Loan Summary:
Loan Amount: –
Total Interest Cost: –
Total Cost of Loan: –
function calculateMortgage() {
// 1. Get Input Values
var homePrice = parseFloat(document.getElementById('mcHomePrice').value);
var downPayment = parseFloat(document.getElementById('mcDownPayment').value);
var interestRate = parseFloat(document.getElementById('mcInterestRate').value);
var loanTerm = parseFloat(document.getElementById('mcLoanTerm').value);
var annualTax = parseFloat(document.getElementById('mcPropertyTax').value);
var annualInsurance = parseFloat(document.getElementById('mcInsurance').value);
// 2. Validate Inputs
if (isNaN(homePrice) || homePrice <= 0) {
alert("Please enter a valid Home Price.");
return;
}
if (isNaN(downPayment) || downPayment < 0) {
downPayment = 0; // Default to 0 if empty/invalid
}
if (isNaN(interestRate) || interestRate < 0) {
alert("Please enter a valid Interest Rate.");
return;
}
if (isNaN(loanTerm) || loanTerm <= 0) {
alert("Please enter a valid Loan Term in years.");
return;
}
if (isNaN(annualTax)) annualTax = 0;
if (isNaN(annualInsurance)) annualInsurance = 0;
// 3. Core Calculations
var principal = homePrice – downPayment;
if (principal <= 0) {
alert("Down payment cannot be greater than or equal to Home Price.");
return;
}
// Monthly Interest Rate (r) = Annual Rate / 100 / 12
var r = (interestRate / 100) / 12;
// Total Number of Payments (n) = Years * 12
var n = loanTerm * 12;
var monthlyPI = 0;
// Amortization Formula: M = P [ r(1+r)^n ] / [ (1+r)^n – 1 ]
if (interestRate === 0) {
monthlyPI = principal / n;
} else {
monthlyPI = principal * ( (r * Math.pow(1 + r, n)) / (Math.pow(1 + r, n) – 1) );
}
var monthlyTax = annualTax / 12;
var monthlyInsurance = annualInsurance / 12;
var totalMonthlyPayment = monthlyPI + monthlyTax + monthlyInsurance;
var totalTotalPayment = monthlyPI * n;
var totalInterest = totalTotalPayment – principal;
// 4. Update UI
document.getElementById('resPrincipalInterest').innerHTML = "$" + formatMoney(monthlyPI);
document.getElementById('resTax').innerHTML = "$" + formatMoney(monthlyTax);
document.getElementById('resInsurance').innerHTML = "$" + formatMoney(monthlyInsurance);
document.getElementById('resTotal').innerHTML = "$" + formatMoney(totalMonthlyPayment);
document.getElementById('resLoanAmount').innerHTML = "$" + formatMoney(principal);
document.getElementById('resTotalInterest').innerHTML = "$" + formatMoney(totalInterest);
document.getElementById('resTotalCost').innerHTML = "$" + formatMoney(totalTotalPayment + (annualTax * loanTerm) + (annualInsurance * loanTerm));
// Show results container
document.getElementById('mc-results').style.display = "block";
}
function formatMoney(num) {
return num.toLocaleString('en-US', { minimumFractionDigits: 2, maximumFractionDigits: 2 });
}
Understanding Your Mortgage Payment
Buying a home is one of the largest financial decisions most people make. A mortgage payment calculator is an essential tool for prospective homeowners to understand their potential monthly financial commitment. While the listing price of a home gives you a ballpark figure, your actual monthly out-of-pocket expenses include several other critical factors collectively known as PITI: Principal, Interest, Taxes, and Insurance.
What Goes Into Your Monthly Payment?
Using our calculator above, you can see how different variables impact your budget. Here is a breakdown of the components:
Principal: The portion of your payment that goes toward paying down the loan balance (the home price minus your down payment).
Interest: The cost of borrowing money from the lender. In the early years of a mortgage, a larger percentage of your payment goes toward interest.
Property Taxes: Taxes charged by your local government based on the value of your property. These are often bundled into your monthly mortgage payment.
Homeowners Insurance: Insurance that protects your home against damage. Lenders typically require this to protect their asset.
How Interest Rates Affect Affordability
Even a small change in interest rates can significantly affect your buying power. For example, on a $300,000 loan, a difference of just 1% in the interest rate can change your monthly payment by hundreds of dollars and the total cost of the loan by tens of thousands over 30 years. Using this calculator allows you to stress-test your budget against rising rates.
The Impact of Your Down Payment
Your down payment plays a dual role: it reduces the principal loan amount and demonstrates financial stability to lenders. putting down at least 20% can also help you avoid Private Mortgage Insurance (PMI), which is an additional cost not included in the standard Principal and Interest calculation. Adjust the "Down Payment" field in the calculator to see how different cash contributions lower your monthly obligation.
Choosing the Right Loan Term
Most buyers opt for a 30-year fixed-rate mortgage, which offers lower monthly payments but results in higher total interest costs over the life of the loan. A 15-year term will have higher monthly payments but can save you a substantial amount in interest. Use the "Loan Term" input to compare 15-year vs. 30-year scenarios to find the best fit for your financial goals.