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Principal & Interest:$0.00
Monthly Property Tax:$0.00
Monthly Home Insurance:$0.00
Total Monthly Payment:$0.00
Understanding How Your Mortgage Payment is Calculated
Calculating your monthly mortgage payment is a critical step in the home buying process. It helps you determine exactly how much house you can afford and ensures you remain within your financial budget. This calculator breaks down the total monthly cost into its primary components: principal, interest, taxes, and insurance (PITI).
Key Factors Affecting Your Mortgage
Home Price & Down Payment: The difference between these two figures gives you your loan amount (the Principal). A larger down payment reduces the principal and typically lowers your monthly payment and interest costs.
Interest Rate: This is the cost of borrowing money. Even a small difference in percentage points (e.g., 6.0% vs 6.5%) can add up to tens of thousands of dollars over the life of a 30-year loan.
Loan Term: Most mortgages are 15 or 30 years. A 30-year term offers lower monthly payments but results in higher total interest paid compared to a 15-year term.
Escrow Costs (Taxes & Insurance): Lenders often require you to pay a portion of your annual property taxes and homeowners insurance each month. These funds are held in an escrow account and paid on your behalf when due.
The Mortgage Formula Explained
While our calculator does the heavy lifting instantly, the math behind a standard fixed-rate mortgage uses the following amortization formula:
M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
Where:
M = Total monthly payment
P = Principal loan amount
i = Monthly interest rate (Annual rate divided by 12)
n = Number of payments (Loan term in years multiplied by 12)
Why Use a Mortgage Calculator?
Using a reliable mortgage calculator allows you to experiment with different scenarios. You can see how increasing your down payment affects your monthly out-of-pocket costs, or compare the monthly savings of a lower interest rate versus the cost of buying "points." By inputting your specific property tax and insurance estimates, you get a realistic view of your actual monthly housing obligation, not just the bank loan repayment.
function calculateMortgage() {
// 1. Get DOM elements
var elHomePrice = document.getElementById('mc_home_price');
var elDownPayment = document.getElementById('mc_down_payment');
var elInterestRate = document.getElementById('mc_interest_rate');
var elLoanTerm = document.getElementById('mc_loan_term');
var elPropertyTax = document.getElementById('mc_property_tax');
var elInsurance = document.getElementById('mc_insurance');
var elResults = document.getElementById('mc_results');
var elError = document.getElementById('mc_error');
// 2. Parse values
var homePrice = parseFloat(elHomePrice.value);
var downPayment = parseFloat(elDownPayment.value);
var interestRate = parseFloat(elInterestRate.value);
var loanTerm = parseFloat(elLoanTerm.value);
var yearlyTax = parseFloat(elPropertyTax.value);
var yearlyInsurance = parseFloat(elInsurance.value);
// 3. Validation
// Assume Tax and Insurance can be 0, but Loan details need to be positive.
if (isNaN(homePrice) || isNaN(downPayment) || isNaN(interestRate) || isNaN(loanTerm) ||
homePrice <= 0 || loanTerm <= 0) {
elError.style.display = 'block';
elResults.style.display = 'none';
return;
}
// Handle optional fields if empty
if (isNaN(yearlyTax)) yearlyTax = 0;
if (isNaN(yearlyInsurance)) yearlyInsurance = 0;
elError.style.display = 'none';
// 4. Calculation Logic
var principal = homePrice – downPayment;
// Handle edge case where principal is 0 or less
if (principal <= 0) {
// Technically no loan needed
var monthlyPI = 0;
} else {
var monthlyRate = (interestRate / 100) / 12;
var numberOfPayments = loanTerm * 12;
// Amortization Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
if (monthlyRate === 0) {
var monthlyPI = principal / numberOfPayments;
} else {
var numerator = monthlyRate * Math.pow((1 + monthlyRate), numberOfPayments);
var denominator = Math.pow((1 + monthlyRate), numberOfPayments) – 1;
var monthlyPI = principal * (numerator / denominator);
}
}
var monthlyTax = yearlyTax / 12;
var monthlyInsurance = yearlyInsurance / 12;
var totalMonthly = monthlyPI + monthlyTax + monthlyInsurance;
// 5. Update UI
document.getElementById('res_principal_interest').innerHTML = '$' + formatMoney(monthlyPI);
document.getElementById('res_tax').innerHTML = '$' + formatMoney(monthlyTax);
document.getElementById('res_insurance').innerHTML = '$' + formatMoney(monthlyInsurance);
document.getElementById('res_total').innerHTML = '$' + formatMoney(totalMonthly);
elResults.style.display = 'block';
}
function formatMoney(num) {
return num.toFixed(2).replace(/\d(?=(\d{3})+\.)/g, '$&,');
}