Loan Summary:
Loan Amount: $0 |
Total Interest Paid: $0 |
Total Cost of Loan: $0
Understanding Your Mortgage Calculation
Calculating your monthly mortgage payment is one of the most critical steps in the home-buying process. This calculator helps you understand exactly how much you can afford by breaking down the costs associated with owning a home, beyond just the listing price.
Components of a Mortgage Payment (PITI)
Your monthly payment is typically composed of four main parts, often referred to as PITI:
Principal: The portion of your payment that goes toward paying down the loan balance.
Interest: The cost of borrowing money from your lender. In the early years of a mortgage, a larger percentage of your payment goes toward interest.
Taxes: Property taxes assessed by your local government, usually held in an escrow account by your lender.
Insurance: Homeowners insurance protects your property against damage. Lenders require this to protect their investment.
How Interest Rate Affects Your Payment
Even a small difference in interest rates can significantly impact your monthly payment and the total cost of the loan over time. For example, on a $300,000 loan, a 1% increase in interest rate can increase your monthly payment by over $180 and cost you over $65,000 in additional interest over a 30-year term.
PMI and HOA Fees
If your down payment is less than 20% of the home price, you may be required to pay Private Mortgage Insurance (PMI), which protects the lender if you default. Additionally, if you buy a condo or a home in a planned community, you will likely have to pay Homeowners Association (HOA) fees. These are separate from your mortgage but impact your monthly affordability.
function calculateMortgage() {
// 1. Get Input Values
var homePrice = parseFloat(document.getElementById('homePrice').value);
var downPayment = parseFloat(document.getElementById('downPayment').value);
var loanTermYears = parseFloat(document.getElementById('loanTerm').value);
var interestRateAnnual = parseFloat(document.getElementById('interestRate').value);
var propertyTaxAnnual = parseFloat(document.getElementById('propertyTax').value);
var homeInsuranceAnnual = parseFloat(document.getElementById('homeInsurance').value);
var hoaFeesMonthly = parseFloat(document.getElementById('hoaFees').value);
// 2. Input Validation
if (isNaN(homePrice) || homePrice <= 0) {
alert("Please enter a valid Home Price.");
return;
}
if (isNaN(downPayment) || downPayment < 0) {
downPayment = 0;
}
if (isNaN(loanTermYears) || loanTermYears <= 0) {
alert("Please enter a valid Loan Term.");
return;
}
if (isNaN(interestRateAnnual) || interestRateAnnual < 0) {
interestRateAnnual = 0;
}
if (isNaN(propertyTaxAnnual)) propertyTaxAnnual = 0;
if (isNaN(homeInsuranceAnnual)) homeInsuranceAnnual = 0;
if (isNaN(hoaFeesMonthly)) hoaFeesMonthly = 0;
// 3. Core Calculations
var loanAmount = homePrice – downPayment;
// Handle case where down payment exceeds home price
if (loanAmount 0) {
if (monthlyInterestRate > 0) {
var numerator = loanAmount * monthlyInterestRate * Math.pow(1 + monthlyInterestRate, numPayments);
var denominator = Math.pow(1 + monthlyInterestRate, numPayments) – 1;
monthlyPrincipalAndInterest = numerator / denominator;
} else {
// If interest rate is 0
monthlyPrincipalAndInterest = loanAmount / numPayments;
}
}
var monthlyTax = propertyTaxAnnual / 12;
var monthlyInsurance = homeInsuranceAnnual / 12;
var totalMonthlyPayment = monthlyPrincipalAndInterest + monthlyTax + monthlyInsurance + hoaFeesMonthly;
var totalPaymentOverTerm = monthlyPrincipalAndInterest * numPayments;
var totalInterestPaid = totalPaymentOverTerm – loanAmount;
var totalCostOfLoan = totalPaymentOverTerm + (monthlyTax * numPayments) + (monthlyInsurance * numPayments) + (hoaFeesMonthly * numPayments) + downPayment;
// Use loan cost strictly as P+I total for standard summary
var totalLoanCostPI = totalPaymentOverTerm;
// 4. Update UI
// Helper for currency formatting
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
minimumFractionDigits: 2
});
document.getElementById('totalMonthlyPayment').innerHTML = formatter.format(totalMonthlyPayment);
document.getElementById('piDisplay').innerHTML = formatter.format(monthlyPrincipalAndInterest);
document.getElementById('taxDisplay').innerHTML = formatter.format(monthlyTax);
document.getElementById('insDisplay').innerHTML = formatter.format(monthlyInsurance);
document.getElementById('hoaDisplay').innerHTML = formatter.format(hoaFeesMonthly);
document.getElementById('loanAmountDisplay').innerHTML = formatter.format(loanAmount);
document.getElementById('totalInterestDisplay').innerHTML = formatter.format(totalInterestPaid);
document.getElementById('totalCostDisplay').innerHTML = formatter.format(totalLoanCostPI);
// Show result box
document.getElementById('mortgageResult').style.display = 'block';
}