function calculateMortgage() {
// 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);
// Validation
if (isNaN(homePrice) || isNaN(downPayment) || isNaN(loanTermYears) || isNaN(interestRateAnnual)) {
alert("Please enter valid numbers for all required fields.");
return;
}
if (downPayment >= homePrice) {
alert("Down payment cannot be greater than or equal to the home price.");
return;
}
// Calculations
var loanAmount = homePrice – downPayment;
var monthlyInterestRate = (interestRateAnnual / 100) / 12;
var numberOfPayments = loanTermYears * 12;
var monthlyPI = 0;
// Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
if (interestRateAnnual === 0) {
monthlyPI = loanAmount / numberOfPayments;
} else {
monthlyPI = loanAmount * (monthlyInterestRate * Math.pow(1 + monthlyInterestRate, numberOfPayments)) / (Math.pow(1 + monthlyInterestRate, numberOfPayments) – 1);
}
var monthlyTax = 0;
if (!isNaN(propertyTaxAnnual)) {
monthlyTax = propertyTaxAnnual / 12;
}
var monthlyInsurance = 0;
if (!isNaN(homeInsuranceAnnual)) {
monthlyInsurance = homeInsuranceAnnual / 12;
}
var totalMonthlyPayment = monthlyPI + monthlyTax + monthlyInsurance;
var totalInterest = (monthlyPI * numberOfPayments) – loanAmount;
// Display Results
document.getElementById('result').style.display = "block";
document.getElementById('res-pi').innerHTML = "$" + monthlyPI.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('res-tax').innerHTML = "$" + monthlyTax.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('res-ins').innerHTML = "$" + monthlyInsurance.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('res-total').innerHTML = "$" + totalMonthlyPayment.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('res-loan-amount').innerHTML = "$" + loanAmount.toLocaleString('en-US');
document.getElementById('res-total-interest').innerHTML = "$" + totalInterest.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
}
Understanding Your Mortgage Calculation
Purchasing a home is one of the most significant financial decisions you will make. Understanding how your monthly mortgage payment is calculated is essential for budgeting and determining what price range is affordable for you. This calculator helps you break down the various components that make up your monthly housing obligation.
Components of a Mortgage Payment
Your monthly payment is typically referred to as PITI, which stands for:
Principal: The portion of the payment that goes toward paying down the money you borrowed. In the early years of a mortgage, this amount is small, but it increases over time.
Interest: The cost of borrowing money. This is paid to the lender. In the beginning of your loan term, the majority of your payment goes toward interest.
Taxes: Property taxes assessed by your local government. These are often collected by the lender in an escrow account and paid on your behalf annually or semi-annually.
Insurance: Homeowners insurance protects your property against damage. Like taxes, this is often bundled into your monthly payment via escrow.
How Interest Rates Affect Your Payment
The interest rate is perhaps the most critical factor in your monthly cost. Even a small difference in percentage points can result in thousands of dollars of extra interest paid over the life of the loan. For example, on a $300,000 loan, the difference between a 6% and a 7% interest rate can change your monthly payment by hundreds of dollars.
The Impact of the Loan Term
Most homebuyers choose between a 15-year and a 30-year mortgage term. A 30-year term offers lower monthly payments because the repayment is spread out over a longer period, but you will pay significantly more in total interest. A 15-year term has higher monthly payments, but you build equity faster and pay much less interest overall.
Why Your Down Payment Matters
Your down payment reduces the principal amount of the loan. A larger down payment (typically 20% or more) not only lowers your monthly Principal & Interest payment but also helps you avoid Private Mortgage Insurance (PMI). PMI is an additional cost lenders charge borrowers who have less than 20% equity in their home to protect the lender against default.
Using This Calculator
To get the most accurate estimate, enter the specific property tax and insurance rates for the area where you are looking to buy. While national averages can give you a rough idea, local tax rates vary significantly and can drastically alter your monthly affordability.