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 propertyTaxYearly = parseFloat(document.getElementById('propertyTax').value);
var homeInsuranceYearly = parseFloat(document.getElementById('homeInsurance').value);
var hoaFeesMonthly = parseFloat(document.getElementById('hoaFees').value);
// 2. Validate inputs
if (isNaN(homePrice) || homePrice < 0) homePrice = 0;
if (isNaN(downPayment) || downPayment < 0) downPayment = 0;
if (isNaN(interestRate) || interestRate < 0) interestRate = 0;
if (isNaN(propertyTaxYearly) || propertyTaxYearly < 0) propertyTaxYearly = 0;
if (isNaN(homeInsuranceYearly) || homeInsuranceYearly < 0) homeInsuranceYearly = 0;
if (isNaN(hoaFeesMonthly) || hoaFeesMonthly < 0) hoaFeesMonthly = 0;
// 3. Core Calculations
var loanAmount = homePrice – downPayment;
// Prevent negative loan amounts
if (loanAmount < 0) loanAmount = 0;
var monthlyInterestRate = (interestRate / 100) / 12;
var numberOfPayments = loanTermYears * 12;
var monthlyPrincipalInterest = 0;
// Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
if (interestRate === 0) {
monthlyPrincipalInterest = loanAmount / numberOfPayments;
} else {
var mathPower = Math.pow(1 + monthlyInterestRate, numberOfPayments);
monthlyPrincipalInterest = loanAmount * ((monthlyInterestRate * mathPower) / (mathPower – 1));
}
if (isNaN(monthlyPrincipalInterest)) monthlyPrincipalInterest = 0;
var monthlyPropertyTax = propertyTaxYearly / 12;
var monthlyHomeInsurance = homeInsuranceYearly / 12;
var totalMonthlyPayment = monthlyPrincipalInterest + monthlyPropertyTax + monthlyHomeInsurance + hoaFeesMonthly;
var totalAmountPaid = (monthlyPrincipalInterest * numberOfPayments);
var totalInterestPaid = totalAmountPaid – loanAmount;
// 4. Update DOM with formatting
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
minimumFractionDigits: 2
});
document.getElementById('monthlyTotal').innerHTML = formatter.format(totalMonthlyPayment);
document.getElementById('piResult').innerHTML = formatter.format(monthlyPrincipalInterest);
document.getElementById('taxResult').innerHTML = formatter.format(monthlyPropertyTax);
document.getElementById('insResult').innerHTML = formatter.format(monthlyHomeInsurance);
document.getElementById('hoaResult').innerHTML = formatter.format(hoaFeesMonthly);
document.getElementById('loanAmountResult').innerHTML = formatter.format(loanAmount);
document.getElementById('totalInterestResult').innerHTML = formatter.format(totalInterestPaid);
document.getElementById('totalCostResult').innerHTML = formatter.format(totalAmountPaid);
// Show results
document.getElementById('results').style.display = 'block';
}
Understanding Your Mortgage Calculation
Using a comprehensive Mortgage Payment Calculator is an essential step in the home buying process. Unlike simple calculators that only look at principal and interest, this tool provides a realistic view of your monthly financial commitment by including property taxes, homeowners insurance, and HOA fees—commonly referred to as PITI (Principal, Interest, Taxes, and Insurance).
The Components of Your Monthly Payment
When you take out a mortgage, your monthly check goes toward several different buckets. Understanding these will help you budget more effectively:
Principal: The portion of the payment that pays down the actual money you borrowed. In the early years of a 30-year mortgage, this amount is small.
Interest: The fee charged by the lender for borrowing the money. Initially, this makes up the bulk of your payment.
Escrow (Taxes & Insurance): Most lenders collect 1/12th of your yearly property tax and insurance bills each month and hold them in an escrow account to pay the bills when they are due.
HOA Fees: If you live in a community with a Homeowners Association, these dues are often paid separately, but we include them here for a complete budget picture.
How Interest Rates Affect Your Buying Power
Even a small change in interest rates can significantly impact your monthly payment and the total cost of your loan. For example, on a $300,000 loan, a 1% increase in interest rate can raise your monthly payment by over $180 and cost you over $60,000 in additional interest over the life of a 30-year loan.
💡 Pro Tip for Borrowers
Consider making one extra mortgage payment per year. This simple strategy applies directly to your principal balance and can shave years off your loan term, saving you tens of thousands of dollars in interest without refinancing.
Loan Term: 15-Year vs. 30-Year
Choosing your loan term is a balancing act between monthly affordability and long-term savings.
30-Year Fixed: Offers lower monthly payments, allowing you to buy a more expensive home or keep monthly cash flow free for other investments. However, you pay significantly more interest over time.
15-Year Fixed: Comes with higher monthly payments, but usually has a lower interest rate and builds equity much faster. This is ideal if you can comfortably afford the higher payment.
Why Include Taxes and Insurance?
Many first-time homebuyers are shocked when their actual mortgage bill is 20-30% higher than the "Principal and Interest" number they saw on basic calculators. Property taxes vary wildly by location—from under 0.5% to over 2.5% of the home's value annually. Ignoring these costs can lead to purchasing a home that is technically outside your comfortable budget.