function calculateMortgage() {
// Get Input Values
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 loanTermYears = parseInt(document.getElementById('mc_loan_term').value);
var annualTax = parseFloat(document.getElementById('mc_tax').value);
var annualInsurance = parseFloat(document.getElementById('mc_insurance').value);
var monthlyHOA = parseFloat(document.getElementById('mc_hoa').value);
var errorDiv = document.getElementById('mc_error');
var resultsDiv = document.getElementById('mc_results');
// Validation
if (isNaN(homePrice) || isNaN(downPayment) || isNaN(interestRate) || isNaN(annualTax) || isNaN(annualInsurance)) {
errorDiv.style.display = 'block';
resultsDiv.style.display = 'none';
return;
}
// Handle HOA default if empty or NaN
if (isNaN(monthlyHOA)) monthlyHOA = 0;
errorDiv.style.display = 'none';
// Calculations
var loanAmount = homePrice – downPayment;
// Handle case where down payment >= home price
if (loanAmount <= 0) {
loanAmount = 0;
var monthlyPI = 0;
var totalInterest = 0;
} else {
var monthlyRate = (interestRate / 100) / 12;
var numberOfPayments = loanTermYears * 12;
// Mortgage Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
if (interestRate === 0) {
var monthlyPI = loanAmount / numberOfPayments;
} else {
var monthlyPI = loanAmount * (monthlyRate * Math.pow(1 + monthlyRate, numberOfPayments)) / (Math.pow(1 + monthlyRate, numberOfPayments) – 1);
}
var totalPaymentPI = monthlyPI * numberOfPayments;
var totalInterest = totalPaymentPI – loanAmount;
}
var monthlyTax = annualTax / 12;
var monthlyInsurance = annualInsurance / 12;
var totalMonthlyPayment = monthlyPI + monthlyTax + monthlyInsurance + monthlyHOA;
var totalCost = (monthlyPI * (loanTermYears * 12)) + (monthlyTax * (loanTermYears * 12)) + (monthlyInsurance * (loanTermYears * 12)) + (monthlyHOA * (loanTermYears * 12));
// Date Calculation
var today = new Date();
var payoffDate = new Date(today.setMonth(today.getMonth() + (loanTermYears * 12)));
var options = { month: 'long', year: 'numeric' };
var payoffDateString = payoffDate.toLocaleDateString('en-US', options);
// Display Results
resultsDiv.style.display = 'block';
// Helper for currency formatting
var fmt = new Intl.NumberFormat('en-US', { style: 'currency', currency: 'USD' });
document.getElementById('res_total_monthly').innerHTML = fmt.format(totalMonthlyPayment);
document.getElementById('res_pi').innerHTML = fmt.format(monthlyPI);
document.getElementById('res_tax').innerHTML = fmt.format(monthlyTax);
document.getElementById('res_ins').innerHTML = fmt.format(monthlyInsurance);
document.getElementById('res_hoa').innerHTML = fmt.format(monthlyHOA);
document.getElementById('res_loan_amount').innerHTML = fmt.format(loanAmount);
document.getElementById('res_total_interest').innerHTML = fmt.format(totalInterest);
document.getElementById('res_total_cost').innerHTML = fmt.format(totalCost); // Approximation for total cost over term
document.getElementById('res_payoff_date').innerHTML = payoffDateString;
}
Understanding Your Mortgage Payment
Buying a home is one of the largest financial decisions most people make. Understanding how your monthly mortgage payment is calculated is crucial for budgeting and long-term financial planning. This calculator uses the PITI model (Principal, Interest, Taxes, and Insurance) to give you a comprehensive view of your potential housing costs.
What is PITI?
PITI stands for the four main components of a monthly mortgage payment:
Principal: The portion of your payment that goes toward paying down the loan balance (the amount you borrowed).
Interest: The cost of borrowing money, paid to the lender. In the early years of a mortgage, a larger 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 annual bill for you.
Insurance: Homeowners insurance protects your property against damage. Like taxes, this is often collected monthly in escrow.
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 your 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 tens of thousands over 30 years. Using the calculator above, you can experiment with different rates to see how they impact your affordability.
The Role of the Down Payment
Your down payment reduces the loan amount, which lowers your monthly principal and interest payments. Furthermore, if you put down less than 20% of the home's value, you may be required to pay Private Mortgage Insurance (PMI), which is an additional cost not always included in standard calculations. A larger down payment also provides immediate equity in your home.
Loan Term: 15 vs. 30 Years
The term of your loan determines how long you have to pay it back. A 30-year term is standard and offers lower monthly payments, spreading the cost over a longer period. However, you will pay significantly more in interest. A 15-year term will have higher monthly payments, but you will build equity faster and pay far less interest over the life of the loan.
Don't Forget HOA Fees
If you are buying a condo or a home in a planned community, you will likely have to pay Homeowners Association (HOA) fees. These are usually paid directly to the association, not the lender, but lenders consider them when calculating your debt-to-income ratio. We included an HOA field in this calculator to ensure you see the full picture of your monthly housing obligation.