Your monthly mortgage payment is calculated from three variables: loan amount, interest rate, and loan term. Formula: M = P × [r(1+r)^n] / [(1+r)^n − 1]. For a $350,000 loan at 6.5% for 30 years, the principal and interest payment is $2,212. But most lenders require PITI payments — Principal, Interest, Taxes, and Insurance — which typically add $400–$800 more per month.
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What Is a Mortgage Payment Calculator?
A mortgage payment calculator computes your monthly housing cost from a home purchase, including the principal and interest payment derived from the loan amount, interest rate, and term, plus the optional escrow components: property taxes, homeowners insurance, and PMI (Private Mortgage Insurance).
The principal and interest (P&I) portion is calculated using the standard amortization formula: M = P × [r(1+r)^n] / [(1+r)^n − 1], where P is the loan amount, r is the monthly interest rate (annual rate ÷ 12), and n is the total number of monthly payments. This produces a fixed monthly payment that remains constant for the life of a fixed-rate mortgage.
Most lenders collect taxes and insurance through an escrow account alongside the P&I payment. Property taxes (typically 0.5–2.5% of home value per year, divided by 12) and homeowners insurance (typically $100–$200/month) can easily add $400–$800 per month on top of P&I. If your down payment is under 20%, PMI (Private Mortgage Insurance) adds another $100–$300/month until you reach 80% loan-to-value.
Understanding the full PITI payment is essential for accurate affordability planning. Many buyers are surprised when their actual monthly payment significantly exceeds the P&I estimate they calculated or saw advertised. The calculator on RoughTools includes all four components for a complete and realistic housing cost estimate.
How to Use the Mortgage Calculator
The calculator accepts all the inputs needed for a complete payment estimate.
- Enter the home price. The total purchase price of the home.
- Enter the down payment. Dollar amount or percentage. The loan amount = home price minus down payment.
- Enter the interest rate. The annual interest rate from your lender or a rate comparison site. For the most accurate estimate, get a pre-approval rate rather than using advertised rates — your actual rate depends on your credit score and debt-to-income ratio.
- Enter the loan term. 30 years is standard; 15-year mortgages are also common and generate significant interest savings.
- Enter property taxes (optional). Annual property taxes divided by 12 for the monthly escrow component. Find this on the county assessor's website or prior-year tax bill.
- Enter homeowners insurance (optional). Annual premium divided by 12. Average US home insurance runs $1,400–$2,000/year depending on location and coverage.
- Enter PMI rate (if applicable). PMI (Private Mortgage Insurance) is required when the down payment is under 20%. Typically 0.5–1.5% of the loan amount annually.
- Read the results. Complete monthly PITI payment, amortization schedule, total interest paid over the life of the loan, and payoff date.
The amortization table reveals something counterintuitive: in the early years of a mortgage, almost all of your payment is interest. Month 1 of a $350,000, 6.5%, 30-year mortgage: $2,212 payment = $245 principal + $1,896 interest. It takes roughly 10 years before more than half of each monthly payment goes toward principal.
The Mortgage Payment Formula
Principal and Interest (P&I) payment:
M = P × [r(1+r)^n] / [(1+r)^n - 1]
Where:
- M = monthly payment
- P = principal loan amount
- r = monthly interest rate (annual rate ÷ 12)
- n = total number of payments (years × 12)
Worked example — $350,000 home purchase:
| Input | Value | |-------|-------| | Home price | $350,000 | | Down payment | $70,000 (20%) | | Loan amount (P) | $280,000 | | Annual interest rate | 6.5% | | Monthly rate (r) | 0.065 ÷ 12 = 0.005417 | | Loan term | 30 years | | Number of payments (n) | 360 |
Calculation:
r(1+r)^n = 0.005417 × (1.005417)^360
= 0.005417 × 6.8485
= 0.037097
(1+r)^n - 1 = 6.8485 - 1 = 5.8485
M = $280,000 × (0.037097 / 5.8485)
M = $280,000 × 0.006343
M = $1,776/month (P&I only)
Adding PITI components:
| Component | Monthly Amount | |-----------|---------------| | Principal & Interest | $1,776 | | Property Taxes ($4,800/yr) | $400 | | Homeowners Insurance ($1,500/yr) | $125 | | PMI (20% down → $0) | $0 | | Total PITI | $2,301/month |
How Amortization Works
Amortization is the process of gradually paying off a loan through regular payments. Each payment covers interest on the remaining balance plus a portion of principal. As the balance decreases, the interest portion shrinks and the principal portion grows — even though the monthly payment stays constant.
