Purchasing a home is likely the largest financial commitment you will make in your lifetime. Using a comprehensive Mortgage Payment Calculator is the first step in financial planning for homeownership. This tool helps you estimate your monthly mortgage payment based on the home's purchase price, your down payment, the interest rate, and the length of the loan term.
How is Your Monthly Payment Calculated?
The standard formula used for calculating fixed-rate mortgages is the amortization formula. While the math can be complex, it determines exactly how much of your monthly payment goes toward the principal balance versus the interest charged by the lender. In the early years of a mortgage, a significant portion of your payment goes toward interest, while in the later years, more is applied to the principal.
Key Factors Affecting Affordability
Home Price: The total cost of the property.
Down Payment: The upfront cash you pay. A larger down payment (typically 20%) reduces the loan amount and eliminates the need for Private Mortgage Insurance (PMI).
Interest Rate: Even a small difference in the interest rate (e.g., 6.0% vs 6.5%) can add up to tens of thousands of dollars over the life of a 30-year loan.
Loan Term: A 30-year term offers lower monthly payments, while a 15-year term saves you significant money on total interest costs but comes with a higher monthly obligation.
Principal vs. Interest
When you look at the "Total Interest Cost" in the results above, you might be surprised to see that it often equals or exceeds the original loan amount, especially with high interest rates over 30 years. This highlights the importance of securing the lowest possible rate or considering making extra principal payments to shorten the loan term.
function calculateMortgage() {
// 1. Get input values exactly by ID
var priceInput = document.getElementById("homePrice").value;
var downInput = document.getElementById("downPayment").value;
var rateInput = document.getElementById("interestRate").value;
var termInput = document.getElementById("loanTerm").value;
// 2. Clear previous error state
var errorDiv = document.getElementById("errorMsg");
var resultDiv = document.getElementById("resultDisplay");
errorDiv.style.display = "none";
resultDiv.style.display = "none";
// 3. Parse values
var price = parseFloat(priceInput);
var down = parseFloat(downInput);
var rate = parseFloat(rateInput);
var term = parseFloat(termInput);
// 4. Validation Logic
if (isNaN(price) || isNaN(down) || isNaN(rate) || isNaN(term) || price <= 0 || term <= 0) {
errorDiv.innerText = "Please enter valid positive numbers for all fields.";
errorDiv.style.display = "block";
return;
}
// 5. Calculation Logic
var principal = price – down;
if (principal <= 0) {
errorDiv.innerText = "Down payment cannot be greater than or equal to Home Price.";
errorDiv.style.display = "block";
return;
}
// Convert annual rate to monthly decimal
var monthlyRate = (rate / 100) / 12;
// Total number of payments (months)
var numberOfPayments = term * 12;
var monthlyPayment = 0;
// Handle zero interest rate edge case
if (rate === 0) {
monthlyPayment = principal / numberOfPayments;
} else {
// Standard Mortgage Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
var mathPower = Math.pow(1 + monthlyRate, numberOfPayments);
monthlyPayment = principal * ((monthlyRate * mathPower) / (mathPower – 1));
}
var totalPayment = monthlyPayment * numberOfPayments;
var totalInterest = totalPayment – principal;
// 6. Formatting Results
// Using NumberFormat for currency display
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
minimumFractionDigits: 2
});
// 7. Update DOM
document.getElementById("monthlyPaymentResult").innerText = formatter.format(monthlyPayment);
document.getElementById("totalRepaymentResult").innerText = formatter.format(totalPayment);
document.getElementById("totalInterestResult").innerText = formatter.format(totalInterest);
// Show results
resultDiv.style.display = "block";
}