// Initialize date input to today
(function() {
var today = new Date();
var dd = String(today.getDate()).padStart(2, '0');
var mm = String(today.getMonth() + 1).padStart(2, '0'); //January is 0!
var yyyy = today.getFullYear();
document.getElementById('mc-start-date').value = yyyy + '-' + mm + '-' + dd;
})();
function calculateMortgage() {
// 1. Get Inputs
var homePrice = parseFloat(document.getElementById('mc-home-price').value);
var downPayment = parseFloat(document.getElementById('mc-down-payment').value);
var interestRate = parseFloat(document.getElementById('mc-interest-rate').value);
var years = parseInt(document.getElementById('mc-loan-term').value);
var annualTax = parseFloat(document.getElementById('mc-property-tax').value);
var annualInsurance = parseFloat(document.getElementById('mc-home-insurance').value);
var monthlyHOA = parseFloat(document.getElementById('mc-hoa-fees').value);
var startDateVal = document.getElementById('mc-start-date').value;
// 2. Validation
if (isNaN(homePrice) || isNaN(downPayment) || isNaN(interestRate) || isNaN(years)) {
alert("Please enter valid numbers for the main loan details.");
return;
}
if (isNaN(annualTax)) annualTax = 0;
if (isNaN(annualInsurance)) annualInsurance = 0;
if (isNaN(monthlyHOA)) monthlyHOA = 0;
// 3. Core Calculations
var principal = homePrice – downPayment;
if (principal <= 0) {
alert("Down payment cannot be greater than or equal to home price.");
return;
}
var monthlyRate = (interestRate / 100) / 12;
var numberOfPayments = years * 12;
// Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
var monthlyPI = 0;
if (interestRate === 0) {
monthlyPI = principal / numberOfPayments;
} else {
monthlyPI = principal * (monthlyRate * Math.pow(1 + monthlyRate, numberOfPayments)) / (Math.pow(1 + monthlyRate, numberOfPayments) – 1);
}
// Monthly components
var monthlyTax = annualTax / 12;
var monthlyInsurance = annualInsurance / 12;
var totalMonthlyPayment = monthlyPI + monthlyTax + monthlyInsurance + monthlyHOA;
// Totals
var totalPaymentOverLife = (monthlyPI * numberOfPayments);
var totalInterest = totalPaymentOverLife – principal;
var totalCost = totalPaymentOverLife + (monthlyTax * numberOfPayments) + (monthlyInsurance * numberOfPayments) + (monthlyHOA * numberOfPayments);
// Date Calculation
var payoffDateStr = "-";
if (startDateVal) {
var date = new Date(startDateVal);
date.setMonth(date.getMonth() + numberOfPayments);
var options = { year: 'numeric', month: 'long', day: 'numeric' };
payoffDateStr = date.toLocaleDateString('en-US', options);
}
// 4. Formatting helper
function formatMoney(num) {
return '$' + num.toFixed(2).replace(/\d(?=(\d{3})+\.)/g, '$&,');
}
// 5. Update DOM
document.getElementById('mc-pi-val').innerText = formatMoney(monthlyPI);
document.getElementById('mc-tax-val').innerText = formatMoney(monthlyTax);
document.getElementById('mc-ins-val').innerText = formatMoney(monthlyInsurance);
document.getElementById('mc-hoa-val').innerText = formatMoney(monthlyHOA);
document.getElementById('mc-monthly-total').innerText = formatMoney(totalMonthlyPayment);
document.getElementById('mc-total-interest').innerText = formatMoney(totalInterest);
document.getElementById('mc-total-cost').innerText = formatMoney(totalCost); // Total PITI+HOA over life
document.getElementById('mc-payoff-date').innerText = payoffDateStr;
// Show results
document.getElementById('mc-results').style.display = 'block';
}
Understanding Your Mortgage Amortization & Monthly Payments
Buying a home is one of the most significant financial decisions you will ever make. Understanding how your mortgage payment is calculated is crucial for budgeting and long-term financial planning. This Mortgage Payment Calculator breaks down the PITI (Principal, Interest, Taxes, and Insurance) to give you a clear picture of your monthly obligations.
What is PITI?
Your monthly mortgage payment is typically composed of four main parts, known by the acronym PITI:
Principal: The portion of your payment that goes toward paying down the loan balance (the amount you borrowed).
Interest: The fee charged by the lender for borrowing the money. In the early years of a mortgage, a large percentage of your payment goes toward interest.
Taxes: Property taxes assessed by your local government. Lenders often 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 often collected monthly in escrow.
How Interest Rates Impact 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, a 1% increase in interest rate can increase your monthly payment by hundreds of dollars and your total interest paid by over $50,000 or more over the life of a 30-year loan.
The Power of the Down Payment
Your down payment directly reduces the Principal amount of the loan. A larger down payment (typically 20% or more) has several benefits:
It lowers your monthly principal and interest payment.
It may help you avoid Private Mortgage Insurance (PMI), an extra cost lenders charge borrowers with smaller down payments.
It builds immediate equity in your home.
Short-Term vs. Long-Term Loans
This calculator allows you to compare different loan terms, such as 15-year vs. 30-year mortgages. While a 30-year mortgage offers lower monthly payments, you will pay significantly more in interest over the life of the loan. A 15-year mortgage has higher monthly payments but builds equity faster and saves you thousands in interest.
How to Use This Calculator
To get the most accurate estimate, gather your financial details before starting:
Enter Home Price: The purchase price of the property.
Input Down Payment: The cash amount you are paying upfront.
Set Interest Rate: Enter your expected annual interest rate.
Adjust Taxes & Insurance: These vary by location. Check local property tax rates and get an insurance quote for accuracy.
Don't Forget HOA: If buying a condo or in a planned community, add the Homeowners Association fees.
Use the results to determine your "Payoff Date" and see the total interest cost to decide which loan structure is best for your financial goals.