function calculateMortgage() {
var homePrice = parseFloat(document.getElementById('homePrice').value);
var downPayment = parseFloat(document.getElementById('downPayment').value);
var loanTermYears = parseFloat(document.getElementById('loanTerm').value);
var annualRate = parseFloat(document.getElementById('interestRate').value);
var errorDiv = document.getElementById('calcError');
var resultDiv = document.getElementById('mortgageResult');
// Validation
if (isNaN(homePrice) || isNaN(downPayment) || isNaN(loanTermYears) || isNaN(annualRate) || homePrice <= 0 || loanTermYears <= 0) {
errorDiv.style.display = "block";
resultDiv.style.display = "none";
return;
}
errorDiv.style.display = "none";
// Calculations
var principal = homePrice – downPayment;
var monthlyRate = (annualRate / 100) / 12;
var numberOfPayments = loanTermYears * 12;
var monthlyPayment = 0;
// Handle 0% interest edge case
if (annualRate === 0) {
monthlyPayment = principal / numberOfPayments;
} else {
monthlyPayment = principal * (monthlyRate * Math.pow(1 + monthlyRate, numberOfPayments)) / (Math.pow(1 + monthlyRate, numberOfPayments) – 1);
}
// Secondary metrics
var totalCost = monthlyPayment * numberOfPayments;
var totalInterest = totalCost – principal;
// Display Results with formatting
document.getElementById('monthlyPaymentDisplay').innerHTML = "$" + monthlyPayment.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('loanAmountDisplay').innerHTML = "$" + principal.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('totalInterestDisplay').innerHTML = "$" + totalInterest.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('totalCostDisplay').innerHTML = "$" + totalCost.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
resultDiv.style.display = "block";
}
How to Use This Mortgage Payment Calculator
Understanding your potential monthly mortgage payment is the first critical step in the home-buying process. Our Mortgage Payment Calculator is designed to give you a precise estimate of your Principal and Interest (P&I) payments based on current market conditions and your personal financial inputs.
Input Explanations
Home Price: The total purchase price of the real estate property you intend to buy.
Down Payment: The upfront cash amount you pay at closing. A larger down payment reduces your loan principal and monthly interest costs.
Loan Term: The duration of the loan in years. Standard terms are 15 or 30 years. Shorter terms generally have higher monthly payments but lower total interest costs.
Interest Rate: The annual percentage rate (APR) charged by the lender. Even a small difference in rate can significantly impact your monthly payment.
Understanding Your Mortgage Results
When you calculate your mortgage payment, it is essential to understand exactly what the numbers represent. The primary result shown above is your P&I (Principal and Interest).
The Principal is the money that goes towards paying off the loan balance itself, while the Interest is the fee paid to the bank for borrowing that money. In the early years of a standard amortization schedule, a larger portion of your payment goes toward interest. As the loan matures, more of your payment is applied to the principal.
Factors That Influence Your Monthly Payment
Several variables can change your borrowing costs:
Credit Score: Borrowers with higher credit scores typically qualify for lower interest rates.
Down Payment Size: Putting down less than 20% often triggers Private Mortgage Insurance (PMI), which is an additional monthly cost not calculated in the basic P&I figure above.
Loan Type: FHA, VA, and USDA loans have different fee structures and rate averages compared to conventional loans.
Why Calculate Total Interest?
Many homebuyers focus solely on the monthly payment, but the "Total Interest Paid" metric is equally important. By adjusting the loan term from 30 years to 15 years, or by securing a slightly lower interest rate, you can often save tens of thousands of dollars over the life of the loan. Use this tool to experiment with different scenarios to find the financial strategy that best suits your long-term goals.