Calculating your potential mortgage payment is a critical first step in the home buying process. This advanced mortgage calculator helps you estimate your monthly costs by factoring in not just the loan principal and interest, but also the "hidden" costs of homeownership like property taxes, homeowners insurance, and HOA fees. Collectively, these components are known as PITI.
The Components of PITI
When lenders look at your ability to afford a home, they look at four specific factors:
Principal: The portion of your payment that goes toward paying down the loan balance.
Interest: The cost of borrowing the money, paid to the lender.
Taxes: Property taxes paid to your local government, usually held in escrow.
Insurance: Homeowners insurance to protect the property against damage.
How Interest Rates Affect Affordability
Even a small change in interest rates can significantly impact your monthly payment and the total amount you pay over the life of the loan. For example, on a $300,000 loan, a 1% increase in the interest rate can increase your monthly payment by hundreds of dollars and your total interest paid by tens of thousands. Use the calculator above to test different rate scenarios to see what you can comfortably afford.
The Impact of the Loan Term
Most homebuyers opt for a 30-year fixed-rate mortgage because it offers the lowest monthly payment. However, a 15-year term will save you a substantial amount in interest charges, though the monthly payments will be higher. This calculator allows you to toggle between 15 and 30-year terms (and others) to compare the financial impact of paying off your home sooner.
function calculateMortgage() {
// Get input values using var
var homePrice = parseFloat(document.getElementById("homePrice").value);
var downPayment = parseFloat(document.getElementById("downPayment").value);
var interestRate = parseFloat(document.getElementById("interestRate").value);
var loanTerm = parseInt(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 to prevent NaN errors
if (isNaN(homePrice) || isNaN(downPayment) || isNaN(interestRate) || isNaN(loanTerm)) {
alert("Please enter valid numbers for Price, Down Payment, Rate, and Term.");
return;
}
// Handle empty optional fields by setting to 0
if (isNaN(propertyTax)) propertyTax = 0;
if (isNaN(homeInsurance)) homeInsurance = 0;
if (isNaN(hoaFees)) hoaFees = 0;
// Core Calculations
var loanAmount = homePrice – downPayment;
var monthlyRate = (interestRate / 100) / 12;
var numberOfPayments = loanTerm * 12;
var monthlyPI = 0;
// Amortization Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
if (interestRate === 0) {
monthlyPI = loanAmount / numberOfPayments;
} else {
var x = Math.pow(1 + monthlyRate, numberOfPayments);
monthlyPI = (loanAmount * x * monthlyRate) / (x – 1);
}
// Monthly Tax and Insurance calculations
var monthlyTax = propertyTax / 12;
var monthlyInsurance = homeInsurance / 12;
// Total Monthly Payment
var totalMonthlyPayment = monthlyPI + monthlyTax + monthlyInsurance + hoaFees;
// Total Interest Calculation
var totalCostOfLoan = monthlyPI * numberOfPayments;
var totalInterest = totalCostOfLoan – loanAmount;
// Update the DOM with results
// Use toLocaleString for currency formatting
document.getElementById("res-pi").innerText = "$" + monthlyPI.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById("res-tax").innerText = "$" + monthlyTax.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById("res-ins").innerText = "$" + monthlyInsurance.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById("res-hoa").innerText = "$" + hoaFees.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById("res-total").innerText = "$" + totalMonthlyPayment.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById("res-total-interest").innerText = "$" + totalInterest.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
// Show results section
document.getElementById("results").style.display = "block";
}