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Understanding Your Mortgage Payment
Calculating your mortgage payment is a crucial first step in the home-buying process. This Mortgage Payment Calculator helps you estimate your monthly financial commitment by factoring in the home price, down payment, interest rate, and loan term.
How the Mortgage Formula Works
The core of a fixed-rate mortgage calculation uses the standard amortization formula:
M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
- M = Total monthly payment
- P = Principal loan amount (Home Price minus Down Payment)
- i = Monthly interest rate (Annual rate divided by 12)
- n = Number of payments (Loan term in years multiplied by 12)
Factors That Impact Your Monthly Payment
Several variables can significantly alter your monthly mortgage costs:
- Down Payment: A larger down payment reduces your principal loan amount, which lowers your monthly payments and the total interest paid over the life of the loan.
- Interest Rate: Even a fraction of a percentage point difference can result in thousands of dollars in savings or extra costs over a 30-year term. Rates fluctuate based on the economy and your credit score.
- Loan Term: A 30-year term offers lower monthly payments but results in higher total interest costs compared to a 15-year term.
- Taxes and Insurance: Most lenders require you to pay property taxes and homeowners insurance into an escrow account. These are added to your Principal and Interest (P&I) payment.
Why Use a Mortgage Calculator?
Before applying for a loan, it is essential to know what you can afford. Lenders look at your Debt-to-Income (DTI) ratio to approve loans. Using this calculator allows you to test different scenarios—such as putting more money down or finding a cheaper home—to see how they affect your monthly budget.
function calculateMortgage() {
// Get Input Values
var homePrice = parseFloat(document.getElementById('homePrice').value);
var downPayment = parseFloat(document.getElementById('downPayment').value);
var annualRate = parseFloat(document.getElementById('interestRate').value);
var years = parseInt(document.getElementById('loanTerm').value);
var yearlyTax = parseFloat(document.getElementById('propertyTax').value);
var yearlyInsurance = parseFloat(document.getElementById('homeInsurance').value);
// Validate Inputs
if (isNaN(homePrice) || homePrice <= 0) {
alert("Please enter a valid Home Price.");
return;
}
if (isNaN(downPayment) || downPayment < 0) {
downPayment = 0;
}
if (isNaN(annualRate) || annualRate < 0) {
alert("Please enter a valid Interest Rate.");
return;
}
if (isNaN(yearlyTax)) yearlyTax = 0;
if (isNaN(yearlyInsurance)) yearlyInsurance = 0;
// Calculate Loan Variables
var principal = homePrice – downPayment;
if (principal <= 0) {
alert("Down payment cannot be greater than or equal to the home price.");
return;
}
var monthlyRate = (annualRate / 100) / 12;
var numberOfPayments = years * 12;
// Calculate Monthly Principal & Interest (Standard Formula)
// M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
var monthlyPI = 0;
if (annualRate === 0) {
monthlyPI = principal / numberOfPayments;
} else {
var x = Math.pow(1 + monthlyRate, numberOfPayments);
monthlyPI = principal * ((monthlyRate * x) / (x – 1));
}
// Calculate Taxes and Insurance
var monthlyTax = yearlyTax / 12;
var monthlyInsurance = yearlyInsurance / 12;
var totalMonthlyTaxIns = monthlyTax + monthlyInsurance;
// Total Monthly Payment
var totalMonthlyPayment = monthlyPI + totalMonthlyTaxIns;
// Total Interest
var totalPaymentOverLife = (monthlyPI * numberOfPayments);
var totalInterest = totalPaymentOverLife – principal;
// Calculate Payoff Date
var today = new Date();
var payoffYear = today.getFullYear() + years;
var payoffMonth = today.toLocaleString('default', { month: 'long' });
var payoffDateString = payoffMonth + " " + payoffYear;
// Format Currency Function
function formatCurrency(num) {
return "$" + num.toFixed(2).replace(/\d(?=(\d{3})+\.)/g, '$&,');
}
// Display Results
document.getElementById('monthlyPIDisplay').innerHTML = formatCurrency(monthlyPI);
document.getElementById('monthlyTaxInsDisplay').innerHTML = formatCurrency(totalMonthlyTaxIns);
document.getElementById('totalMonthlyDisplay').innerHTML = formatCurrency(totalMonthlyPayment);
document.getElementById('loanAmountDisplay').innerHTML = formatCurrency(principal);
document.getElementById('totalInterestDisplay').innerHTML = formatCurrency(totalInterest);
document.getElementById('payoffDateDisplay').innerHTML = payoffDateString;
// Show Result Section
document.getElementById('mcResultSection').style.display = 'block';
// smooth scroll to results (optional but good UX)
document.getElementById('mcResultSection').scrollIntoView({ behavior: 'smooth', block: 'nearest' });
}