Please enter valid positive numbers for Home Price, Down Payment, and Interest Rate.
Principal & Interest:$0.00
Monthly Tax:$0.00
Monthly Insurance:$0.00
Total Monthly Payment:$0.00
Total Interest Paid: | Payoff Date:
function calculateMortgage() {
// 1. Get Elements
var priceInput = document.getElementById("mc_price");
var downInput = document.getElementById("mc_down");
var termInput = document.getElementById("mc_term");
var rateInput = document.getElementById("mc_rate");
var taxInput = document.getElementById("mc_tax");
var insInput = document.getElementById("mc_ins");
var resultBox = document.getElementById("mc_result");
var errorBox = document.getElementById("mc_error");
// 2. Parse Values
var price = parseFloat(priceInput.value);
var down = parseFloat(downInput.value);
var years = parseInt(termInput.value);
var rate = parseFloat(rateInput.value);
var taxYearly = parseFloat(taxInput.value);
var insYearly = parseFloat(insInput.value);
// 3. Validation
if (isNaN(price) || isNaN(down) || isNaN(years) || isNaN(rate) || price <= 0 || rate < 0) {
errorBox.style.display = "block";
resultBox.style.display = "none";
return;
}
// Handle optional fields being empty
if (isNaN(taxYearly)) taxYearly = 0;
if (isNaN(insYearly)) insYearly = 0;
errorBox.style.display = "none";
// 4. Calculations
var principal = price – down;
if (principal <= 0) {
alert("Down payment cannot be greater than or equal to the Home Price.");
return;
}
var monthlyInterest = 0;
var monthlyPayment = 0;
var totalInterest = 0;
// Check for zero interest rate edge case
if (rate === 0) {
monthlyPayment = principal / (years * 12);
totalInterest = 0;
} else {
var r = rate / 100 / 12; // Monthly interest rate
var n = years * 12; // Total number of payments
// Amortization Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
monthlyPayment = principal * ( (r * Math.pow(1 + r, n)) / (Math.pow(1 + r, n) – 1) );
totalInterest = (monthlyPayment * n) – principal;
}
var monthlyTax = taxYearly / 12;
var monthlyIns = insYearly / 12;
var totalMonthly = monthlyPayment + monthlyTax + monthlyIns;
// Calculate Payoff Date
var today = new Date();
var payoffDate = new Date(today.setFullYear(today.getFullYear() + years));
var dateString = payoffDate.toLocaleDateString('en-US', { month: 'short', year: 'numeric' });
// 5. Update UI
document.getElementById("res_pi").innerText = "$" + monthlyPayment.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById("res_tax").innerText = "$" + monthlyTax.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById("res_ins").innerText = "$" + monthlyIns.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById("res_total").innerText = "$" + totalMonthly.toLocaleString('en-US', {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById("res_total_interest").innerText = "$" + totalInterest.toLocaleString('en-US', {minimumFractionDigits: 0, maximumFractionDigits: 0});
document.getElementById("res_date").innerText = dateString;
resultBox.style.display = "block";
}
Understanding How Your Mortgage Is Calculated
Buying a home is one of the largest financial commitments you will make in your lifetime. Understanding how your monthly mortgage payment is calculated is essential for budgeting and ensuring you choose a loan structure that fits your financial goals. This calculator breaks down the principal, interest, taxes, and insurance (PITI) to give you a clear picture of your monthly obligations.
The Core Components: Principal and Interest
The majority of your monthly payment goes toward the Principal and Interest.
Principal is the money you borrowed to buy the house.
Interest is the fee the lender charges you for borrowing that money.
In the early years of a standard 30-year fixed-rate mortgage, a larger portion of your payment goes toward interest. As time passes, the balance shifts, and you begin paying down more of the principal, building equity in your home.
The Impact of Loan Term
The term of your loan significantly affects your monthly payment and the total interest paid.
30-Year Term: Offers lower monthly payments because the loan is spread out over a longer period. However, you will pay significantly more in interest over the life of the loan.
15-Year Term: Comes with higher monthly payments but typically offers a lower interest rate. You will pay off your home faster and save a substantial amount on interest.
Don't Forget Taxes and Insurance
Many first-time homebuyers focus solely on the mortgage rate but forget about property taxes and homeowners insurance. These are often bundled into your monthly payment through an escrow account. Depending on your location, property taxes can add several hundred dollars to your monthly bill. Our calculator includes fields for these expenses to provide a realistic estimate of your "out-the-door" monthly cost.
How to Lower Your Mortgage Payment
If the estimated payment is higher than your budget allows, consider these strategies:
1. Increase your down payment: This lowers the principal amount you need to borrow.
2. Improve your credit score: A higher score often qualifies you for a lower interest rate.
3. Shop around: Even a 0.5% difference in interest rate can save you thousands over the life of the loan.