function calculateMortgage() {
// 1. Get Inputs
var price = parseFloat(document.getElementById('homePrice').value);
var down = parseFloat(document.getElementById('downPayment').value);
var rate = parseFloat(document.getElementById('interestRate').value);
var term = parseFloat(document.getElementById('loanTerm').value);
var taxYearly = parseFloat(document.getElementById('propertyTax').value);
var insYearly = parseFloat(document.getElementById('homeInsurance').value);
// 2. Validate Inputs
if (isNaN(price) || price < 0) price = 0;
if (isNaN(down) || down < 0) down = 0;
if (isNaN(rate) || rate < 0) rate = 0;
if (isNaN(term) || term <= 0) term = 30; // Default to 30 if invalid
if (isNaN(taxYearly) || taxYearly < 0) taxYearly = 0;
if (isNaN(insYearly) || insYearly < 0) insYearly = 0;
// 3. Calculation Logic
var principal = price – down;
// Handle case where down payment is greater than price
if (principal < 0) principal = 0;
var monthlyInterestRate = rate / 100 / 12;
var numberOfPayments = term * 12;
var monthlyPrincipalInterest = 0;
if (rate === 0) {
// Simple division if 0% interest
monthlyPrincipalInterest = principal / numberOfPayments;
} else {
// Standard Mortgage Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
monthlyPrincipalInterest = principal *
(monthlyInterestRate * Math.pow(1 + monthlyInterestRate, numberOfPayments)) /
(Math.pow(1 + monthlyInterestRate, numberOfPayments) – 1);
}
// Check for NaN / Infinite results (e.g., if inputs are zero)
if (!isFinite(monthlyPrincipalInterest)) monthlyPrincipalInterest = 0;
var monthlyTax = taxYearly / 12;
var monthlyInsurance = insYearly / 12;
var totalMonthlyPayment = monthlyPrincipalInterest + monthlyTax + monthlyInsurance;
// Calculate Lifetime metrics
var totalPaid = monthlyPrincipalInterest * numberOfPayments;
var totalInterest = totalPaid – principal;
if (totalInterest < 0) totalInterest = 0;
// 4. Update UI
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
minimumFractionDigits: 2
});
document.getElementById('res-pi').innerHTML = formatter.format(monthlyPrincipalInterest);
document.getElementById('res-tax').innerHTML = formatter.format(monthlyTax);
document.getElementById('res-ins').innerHTML = formatter.format(monthlyInsurance);
document.getElementById('res-total').innerHTML = formatter.format(totalMonthlyPayment);
document.getElementById('res-lifetime-interest').innerHTML = formatter.format(totalInterest);
// Show results
document.getElementById('results-area').style.display = 'block';
}
Understanding Your Mortgage Payments
Buying a home is one of the most significant financial decisions you will make in your lifetime. Understanding exactly where your money goes each month is crucial for long-term financial stability. This Mortgage Payment Calculator helps you break down the costs associated with homeownership, specifically focusing on the PITI (Principal, Interest, Taxes, and Insurance) model.
How the Mortgage Formula Works
Most fixed-rate mortgages use a standard amortization formula to determine your monthly principal and interest payments. While the math can seem complex, the concept is straightforward: you are paying back the money you borrowed (principal) plus the cost of borrowing that money (interest) over a set period (the loan term).
In the early years of your mortgage, a larger portion of your payment goes toward interest. As time passes and the principal balance decreases, more of your payment goes toward building equity in your home.
Components of Your Monthly Payment (PITI)
When lenders look at your ability to afford a home, they look at four specific numbers, collectively known as PITI:
Principal: The money that goes directly toward reducing your loan balance.
Interest: The fee charged by the lender for the use of their money. This is calculated based on your Annual Percentage Rate (APR).
Taxes: Property taxes assessed by your local government. These are often collected by the lender in an escrow account and paid on your behalf annually.
Insurance: Homeowners insurance protects your property against damage. Like taxes, this is usually divided into monthly installments and held in escrow.
Impact of the Down Payment
Your down payment significantly influences your monthly costs. A larger down payment reduces the principal loan amount, which lowers your monthly principal and interest payment. Additionally, if you put down less than 20% of the home's value, you may be required to pay Private Mortgage Insurance (PMI), which would increase your monthly costs further. This calculator assumes a standard loan structure without PMI calculations, so be sure to factor that in if you are planning a low down payment.
Choosing the Right Loan Term
The loan term is the length of time you have to repay the loan. The most common terms in the United States are 15 years and 30 years.
30-Year Fixed: Offers lower monthly payments but results in paying significantly more interest over the life of the loan.
15-Year Fixed: Requires higher monthly payments but allows you to build equity faster and save thousands of dollars in interest.
Use the calculator above to compare how changing the loan term from 30 to 15 years affects both your monthly budget and the total interest you will pay.