function calculateMortgage() {
var homePrice = parseFloat(document.getElementById('homePrice').value);
var downPayment = parseFloat(document.getElementById('downPayment').value);
var annualRate = parseFloat(document.getElementById('interestRate').value);
var years = parseFloat(document.getElementById('loanTerm').value);
if (isNaN(homePrice) || isNaN(downPayment) || isNaN(annualRate) || isNaN(years) || homePrice <= 0) {
alert("Please enter valid positive numbers for all fields.");
return;
}
var principal = homePrice – downPayment;
if (principal <= 0) {
alert("Down payment cannot be greater than or equal to the home price.");
return;
}
var monthlyRate = (annualRate / 100) / 12;
var numberOfPayments = years * 12;
var monthlyPayment = 0;
if (monthlyRate === 0) {
monthlyPayment = principal / numberOfPayments;
} else {
var x = Math.pow(1 + monthlyRate, numberOfPayments);
monthlyPayment = (principal * x * monthlyRate) / (x – 1);
}
var totalCost = monthlyPayment * numberOfPayments;
var totalInterest = totalCost – principal;
document.getElementById('resMonthlyPayment').innerHTML = "$" + monthlyPayment.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2}) + "Estimated Monthly Payment";
document.getElementById('resPrincipal').innerText = "$" + principal.toLocaleString();
document.getElementById('resTotalInterest').innerText = "$" + totalInterest.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('resTotalCost').innerText = "$" + totalCost.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('mortgage-result-area').style.display = 'block';
}
How to Use the Mortgage Repayment Calculator
Planning to buy a home is one of the biggest financial decisions you will ever make. Our Mortgage Repayment Calculator is designed to provide you with a clear picture of your potential monthly commitments. By inputting just a few key variables, you can estimate your monthly principal and interest payments, helping you budget effectively for your future home.
Understanding the Key Inputs
Home Price: The total purchase price of the property you are looking to buy.
Down Payment: The amount of cash you pay upfront. A higher down payment reduces your loan principal and monthly interest.
Annual Interest Rate: The percentage charged by the lender for the loan. This varies based on market conditions and your credit score.
Loan Term: The duration of the loan, typically 15 or 30 years for residential mortgages.
The Mortgage Formula
Lenders use a specific formula to determine your fixed monthly payment. The math behind the calculator is as follows:
M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
Where:
M = Total monthly payment
P = Principal loan amount (Home Price – Down Payment)
i = Monthly interest rate (Annual Rate / 12 months)
n = Number of months (Loan Term in Years × 12)
Example Calculation
Suppose you are purchasing a home for $400,000 with a $80,000 (20%) down payment. You secure a 30-year fixed-rate mortgage at an interest rate of 6%.
Principal: $320,000
Monthly Rate: 0.005 (6% / 12)
Number of Payments: 360 (30 years × 12)
Using the formula, your monthly payment would be approximately $1,918.56. Over the life of the loan, you would pay a total of $370,682 in interest.
Why Your Monthly Payment Might Be Higher
Note that this calculator estimates Principal and Interest (P&I). In a real-world scenario, your monthly mortgage bill may also include:
Property Taxes: Levied by your local government.
Homeowners Insurance: Required by lenders to protect the asset.
Private Mortgage Insurance (PMI): Usually required if your down payment is less than 20%.
HOA Fees: If you are moving into a managed community or condo.
Frequently Asked Questions
How does a longer loan term affect my payment?
A longer term (like 30 years) spreads the principal over more months, resulting in a lower monthly payment compared to a 15-year term. However, you will pay significantly more in total interest over the life of the longer loan.
Does this calculator include closing costs?
No. Closing costs, which typically range from 2% to 5% of the purchase price, are usually paid upfront at the time of purchase and are not factored into the monthly repayment calculation unless they are rolled into the loan amount.