function calculateMortgage() {
// 1. Get Input Values
var price = parseFloat(document.getElementById('mc_price').value);
var down = parseFloat(document.getElementById('mc_down').value);
var years = parseInt(document.getElementById('mc_term').value);
var rate = parseFloat(document.getElementById('mc_rate').value);
var taxYear = parseFloat(document.getElementById('mc_tax').value);
var insYear = parseFloat(document.getElementById('mc_ins').value);
var hoa = parseFloat(document.getElementById('mc_hoa').value);
// 2. Validate Inputs
if (isNaN(price) || isNaN(down) || isNaN(years) || isNaN(rate) || isNaN(taxYear) || isNaN(insYear)) {
alert("Please enter valid numbers in all fields.");
return;
}
if (down >= price) {
alert("Down payment cannot be greater than or equal to the home price.");
return;
}
// 3. Handle HOA default if empty
if (isNaN(hoa)) hoa = 0;
// 4. Core Calculations
var loanAmount = price – down;
var monthlyRate = (rate / 100) / 12;
var totalPayments = years * 12;
// P&I Calculation Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
var monthlyPI = 0;
if (rate === 0) {
monthlyPI = loanAmount / totalPayments;
} else {
monthlyPI = loanAmount * (monthlyRate * Math.pow(1 + monthlyRate, totalPayments)) / (Math.pow(1 + monthlyRate, totalPayments) – 1);
}
var monthlyTax = taxYear / 12;
var monthlyIns = insYear / 12;
var totalMonthlyPayment = monthlyPI + monthlyTax + monthlyIns + hoa;
var totalCost = (monthlyPI * totalPayments);
var totalInterest = totalCost – loanAmount;
// 5. Update UI
// Helper for currency
var fmt = new Intl.NumberFormat('en-US', { style: 'currency', currency: 'USD' });
document.getElementById('mc_monthly_display').innerText = fmt.format(totalMonthlyPayment);
document.getElementById('mc_pi_display').innerText = fmt.format(monthlyPI);
document.getElementById('mc_tax_display').innerText = fmt.format(monthlyTax);
document.getElementById('mc_ins_display').innerText = fmt.format(monthlyIns);
document.getElementById('mc_hoa_display').innerText = fmt.format(hoa);
document.getElementById('mc_loan_amount').innerText = fmt.format(loanAmount);
document.getElementById('mc_total_interest').innerText = fmt.format(totalInterest);
document.getElementById('mc_total_cost').innerText = fmt.format(totalCost + down); // Total cost includes down payment
// Show results
document.getElementById('mc_results').style.display = 'block';
}
Understanding Your Mortgage Payment
Calculating your monthly mortgage payment is one of the first and most critical steps in the home-buying process. While many people focus solely on the principal and interest, a true "out-the-door" monthly cost often includes property taxes, homeowner's insurance, and potentially Homeowner Association (HOA) fees. This is often referred to as PITI.
What is Included in PITI?
When you use our Mortgage Calculator, we break down the payment into four key components:
Principal: The portion of your payment that goes toward paying down the loan balance ($350,000 house price minus down payment).
Interest: The cost of borrowing money, calculated based on your Annual Percentage Rate (APR).
Taxes: Property taxes assessed by your local government, typically paid into an escrow account monthly.
Insurance: Hazard insurance to protect the property, also usually escrowed.
How Interest Rates Affect Affordability
Even a small change in interest rates can drastically alter your monthly payment and the total interest paid over the life of the loan. For example, on a $300,000 loan, the difference between a 6.0% and a 7.0% interest rate is roughly $200 per month. Over a 30-year term, that adds up to over $70,000 in extra cost.
30-Year vs. 15-Year Terms
Choosing your loan term is a balance between monthly cash flow and total savings. A 30-year term offers lower monthly payments, making the home more affordable day-to-day. However, you will pay significantly more interest over the life of the loan. A 15-year term requires higher monthly payments but builds equity much faster and offers a lower interest rate in most market conditions.
The Impact of Your Down Payment
Your down payment determines your Loan-to-Value (LTV) ratio. A larger down payment (typically 20% or more) allows you to avoid Private Mortgage Insurance (PMI), reduces your monthly payment, and may qualify you for better interest rates. If you put down less than 20%, remember that lenders will likely add a PMI premium to your monthly bill, which this calculator does not estimate automatically.
Using This Calculator
To get the most accurate result, check your local county assessor's website for average property tax rates in your desired neighborhood, and call an insurance agent for a quick quote on a home of similar size and age.