function calculateMortgage() {
// 1. Get input values
var homePrice = parseFloat(document.getElementById('homePrice').value);
var downPayment = parseFloat(document.getElementById('downPayment').value);
var interestRate = parseFloat(document.getElementById('interestRate').value);
var loanTermYears = parseFloat(document.getElementById('loanTerm').value);
var annualTax = parseFloat(document.getElementById('propertyTax').value);
var annualInsurance = parseFloat(document.getElementById('homeInsurance').value);
// 2. Validation
if (isNaN(homePrice) || homePrice < 0) homePrice = 0;
if (isNaN(downPayment) || downPayment < 0) downPayment = 0;
if (isNaN(interestRate) || interestRate < 0) interestRate = 0;
if (isNaN(loanTermYears) || loanTermYears <= 0) loanTermYears = 30;
if (isNaN(annualTax) || annualTax < 0) annualTax = 0;
if (isNaN(annualInsurance) || annualInsurance < 0) annualInsurance = 0;
// 3. Core Calculations
var loanAmount = homePrice – downPayment;
// Prevent negative loan amount
if (loanAmount 0) {
monthlyPI = loanAmount / numberOfPayments;
} else {
monthlyPI = 0;
}
} else {
var numerator = monthlyInterestRate * Math.pow((1 + monthlyInterestRate), numberOfPayments);
var denominator = Math.pow((1 + monthlyInterestRate), numberOfPayments) – 1;
if (denominator !== 0) {
monthlyPI = loanAmount * (numerator / denominator);
}
}
var monthlyTax = annualTax / 12;
var monthlyInsurance = annualInsurance / 12;
var totalMonthly = monthlyPI + monthlyTax + monthlyInsurance;
// 4. Update UI
// Format Currency Helper
function formatCurrency(num) {
return "$" + num.toFixed(2).replace(/\d(?=(\d{3})+\.)/g, '$&,');
}
document.getElementById('totalMonthlyPayment').innerText = formatCurrency(totalMonthly);
document.getElementById('piPayment').innerText = formatCurrency(monthlyPI);
document.getElementById('taxPayment').innerText = formatCurrency(monthlyTax);
document.getElementById('insPayment').innerText = formatCurrency(monthlyInsurance);
document.getElementById('totalCheck').innerText = formatCurrency(totalMonthly);
document.getElementById('loanAmountResult').innerText = formatCurrency(loanAmount).slice(0, -3); // remove cents for loan amount
// Show results container
document.getElementById('resultsArea').style.display = "block";
}
Understanding Your Mortgage Payment
Calculating your monthly mortgage payment is a critical step in the home buying process. This tool helps you estimate exactly how much you will pay each month, breaking down the costs into Principal & Interest, Property Taxes, and Homeowners Insurance (PITI). Understanding these components ensures you budget accurately for your new home.
The 4 Pillars of Your Monthly Payment (PITI)
Most mortgage payments are made up of four specific costs, often abbreviated as PITI:
Principal: The portion of your payment that goes toward paying down the original loan amount. In the early years of a loan, this amount is small but grows over time.
Interest: The cost of borrowing money from your lender. This usually makes up the majority of your payment at the beginning of the loan term.
Taxes: Property taxes assessed by your local government. Lenders typically collect a portion of this monthly and hold it in an escrow account to pay the bill when it's due.
Insurance: Homeowners insurance protects your property against hazards. Like taxes, this is often paid monthly into an escrow account.
How Interest Rates Affect Affordability
The interest rate is arguably the most significant factor in your monthly payment aside from the home price itself. Even a small difference of 0.5% can result in thousands of dollars in savings or extra costs over the life of a 30-year loan. When using the calculator, try adjusting the interest rate field to see how sensitive your monthly payment is to rate fluctuations.
Choosing the Right Loan Term
The most common loan term is 30 years, which offers lower monthly payments but results in more total interest paid over the life of the loan. A 15-year term will have higher monthly payments, but you will build equity much faster and pay significantly less in total interest. Use this calculator to compare both scenarios to see which fits your budget and financial goals best.
Why Include Taxes and Insurance?
Many simple mortgage calculators only show Principal and Interest. However, because lenders usually require you to escrow funds for taxes and insurance, your actual "check written" amount will be higher. This calculator includes inputs for Annual Property Tax and Home Insurance to give you a realistic view of your total housing obligation.