Please enter valid positive numbers for all fields.
Loan Amount:–
Total Interest:–
Total Cost of Loan:–
Monthly Payment:–
function calculateMortgage() {
var homePriceInput = document.getElementById("homePrice");
var downPaymentInput = document.getElementById("downPayment");
var loanTermInput = document.getElementById("loanTerm");
var interestRateInput = document.getElementById("interestRate");
var errorMsg = document.getElementById("errorMsg");
var resultBox = document.getElementById("resultBox");
var homePrice = parseFloat(homePriceInput.value);
var downPayment = parseFloat(downPaymentInput.value);
var loanTermYears = parseFloat(loanTermInput.value);
var annualRate = parseFloat(interestRateInput.value);
// Validation
if (isNaN(homePrice) || isNaN(downPayment) || isNaN(loanTermYears) || isNaN(annualRate) ||
homePrice < 0 || downPayment < 0 || loanTermYears <= 0 || annualRate < 0) {
errorMsg.style.display = "block";
resultBox.style.display = "none";
return;
}
// Logic
errorMsg.style.display = "none";
var principal = homePrice – downPayment;
// Handle case where down payment is greater than home price
if (principal 0) {
monthlyPayment = principal / numberOfPayments;
} else {
monthlyPayment = principal; // Should not happen given validation
}
} else {
// Formula: M = P * (r(1+r)^n) / ((1+r)^n – 1)
var mathPower = Math.pow(1 + monthlyRate, numberOfPayments);
monthlyPayment = principal * ((monthlyRate * mathPower) / (mathPower – 1));
}
var totalCost = monthlyPayment * numberOfPayments;
var totalInterest = totalCost – principal;
// Formatting Output
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
minimumFractionDigits: 2
});
document.getElementById("displayLoanAmount").innerHTML = formatter.format(principal);
document.getElementById("displayTotalInterest").innerHTML = formatter.format(totalInterest);
document.getElementById("displayTotalCost").innerHTML = formatter.format(totalCost);
document.getElementById("displayMonthlyPayment").innerHTML = formatter.format(monthlyPayment);
resultBox.style.display = "block";
}
Understanding Your Mortgage Payment
Calculating your monthly mortgage payment is one of the most critical steps in the home-buying process. This Mortgage Payment Calculator helps prospective homeowners estimate their monthly financial obligations based on the home's purchase price, down payment, loan term, and interest rate.
Knowing your numbers effectively prevents "house poor" scenarios where too much income is tied up in housing costs. By adjusting the down payment or interest rate in the calculator above, you can see how different scenarios impact your long-term financial health.
How the Mortgage Formula Works
While this calculator handles the complex math instantly, understanding the underlying components is beneficial. A standard fixed-rate mortgage payment is calculated using an amortization formula that considers:
Principal (P): The amount of money you borrow (Home Price minus Down Payment).
Interest Rate (r): The annual percentage rate charged by the lender, divided by 12 to get the monthly rate.
Number of Payments (n): The total number of months in the loan term (e.g., 30 years × 12 months = 360 payments).
Factors That Influence Your Monthly Payment
Several variables can significantly change your monthly outlay:
1. Down Payment Size: A larger down payment reduces the principal loan amount. This not only lowers your monthly payment but may also eliminate the need for Private Mortgage Insurance (PMI) if you put down 20% or more.
2. Loan Term: A 30-year term typically offers lower monthly payments compared to a 15-year term, but you will pay significantly more in interest over the life of the loan. A shorter term saves money in the long run but requires higher monthly cash flow.
3. Interest Rate: Even a small difference of 0.5% in your interest rate can add up to tens of thousands of dollars over the lifespan of a mortgage. Your credit score, the economy, and the loan type all influence the rate you are offered.
Beyond Principal and Interest
Please note that the calculator above computes the Principal and Interest (P&I). However, your actual monthly housing expense often includes other costs known as PITI:
Taxes: Property taxes charged by your local municipality.
Insurance: Homeowners insurance to protect against damage.
When budgeting for a new home, ensure you account for these additional costs alongside the figure generated by this calculator.