function calculateMortgage() {
// Get input values
var homePrice = parseFloat(document.getElementById("homePrice").value);
var downPayment = parseFloat(document.getElementById("downPayment").value);
var interestRate = parseFloat(document.getElementById("interestRate").value);
var loanTerm = parseFloat(document.getElementById("loanTerm").value);
var propertyTax = parseFloat(document.getElementById("propertyTax").value);
var homeInsurance = parseFloat(document.getElementById("homeInsurance").value);
var hoaFees = parseFloat(document.getElementById("hoaFees").value);
// Validation
if (isNaN(homePrice) || isNaN(downPayment) || isNaN(interestRate) || isNaN(loanTerm)) {
alert("Please enter valid numbers for the main loan details.");
return;
}
// Handle optional fields as 0 if empty/NaN
if (isNaN(propertyTax)) propertyTax = 0;
if (isNaN(homeInsurance)) homeInsurance = 0;
if (isNaN(hoaFees)) hoaFees = 0;
// Core Calculations
var principal = homePrice – downPayment;
var monthlyRate = (interestRate / 100) / 12;
var numberOfPayments = loanTerm * 12;
// Mortgage Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
var monthlyPrincipalInterest = 0;
if (interestRate === 0) {
monthlyPrincipalInterest = principal / numberOfPayments;
} else {
var numerator = monthlyRate * Math.pow((1 + monthlyRate), numberOfPayments);
var denominator = Math.pow((1 + monthlyRate), numberOfPayments) – 1;
monthlyPrincipalInterest = principal * (numerator / denominator);
}
// Additional Monthly Costs
var monthlyTax = propertyTax / 12;
var monthlyInsurance = homeInsurance / 12;
var totalMonthlyPayment = monthlyPrincipalInterest + monthlyTax + monthlyInsurance + hoaFees;
// Display Results
document.getElementById("resultPI").innerHTML = "$" + monthlyPrincipalInterest.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById("resultTax").innerHTML = "$" + monthlyTax.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById("resultIns").innerHTML = "$" + monthlyInsurance.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById("resultHOA").innerHTML = "$" + hoaFees.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById("resultTotal").innerHTML = "$" + totalMonthlyPayment.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
// Show result div
document.getElementById("resultsArea").style.display = "block";
}
Understanding Your Mortgage Payment
Buying a home is one of the most significant financial decisions you will make in your lifetime. Understanding exactly how your monthly mortgage payment is calculated is crucial for budgeting and determining "how much house" you can afford. This Mortgage Payment Calculator helps you estimate your monthly obligations by factoring in not just the loan repayment, but also taxes, insurance, and association fees.
Components of a Mortgage Payment (PITI)
Most mortgage payments are made up of four primary components, often referred to by the acronym PITI:
Principal: The portion of your payment that goes toward paying down the original amount you borrowed. In the early years of a loan, this amount is small, but it grows over time.
Interest: The cost of borrowing money from your lender. Interest makes up the bulk of your payment in the early years of a standard 30-year mortgage.
Taxes: Property taxes assessed by your local government. Lenders typically collect this monthly and hold it in an escrow account to pay the bill when it's due.
Insurance: Homeowners insurance protects your property against damage. Like taxes, this is usually paid monthly into an escrow account.
How Interest Rates Affect Your Payment
Even a small difference in interest rates can have a massive impact on your monthly payment and the total cost of the loan. For example, on a $300,000 loan:
At 6.0% interest, the monthly P&I is approximately $1,799.
At 7.0% interest, the monthly P&I jumps to approximately $1,996.
That 1% difference costs nearly $200 extra per month and over $70,000 in additional interest over the life of a 30-year loan.
The Impact of Down Payments
Your down payment affects your mortgage in two ways. First, a larger down payment reduces the principal amount you need to borrow, which lowers your monthly payment. Second, if you put down less than 20% of the home's value, lenders often require Private Mortgage Insurance (PMI). PMI protects the lender if you default and can add 0.5% to 1% of the loan amount to your yearly costs.
Using This Calculator
To get the most accurate estimate, try to find the current property tax rate for the area you are looking to buy in (often between 1% and 2% of the home value annually) and get a quote for homeowners insurance. Don't forget to include HOA fees if you are looking at condos or planned communities, as these are paid separately from your mortgage but affect your monthly debt-to-income ratio.