Amortization schedule — first and last few payments on $280,000 at 6.5%:
| Month | Payment | Principal | Interest | Remaining Balance | |-------|---------|-----------|---------|------------------| | 1 | $1,776 | $261 | $1,515 | $279,739 | | 12 | $1,776 | $276 | $1,500 | $277,283 | | 60 (Yr 5) | $1,776 | $314 | $1,461 | $270,244 | | 120 (Yr 10) | $1,776 | $375 | $1,401 | $259,040 | | 180 (Yr 15) | $1,776 | $449 | $1,327 | $243,789 | | 240 (Yr 20) | $1,776 | $536 | $1,240 | $222,433 | | 300 (Yr 25) | $1,776 | $641 | $1,135 | $192,019 | | 350 | $1,776 | $1,605 | $171 | $29,802 | | 360 | $1,776 | $1,766 | $10 | $0 |
Total interest over 30 years: approximately $359,000 — more than the original loan amount. This is why paying extra toward principal early in the loan dramatically reduces total interest paid.
15-Year vs. 30-Year Mortgage: The Key Tradeoff
The 15-year mortgage is the most powerful interest-saving option most homebuyers have access to.
Same $280,000 loan at 6.0% (shorter term typically gets lower rate):
| | 30-Year | 15-Year | |--|---------|---------| | Monthly P&I | $1,679 | $2,363 | | Total paid | $604,440 | $425,340 | | Total interest | $324,440 | $145,340 | | Interest savings | — | $179,100 |
The 15-year mortgage costs $684/month more but saves $179,100 in total interest. The 15-year also builds equity dramatically faster — by year 7, the 15-year borrower has paid down roughly 40% of the principal; the 30-year borrower has paid down less than 10%.
The 30-year is not the wrong choice — the lower payment provides more cash flow flexibility, which matters for households with variable income or who want to invest the payment difference. But the interest cost is real and substantial.
Frequently Asked Questions
What credit score do I need to get the best mortgage rate? Lenders generally offer their best conventional mortgage rates to borrowers with credit scores of 760 or above. Scores between 680–759 still get good rates; below 680, you either pay a higher rate or are limited to FHA loans. Each 20-point drop in credit score typically increases the rate by 0.125–0.25%. On a $300,000 30-year loan, a 0.5% higher rate costs approximately $31,000 more in total interest — a strong incentive to optimize credit before applying.
What is PMI and when does it go away? Private Mortgage Insurance protects the lender (not you) if you default on a loan with under 20% down. It typically costs 0.5%–1.5% of the loan amount annually, added to your monthly payment. Once your loan-to-value ratio (LTV) reaches 80% (either through payments or appreciation), you can request PMI removal; it is automatically cancelled at 78% LTV under the Homeowners Protection Act. On a $280,000 loan with 0.8% PMI, that is $2,240/year ($187/month) until you reach 80% LTV.
How do points and origination fees affect my mortgage? Discount points are prepaid interest — you pay a percentage of the loan at closing to get a lower rate. One point costs 1% of the loan ($2,800 on a $280,000 mortgage) and typically reduces the rate by 0.25%. The break-even: $2,800 ÷ monthly savings from lower payment = months until you break even. If you plan to stay long-term, points save money. Points make sense for buyers who plan to keep the loan at least 5–7 years.
Is it better to make biweekly mortgage payments? Biweekly payments (26 half-payments per year = 13 full payments instead of 12) accelerate payoff by making one extra principal payment per year. On a $280,000, 6.5%, 30-year mortgage, biweekly payments pay off the loan in approximately 25 years instead of 30 and save roughly $70,000 in interest. Most lenders allow this — just confirm they credit payments immediately rather than holding them until month-end.
What happens if I make extra principal payments? Extra principal payments reduce your loan balance immediately, saving interest on every remaining payment (since interest is calculated on the outstanding balance). On a $280,000, 6.5%, 30-year mortgage, adding $200/month to principal saves approximately $75,000 in interest and shortens the loan by about 7 years. Even a single extra payment per year (an "extra month") saves tens of thousands over the loan life. Any extra payment is specifically applied to principal — verify this with your servicer's payment portal.
Related Free Tools on RoughTools
- Loan Payment Calculator — calculate payments for any loan type
- Home Affordability Calculator — find the maximum home price you can afford based on income
- Rent vs. Buy Calculator — compare total costs of renting vs. buying
- Refinance Calculator — calculate whether refinancing saves you money
Calculate Your Mortgage Payment Now
The free Mortgage Calculator at RoughTools calculates your complete monthly PITI payment, generates a full amortization schedule, and compares 15-year versus 30-year scenarios side by side. Enter home price, down payment, rate, and term to get your full housing cost breakdown. No account needed, completely free.