Mortgage Calculator
How Mortgage Calculations Work
A mortgage calculator helps you estimate your monthly payment and understand the total cost of homeownership. It considers the principal loan amount, interest rate, loan term, and additional costs like property taxes, insurance, and PMI to give you a complete picture of your monthly housing expense.
Understanding your mortgage payment is crucial for budgeting and determining how much house you can afford. The calculation uses an amortization schedule where early payments go mostly toward interest, and later payments apply more toward the principal balance.
Mortgage Calculation Formula
Monthly Principal & Interest Payment
M = P × [r(1+r)^n] / [(1+r)^n - 1]
Where:
M = Monthly payment
P = Principal loan amount (home price - down payment)
r = Monthly interest rate (annual rate ÷ 12)
n = Total number of payments (years × 12)
Total Monthly Payment (PITI)
Total Payment = Principal & Interest + Property Tax + Insurance + PMI
Property Tax (monthly) = Annual Property Tax ÷ 12
Insurance (monthly) = Annual Insurance ÷ 12
PMI (monthly) = If down payment < 20%, typically 0.3-1.5% of loan amount annually
Key Calculations
- Loan Amount: Home Price - Down Payment
- Down Payment %: (Down Payment ÷ Home Price) × 100
- Total Interest: (Monthly Payment × Total Payments) - Principal
- Total Cost: Home Price + Total Interest + Fees
Examples
Example 1: Conventional 30-Year Mortgage
Home Price: $400,000
Down Payment: $80,000 (20%)
Loan Amount: $320,000
Interest Rate: 6.5% annually
Term: 30 years
Monthly P&I: $2,022
Total Interest: $407,920
Example 2: Low Down Payment with PMI
Home Price: $300,000
Down Payment: $15,000 (5%)
Loan Amount: $285,000
Interest Rate: 7.0% annually
PMI: $142/month (0.6% annually)
Monthly P&I: $1,898
Total Monthly (with PMI): $2,040
Example 3: 15-Year Mortgage
Home Price: $350,000
Down Payment: $70,000 (20%)
Loan Amount: $280,000
Interest Rate: 6.0% annually
Term: 15 years
Monthly P&I: $2,366
Total Interest: $145,880
Frequently Asked Questions
When to Use This Calculator
Use this before shopping for a home to set a realistic budget, during the pre-approval process to verify lender numbers, and when comparing fixed vs adjustable-rate mortgage options over different loan terms.
Common Mistakes to Avoid
- Only looking at principal and interest — a mortgage payment also includes property taxes, homeowner's insurance, and possibly HOA fees and PMI.
- Assuming you can afford the maximum loan amount — lenders approve the maximum they will lend, not the maximum you should borrow given your lifestyle.
- Not comparing at least 3 lenders — mortgage rates and fees vary significantly; a 0.5% difference in rate saves thousands over 30 years.