function calculateMortgage() {
// Retrieve inputs using var as requested
var homePriceInput = document.getElementById('mcHomePrice');
var downPaymentInput = document.getElementById('mcDownPayment');
var interestRateInput = document.getElementById('mcInterestRate');
var loanTermInput = document.getElementById('mcLoanTerm');
var homePrice = parseFloat(homePriceInput.value);
var downPaymentPercent = parseFloat(downPaymentInput.value);
var interestRate = parseFloat(interestRateInput.value);
var loanTermYears = parseFloat(loanTermInput.value);
// Validation Logic
if (isNaN(homePrice) || isNaN(downPaymentPercent) || isNaN(interestRate) || isNaN(loanTermYears)) {
alert("Please enter valid numbers in all fields.");
return;
}
if (homePrice <= 0 || loanTermYears <= 0) {
alert("Home price and loan term must be greater than zero.");
return;
}
// Calculation Logic
var downPaymentAmount = homePrice * (downPaymentPercent / 100);
var principal = homePrice – downPaymentAmount;
var monthlyInterestRate = (interestRate / 100) / 12;
var numberOfPayments = loanTermYears * 12;
var monthlyPayment = 0;
// Handle zero interest rate edge case
if (interestRate === 0) {
monthlyPayment = principal / numberOfPayments;
} else {
// Standard Mortgage Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1]
var numerator = monthlyInterestRate * Math.pow((1 + monthlyInterestRate), numberOfPayments);
var denominator = Math.pow((1 + monthlyInterestRate), numberOfPayments) – 1;
monthlyPayment = principal * (numerator / denominator);
}
var totalCost = monthlyPayment * numberOfPayments;
var totalInterest = totalCost – principal;
// Display Logic
var resultDiv = document.getElementById('mc-result');
var monthlyDisplay = document.getElementById('mcMonthlyPayment');
var loanDisplay = document.getElementById('mcLoanAmount');
var interestDisplay = document.getElementById('mcTotalInterest');
var costDisplay = document.getElementById('mcTotalCost');
// Currency Formatting
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
minimumFractionDigits: 2
});
monthlyDisplay.innerHTML = formatter.format(monthlyPayment);
loanDisplay.innerHTML = formatter.format(principal);
interestDisplay.innerHTML = formatter.format(totalInterest);
costDisplay.innerHTML = formatter.format(totalCost);
// Show results
resultDiv.style.display = "block";
}
How to Use This Mortgage Calculator
Calculating your potential monthly mortgage payment is a crucial first step in the home-buying process. Our specialized Mortgage Calculator helps you estimate your financial commitment by factoring in the home price, down payment, interest rate, and loan term.
To get an accurate estimate:
Home Price: Enter the total purchase price of the property.
Down Payment (%): Input the percentage of the home price you plan to pay upfront. A higher down payment reduces your loan principal and monthly payments.
Interest Rate (%): Enter the current annual interest rate offered by your lender. Even a small difference here can significantly impact the total cost of the loan.
Loan Term: Specify how many years you will take to repay the loan (typically 15 or 30 years).
Understanding the Mortgage Formula
This calculator uses the standard amortization formula used by most lenders to determine principal and interest payments. The formula calculates exactly how much needs to be paid monthly to pay off the loan balance and the accrued interest by the end of the term.
Example Scenario: For a $300,000 home with a 20% down payment ($60,000), you would be borrowing $240,000. If you secure a 30-year fixed-rate mortgage at 4.5% interest, your calculation would result in a monthly principal and interest payment of approximately $1,216.04. Over the life of the loan, you would pay a total of roughly $197,776 in interest alone.
Factors That Affect Your Mortgage Payment
While this calculator provides the core Principal and Interest (P&I) payment, keep in mind that your actual monthly housing costs may include other factors:
Property Taxes: Assessed by your local government, usually bundled into your monthly payment via an escrow account.
Homeowners Insurance: Required by lenders to protect the property against damage.
PMI (Private Mortgage Insurance): If your down payment is less than 20%, lenders often require this extra insurance, which increases your monthly cost.
Frequently Asked Questions
How does the loan term affect my payment?
A shorter loan term (e.g., 15 years) will result in higher monthly payments but significantly less total interest paid over the life of the loan. A longer term (e.g., 30 years) lowers the monthly obligation but increases the total interest cost.
What is a good interest rate?
Interest rates fluctuate based on the economy, inflation, and your personal credit score. It is advisable to shop around with multiple lenders to ensure you are getting the most competitive rate for your financial situation.