function calculateMortgage() {
// 1. Get Elements
var homePriceInput = document.getElementById("mc_home_price");
var downPaymentInput = document.getElementById("mc_down_payment");
var interestRateInput = document.getElementById("mc_interest_rate");
var loanTermInput = document.getElementById("mc_loan_term");
var errorDiv = document.getElementById("mc_error");
var resultDiv = document.getElementById("mc_results");
// 2. Parse Values
var homePrice = parseFloat(homePriceInput.value);
var downPayment = parseFloat(downPaymentInput.value);
var annualRate = parseFloat(interestRateInput.value);
var years = parseInt(loanTermInput.value);
// 3. Validation
if (isNaN(homePrice) || isNaN(downPayment) || isNaN(annualRate) || isNaN(years) || homePrice = homePrice) {
errorDiv.innerHTML = "Down payment cannot exceed or equal home price.";
errorDiv.style.display = "block";
resultDiv.style.display = "none";
return;
}
errorDiv.style.display = "none";
// 4. Calculation Logic
var principal = homePrice – downPayment;
var monthlyRate = (annualRate / 100) / 12;
var numberOfPayments = years * 12;
var monthlyPayment = 0;
// Handle zero interest case
if (annualRate === 0) {
monthlyPayment = principal / numberOfPayments;
} else {
// Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
var x = Math.pow(1 + monthlyRate, numberOfPayments);
monthlyPayment = (principal * monthlyRate * x) / (x – 1);
}
var totalCost = monthlyPayment * numberOfPayments;
var totalInterest = totalCost – principal;
// 5. Formatting and Display
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
minimumFractionDigits: 2
});
document.getElementById("mc_res_monthly").innerText = formatter.format(monthlyPayment);
document.getElementById("mc_res_loan_amount").innerText = formatter.format(principal);
document.getElementById("mc_res_total_interest").innerText = formatter.format(totalInterest);
document.getElementById("mc_res_total_cost").innerText = formatter.format(totalCost);
resultDiv.style.display = "block";
}
Understanding Your Mortgage Calculator Results
Using a mortgage calculator is an essential first step in the home-buying process. It helps you estimate your monthly obligations by factoring in the home price, down payment, interest rate, and loan term. This tool specifically calculates the Principal and Interest (P&I) portion of your payment, giving you a clear baseline for affordability.
Key Inputs Explained
Home Price: The total purchase price of the real estate property you intend to buy.
Down Payment: The upfront cash you pay toward the home purchase. A higher down payment reduces the principal loan amount and the interest paid over time.
Interest Rate: The annual percentage rate (APR) charged by the lender. Even a small difference of 0.5% can significantly change your monthly payment and total interest cost.
Loan Term: The duration of the loan. A 30-year term offers lower monthly payments but results in higher total interest paid compared to a 15-year term.
How Interest Impacts Your Loan
Interest is the cost of borrowing money. In the early years of a standard fixed-rate mortgage, a large portion of your monthly payment goes toward paying off interest rather than the principal balance. This is known as amortization. As you progress through the loan term, the portion allocated to interest decreases, and the portion allocated to principal increases.
Why Your Final Payment Might Differ
While this calculator provides an accurate estimate of Principal and Interest, your actual monthly housing costs may include other fees often collected via escrow, such as:
Property Taxes: Assessed by your local government based on property value.
Homeowners Insurance: Protects your property against damage and liability.
PMI (Private Mortgage Insurance): Usually required if your down payment is less than 20% of the home price.
HOA Fees: Monthly or annual fees if the property is part of a Homeowners Association.
Always consult with a qualified mortgage lender to get a pre-approval and a definitive breakdown of your closing costs and monthly liabilities.