I is $1,896.20, but the total PITI payment may be around $2,334 when you include $313/month in property taxes and $125/month in insurance.
How is PMI calculated on a mortgage?
PMI is typically calculated as an annual percentage of the original loan amount, divided by 12 for the monthly cost. PMI rates generally range from 0.5% to 1.5% of the loan amount per year, depending on your credit score, down payment size, and loan-to-value ratio. On a $300,000 loan, PMI at 0.8% costs $2,400 per year or $200 per month. PMI is generally required when your down payment is less than 20% of the home price.
Why does most of my mortgage payment go to interest at first?
Interest is calculated each month on the remaining loan balance. At the start of a mortgage, your balance is at its highest, so the interest charge is large. On a $300,000 loan at 6.5%, month one's interest is $1,625 out of a $1,896.20 payment -- that is 85.7% going to interest. As you pay down the balance each month, less interest accrues, and more of your fixed payment goes toward principal. The crossover point where principal exceeds interest typically occurs around year 20 on a 30-year mortgage at current rates.
How much interest do you pay over 30 years on a $300,000 mortgage?
On a $300,000 mortgage at 6.5% for 30 years, you pay a total of $682,633 over the life of the loan. That means $382,633 goes to interest -- more than the original amount borrowed. Choosing a 15-year term at the same rate reduces total interest to $170,398, saving over $212,000, though the monthly payment increases from $1,896.20 to $2,613.32.
Sources
- Consumer Financial Protection Bureau -- Owning a Home (opens in new tab)
- CFPB -- How Does My Mortgage Amortization Work? (opens in new tab)
- Freddie Mac -- Primary Mortgage Market Survey (PMMS) (opens in new tab)
- CFPB -- What Is Private Mortgage Insurance (PMI)? (opens in new tab)
- CFPB -- When Can I Remove PMI? (opens in new tab)
- Truth in Lending Act (TILA) / Regulation Z (opens in new tab)
- Real Estate Settlement Procedures Act (RESPA) / Regulation X (opens in new tab)
- U.S. Census Bureau -- American Housing Survey (opens in new tab)
- U.S. Department of Housing and Urban Development -- Buying a Home (opens in new tab)
- Tax Foundation -- Property Taxes by State (opens in new tab)