Loan Amount:$0.00 |
Total Interest (Lifetime):$0.00
Understanding Your Mortgage Payment
Using a comprehensive Mortgage Payment Calculator is the first step in the home buying journey. While the sticker price of a home is important, your monthly cash flow is dictated by the specific components of your loan structure, often referred to as PITI (Principal, Interest, Taxes, and Insurance).
How the Calculation Works
This calculator breaks down your monthly obligation into three distinct categories to give you a realistic view of affordability:
Principal & Interest: This is the core of your loan repayment. The calculation uses the standard amortization formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]. As you pay down the loan, the portion going toward principal increases while the interest portion decreases.
Property Taxes: Local municipalities charge taxes based on your property's assessed value. These are typically paid annually but are often escrowed into monthly mortgage payments. This calculator divides your annual estimate by 12.
Homeowners Insurance: Lenders require insurance to protect the asset. Like taxes, this annual premium is divided into monthly installments.
Why Down Payment Matters
Your down payment directly influences your loan-to-value (LTV) ratio. A larger down payment reduces the principal loan amount, which lowers your monthly interest costs. Additionally, putting down less than 20% often triggers Private Mortgage Insurance (PMI), an extra cost not included in standard principal and interest calculations.
Interest Rate Impact
Even a fractional difference in your interest rate can save or cost you tens of thousands of dollars over the life of a 30-year loan. Use the calculator above to experiment with different rates to see how they impact both your monthly payment and the total interest paid over the life of the loan.
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 = parseInt(document.getElementById('loanTerm').value);
var propertyTaxAnnual = parseFloat(document.getElementById('propertyTax').value);
var homeInsuranceAnnual = parseFloat(document.getElementById('homeInsurance').value);
// 2. Element references for Output
var errorMsg = document.getElementById('errorMsg');
var resultsSection = document.getElementById('resultsSection');
// 3. Validation
if (isNaN(homePrice) || isNaN(downPayment) || isNaN(interestRate) || isNaN(propertyTaxAnnual) || isNaN(homeInsuranceAnnual)) {
errorMsg.style.display = 'block';
resultsSection.style.display = 'none';
return;
}
// Additional Logic Checks
if (downPayment >= homePrice) {
errorMsg.innerHTML = "Down payment cannot exceed or equal home price.";
errorMsg.style.display = 'block';
resultsSection.style.display = 'none';
return;
}
errorMsg.style.display = 'none';
// 4. Calculation Logic
var principal = homePrice – downPayment;
var monthlyInterestRate = (interestRate / 100) / 12;
var totalPayments = loanTermYears * 12;
// Amortization Formula
var monthlyPrincipalAndInterest = 0;
if (interestRate === 0) {
monthlyPrincipalAndInterest = principal / totalPayments;
} else {
monthlyPrincipalAndInterest = principal *
(monthlyInterestRate * Math.pow(1 + monthlyInterestRate, totalPayments)) /
(Math.pow(1 + monthlyInterestRate, totalPayments) – 1);
}
var monthlyTax = propertyTaxAnnual / 12;
var monthlyInsurance = homeInsuranceAnnual / 12;
var totalMonthlyPayment = monthlyPrincipalAndInterest + monthlyTax + monthlyInsurance;
var totalCostOfLoan = (monthlyPrincipalAndInterest * totalPayments);
var totalInterest = totalCostOfLoan – principal;
// 5. Update UI
// Helper function for currency formatting
function formatCurrency(num) {
return '$' + num.toFixed(2).replace(/\d(?=(\d{3})+\.)/g, '$&,');
}
document.getElementById('resPrincipalInterest').innerHTML = formatCurrency(monthlyPrincipalAndInterest);
document.getElementById('resTax').innerHTML = formatCurrency(monthlyTax);
document.getElementById('resInsurance').innerHTML = formatCurrency(monthlyInsurance);
document.getElementById('resTotal').innerHTML = formatCurrency(totalMonthlyPayment);
document.getElementById('resLoanAmount').innerHTML = formatCurrency(principal);
document.getElementById('resTotalInterest').innerHTML = formatCurrency(totalInterest);
resultsSection.style.display = 'block';
}