function calculateMortgage() {
// 1. Get Input Values
var price = parseFloat(document.getElementById('mc-home-price').value);
var down = parseFloat(document.getElementById('mc-down-payment').value);
var rate = parseFloat(document.getElementById('mc-interest-rate').value);
var years = parseFloat(document.getElementById('mc-loan-term').value);
var taxYear = parseFloat(document.getElementById('mc-property-tax').value);
var insYear = parseFloat(document.getElementById('mc-insurance').value);
var hoaMonth = parseFloat(document.getElementById('mc-hoa').value);
// Validation
if (isNaN(price) || isNaN(down) || isNaN(rate) || isNaN(years)) {
alert("Please enter valid numbers for the core fields.");
return;
}
// Fallback for optional fields
if (isNaN(taxYear)) taxYear = 0;
if (isNaN(insYear)) insYear = 0;
if (isNaN(hoaMonth)) hoaMonth = 0;
// 2. Core Calculations
var principal = price – down;
// Monthly Interest Rate
var monthlyRate = (rate / 100) / 12;
// Total Number of Payments
var numberOfPayments = years * 12;
// Monthly Principal & Interest (Standard Amortization Formula)
// M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1]
var monthlyPI = 0;
if (rate === 0) {
monthlyPI = principal / numberOfPayments;
} else {
monthlyPI = principal * ( (monthlyRate * Math.pow(1 + monthlyRate, numberOfPayments)) / (Math.pow(1 + monthlyRate, numberOfPayments) – 1) );
}
// Monthly Taxes and Insurance
var monthlyTax = taxYear / 12;
var monthlyIns = insYear / 12;
// Total Monthly Payment
var totalMonthly = monthlyPI + monthlyTax + monthlyIns + hoaMonth;
// Total Loan Stats
var totalPaidPI = monthlyPI * numberOfPayments;
var totalInterest = totalPaidPI – principal;
var totalCost = totalPaidPI + (monthlyTax * numberOfPayments) + (monthlyIns * numberOfPayments) + (hoaMonth * numberOfPayments) + down;
// Payoff Date Calculation
var today = new Date();
var payoffDate = new Date(today.setMonth(today.getMonth() + numberOfPayments));
var options = { year: 'numeric', month: 'long' };
var payoffString = payoffDate.toLocaleDateString('en-US', options);
// 3. Update UI
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
minimumFractionDigits: 2
});
document.getElementById('mc-total-monthly').innerText = formatter.format(totalMonthly);
document.getElementById('mc-pi-monthly').innerText = formatter.format(monthlyPI);
document.getElementById('mc-tax-monthly').innerText = formatter.format(monthlyTax);
document.getElementById('mc-ins-monthly').innerText = formatter.format(monthlyIns);
document.getElementById('mc-hoa-monthly').innerText = formatter.format(hoaMonth);
document.getElementById('mc-total-interest').innerText = formatter.format(totalInterest);
document.getElementById('mc-total-cost').innerText = formatter.format(totalCost);
document.getElementById('mc-payoff-date').innerText = payoffString;
// Show Results
document.getElementById('mc-results').style.display = 'block';
}
Understanding Your Mortgage Payment
Calculating your monthly mortgage payment is a critical first step in the home-buying process. Our Mortgage Payment Calculator breaks down the costs associated with homeownership, helping you determine exactly how much house you can afford. While the loan principal and interest make up the bulk of your payment, other factors like taxes, insurance, and HOA fees can significantly impact your monthly budget.
Principal and Interest (P&I)
The "Principal" is the amount of money you borrow from the lender (Home Price minus Down Payment). "Interest" is the cost of borrowing that money. In the early years of a mortgage, a large portion of your payment goes toward interest. As time passes, more of your payment is applied to the principal balance, a process known as amortization.
The Impact of Interest Rates
Even a small difference in your interest rate can save or cost you tens of thousands of dollars over the life of a loan. Factors affecting your rate include:
Credit Score: Higher scores generally qualify for lower rates.
Down Payment: A larger down payment (20% or more) often secures better terms and avoids Private Mortgage Insurance (PMI).
Loan Term: 15-year loans typically have lower rates than 30-year loans but come with higher monthly payments.
Hidden Costs: Taxes, Insurance, and HOAs
When budgeting for a home, do not overlook the "escrow" items. Lenders often collect property taxes and homeowners insurance premiums monthly along with your mortgage payment. Additionally, if you are buying a condo or a home in a planned community, you may have Homeowners Association (HOA) fees. These fees are usually paid directly to the association but are included in this calculator to give you a complete picture of your monthly housing expense.
How to Lower Your Monthly Payments
If the estimated payment is higher than your budget allows, consider these strategies:
Increase your down payment: This lowers the principal loan amount.
Extend the loan term: Moving from a 15-year to a 30-year term lowers monthly payments, though you will pay more interest in the long run.
Shop for cheaper insurance: Compare quotes from different providers to reduce your homeowners insurance costs.
Buy "points": You can sometimes pay an upfront fee to the lender to lower your interest rate.
Use this calculator to experiment with different scenarios—such as changing the down payment or interest rate—to find a mortgage plan that fits your financial goals.