Mortgage Repayment Calculator
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Mortgage Calculator
Estimate your monthly payments and total interest costs.
$
$
%
Years
Please enter valid positive numbers for all fields.
Estimated Monthly Payment
$0.00
Loan Amount
$0
Total Interest
$0
Total Cost
$0
Payoff Date
–
Understanding Your Mortgage Calculation
Buying a home is one of the most significant financial decisions you will make. This Mortgage Repayment Calculator helps you understand the financial commitment by breaking down your estimated monthly payments based on the home price, down payment, interest rate, and loan term.
How the Mortgage Formula Works
Standard fixed-rate mortgages are calculated using an amortization formula. This ensures that your payment remains constant throughout the life of the loan, though the portion covering principal vs. interest changes over time.
The core formula used is:
M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
M: Total monthly payment
P: Principal loan amount (Home Price minus Down Payment)
i: Monthly interest rate (Annual rate divided by 12)
n: Number of payments (Loan term in years multiplied by 12)
Key Factors Affecting Your Payment
1. Principal and Down Payment
The principal is the amount of money you borrow. The larger your down payment, the lower your principal, which results in lower monthly payments and less interest paid over the life of the loan. A common benchmark is 20%, which also helps you avoid Private Mortgage Insurance (PMI).
2. Interest Rate
Even a small difference in interest rates can drastically affect your monthly payment and total loan cost. For example, on a $300,000 loan, a 1% difference in rate can save or cost you tens of thousands of dollars over 30 years.
3. Loan Term
The loan term is the duration of the mortgage. The most common terms are 15 and 30 years.
30-Year Term: Lower monthly payments, but higher total interest costs.
15-Year Term: Higher monthly payments, but you build equity faster and pay significantly less interest.
Frequently Asked Questions
Does this calculator include taxes and insurance?
This specific calculation focuses on Principal and Interest (P&I). To get a full picture of your housing costs ("PITI"), you should add estimated monthly property taxes, homeowners insurance, and HOA fees to the result provided above.
How much house can I afford?
Financial experts generally recommend that your monthly mortgage payment should not exceed 28% of your gross monthly income. Additionally, your total debt payments (including student loans, car payments, and credit cards) should ideally stay under 36% of your income.
What happens if I make extra payments?
Making extra payments toward your principal reduces the loan balance faster. This shortens the loan term and reduces the total amount of interest you pay. Even one extra payment per year can shave years off a 30-year mortgage.
function calculateMortgage() {
// Get input values
var homePrice = parseFloat(document.getElementById('homePrice').value);
var downPayment = parseFloat(document.getElementById('downPayment').value);
var interestRate = parseFloat(document.getElementById('interestRate').value);
var loanTermYears = parseFloat(document.getElementById('loanTerm').value);
var errorMsg = document.getElementById('errorMsg');
var resultsSection = document.getElementById('resultsSection');
// Validation
if (isNaN(homePrice) || isNaN(downPayment) || isNaN(interestRate) || isNaN(loanTermYears) ||
homePrice <= 0 || interestRate < 0 || loanTermYears = home price
if (principal <= 0) {
document.getElementById('monthlyPayment').innerHTML = "$0.00";
document.getElementById('loanAmountResult').innerHTML = "$0";
document.getElementById('totalInterestResult').innerHTML = "$0";
document.getElementById('totalCostResult').innerHTML = "$" + homePrice.toLocaleString();
document.getElementById('payoffDateResult').innerHTML = "N/A";
resultsSection.style.display = 'block';
return;
}
var monthlyRate = (interestRate / 100) / 12;
var numberOfPayments = loanTermYears * 12;
var monthlyPayment = 0;
// If interest rate is 0, simple division
if (interestRate === 0) {
monthlyPayment = principal / numberOfPayments;
} else {
// Mortgage Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
monthlyPayment = principal * ( (monthlyRate * Math.pow(1 + monthlyRate, numberOfPayments)) / (Math.pow(1 + monthlyRate, numberOfPayments) – 1) );
}
var totalCost = monthlyPayment * numberOfPayments;
var totalInterest = totalCost – principal;
// Calculate Payoff Date
var today = new Date();
var payoffDate = new Date(today.setMonth(today.getMonth() + numberOfPayments));
var monthNames = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"];
var payoffString = monthNames[payoffDate.getMonth()] + " " + payoffDate.getFullYear();
// Update DOM
document.getElementById('monthlyPayment').innerHTML = "$" + monthlyPayment.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('loanAmountResult').innerHTML = "$" + principal.toLocaleString(undefined, {minimumFractionDigits: 0, maximumFractionDigits: 0});
document.getElementById('totalInterestResult').innerHTML = "$" + totalInterest.toLocaleString(undefined, {minimumFractionDigits: 0, maximumFractionDigits: 0});
document.getElementById('totalCostResult').innerHTML = "$" + totalCost.toLocaleString(undefined, {minimumFractionDigits: 0, maximumFractionDigits: 0});
document.getElementById('payoffDateResult').innerHTML = payoffString;
resultsSection.style.display = 'block';
}