function calculateMortgage() {
// Retrieve input values
var homePriceStr = document.getElementById('homePrice').value;
var downPaymentStr = document.getElementById('downPayment').value;
var interestRateStr = document.getElementById('interestRate').value;
var loanTermStr = document.getElementById('loanTerm').value;
var errorDiv = document.getElementById('mp-error');
var resultsDiv = document.getElementById('mp-results');
// Parse values to floats
var homePrice = parseFloat(homePriceStr);
var downPayment = parseFloat(downPaymentStr);
var annualRate = parseFloat(interestRateStr);
var years = parseInt(loanTermStr);
// Validation
if (isNaN(homePrice) || isNaN(downPayment) || isNaN(annualRate) || isNaN(years) || homePrice = homePrice) {
errorDiv.style.display = 'block';
resultsDiv.style.display = 'none';
errorDiv.innerText = "Down payment cannot be equal to or greater than the home price.";
return;
}
errorDiv.style.display = 'none';
// Calculation Logic
var principal = homePrice – downPayment;
var monthlyRate = (annualRate / 100) / 12;
var numberOfPayments = years * 12;
var monthlyPayment = 0;
if (monthlyRate === 0) {
monthlyPayment = principal / numberOfPayments;
} else {
// M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
monthlyPayment = principal * ( (monthlyRate * Math.pow(1 + monthlyRate, numberOfPayments)) / (Math.pow(1 + monthlyRate, numberOfPayments) – 1) );
}
var totalCost = monthlyPayment * numberOfPayments;
var totalInterest = totalCost – principal;
// Formatting currency
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
minimumFractionDigits: 2
});
// Update DOM
document.getElementById('monthlyPaymentResult').innerText = formatter.format(monthlyPayment);
document.getElementById('totalPrincipalResult').innerText = formatter.format(principal);
document.getElementById('totalInterestResult').innerText = formatter.format(totalInterest);
document.getElementById('totalCostResult').innerText = formatter.format(totalCost);
resultsDiv.style.display = 'block';
}
Understanding Your Mortgage Calculation
Calculating your potential monthly mortgage payment is a crucial first step in the home-buying process. Our Mortgage Payment Calculator helps you estimate your monthly financial commitment by factoring in the home price, down payment size, interest rate, and the length of the loan.
How the Mortgage Formula Works
Most fixed-rate mortgages use a standard amortization formula to determine your monthly principal and interest payments. The formula used is:
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: Total number of payments (Loan term in years multiplied by 12)
Key Factors Affecting Your Payment
Several variables can significantly impact your monthly housing costs:
Loan Term: A 30-year term typically offers lower monthly payments compared to a 15-year term, but you will pay significantly more in total interest over the life of the loan.
Interest Rate: Even a small difference of 0.5% in your interest rate can save or cost you tens of thousands of dollars over the duration of the mortgage. Higher credit scores often qualify for lower rates.
Down Payment: Putting more money down upfront reduces the principal loan amount. Additionally, if you put down less than 20%, lenders often require Private Mortgage Insurance (PMI), which increases your monthly costs.
Interpreting Your Results
The "Total Cost" displayed in the calculator represents the sum of every payment you will make over the years. By comparing the "Total Interest Paid" against the "Total Principal," you can visualize the cost of borrowing money. For many homeowners, the interest paid over a 30-year period can sometimes equal or exceed the original value of the loan, highlighting the importance of securing a favorable interest rate or making extra principal payments when possible.