A mortgage is a loan specifically used to purchase real estate, such as a home. The loan is typically repaid over a long period, often 15 to 30 years, with regular payments consisting of both principal and interest. This calculator helps you estimate your monthly mortgage payment based on the loan amount, interest rate, and loan term.
How the Calculation Works
The standard formula used to calculate the monthly mortgage payment (M) is:
M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1]
Where:
P = Principal loan amount (the total amount you borrow).
i = Monthly interest rate. This is your annual interest rate divided by 12. For example, a 5% annual rate becomes 0.05 / 12 = 0.004167.
n = Total number of payments over the loan's lifetime. This is the loan term in years multiplied by 12. For a 30-year mortgage, n = 30 * 12 = 360.
This formula provides an estimate for the principal and interest portion of your monthly mortgage payment. It does not include other costs like property taxes, homeowner's insurance, or private mortgage insurance (PMI), which are often included in your total monthly housing expense (known as PITI: Principal, Interest, Taxes, Insurance).
Key Factors Influencing Your Mortgage Payment
Loan Amount (Principal): The larger the amount you borrow, the higher your monthly payments will be.
Interest Rate: This is one of the most significant factors. A lower interest rate means you pay less in interest over the life of the loan, resulting in a lower monthly payment. Even a small difference in the annual rate can lead to substantial savings over decades.
Loan Term: Mortgages are typically offered with terms like 15, 20, or 30 years. A shorter loan term means you'll pay off the loan faster but will have higher monthly payments. A longer loan term results in lower monthly payments but means you'll pay more interest over time.
Using This Calculator
Enter the total amount you plan to borrow for your home purchase, the expected annual interest rate (as a percentage), and the number of years you intend to take to repay the loan. The calculator will then provide an estimated monthly payment for the principal and interest components. This tool is invaluable for budgeting and comparing different mortgage offers.
function updateInterestRateDisplay() {
var rate = document.getElementById("interestRate").value;
document.getElementById("interestRateValue").innerText = parseFloat(rate).toFixed(2) + "%";
}
function updateLoanTermDisplay() {
var term = document.getElementById("loanTerm").value;
document.getElementById("loanTermValue").innerText = term + " years";
}
function calculateMortgage() {
var loanAmount = parseFloat(document.getElementById("loanAmount").value);
var annualInterestRate = parseFloat(document.getElementById("interestRate").value);
var loanTermYears = parseInt(document.getElementById("loanTerm").value);
var resultDiv = document.getElementById("result");
// Basic validation
if (isNaN(loanAmount) || isNaN(annualInterestRate) || isNaN(loanTermYears) || loanAmount <= 0 || annualInterestRate <= 0 || loanTermYears <= 0) {
resultDiv.innerHTML = "Please enter valid positive numbers for all fields.";
return;
}
var monthlyInterestRate = annualInterestRate / 100 / 12;
var numberOfPayments = loanTermYears * 12;
// Mortgage Payment 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;
if (denominator === 0) { // Avoid division by zero, though unlikely with valid inputs
resultDiv.innerHTML = "Calculation error. Please check inputs.";
return;
}
var monthlyPayment = loanAmount * (numerator / denominator);
if (isNaN(monthlyPayment) || !isFinite(monthlyPayment)) {
resultDiv.innerHTML = "Calculation resulted in an invalid number. Please check inputs.";
} else {
resultDiv.innerHTML = "$" + monthlyPayment.toFixed(2) + " / month";
}
}
// Link sliders to number inputs and update displays on load and input
var interestRateInput = document.getElementById("interestRate");
var interestRateSlider = document.getElementById("interestRateSlider");
var loanTermInput = document.getElementById("loanTerm");
var loanTermSlider = document.getElementById("loanTermSlider");
interestRateSlider.oninput = function() {
interestRateInput.value = this.value;
updateInterestRateDisplay();
};
interestRateInput.oninput = function() {
interestRateSlider.value = this.value;
updateInterestRateDisplay();
};
loanTermSlider.oninput = function() {
loanTermInput.value = this.value;
updateLoanTermDisplay();
};
loanTermInput.oninput = function() {
loanTermSlider.value = this.value;
updateLoanTermDisplay();
};
// Initialize displays on page load
updateInterestRateDisplay();
updateLoanTermDisplay();
// Set initial values for number inputs from sliders
interestRateInput.value = interestRateSlider.value;
loanTermInput.value = loanTermSlider.value;