This is an estimate for principal and interest only.
Additional costs like taxes, insurance, and HOA fees are not included.
Understanding Your Mortgage Payment
A mortgage is a significant financial commitment, and understanding how your monthly payment is calculated is crucial for informed decision-making. This calculator helps you estimate the principal and interest portion of your monthly mortgage payment.
The Mortgage Payment Formula
The standard formula for calculating a fixed-rate mortgage payment (M) is:
M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1]
Where:
M = Your total monthly mortgage payment (Principal & Interest)
P = The principal loan amount (the amount you borrow)
i = Your monthly interest rate. This is your annual interest rate divided by 12. (e.g., 3.5% annual rate becomes 0.035 / 12 = 0.00291667)
n = The total number of payments over the loan's lifetime. This is your loan term in years multiplied by 12. (e.g., a 30-year loan has 30 * 12 = 360 payments)
How the Calculator Works
Our calculator takes your inputs for the loan amount, annual interest rate, and loan term, and applies this formula to provide an estimated monthly payment for principal and interest.
Loan Amount (P): This is the initial amount you're borrowing to purchase your home.
Annual Interest Rate: This is the yearly percentage charged by the lender. The calculator converts this to a monthly rate by dividing by 12.
Loan Term (Years): This is the duration over which you agree to repay the loan. The calculator converts this to the total number of monthly payments.
The calculation determines how much you need to pay each month so that by the end of the loan term, the entire principal is repaid along with all the accrued interest.
Important Considerations
The monthly payment calculated here does not include other essential costs associated with homeownership, often referred to as PITI:
Principal: The amount borrowed.
Interest: The cost of borrowing money.
Taxes: Property taxes levied by your local government.
Insurance: Homeowner's insurance to protect against damage or loss.
Lenders often collect property taxes and homeowner's insurance premiums as part of your monthly payment and hold them in an escrow account. These additional costs can significantly increase your total monthly housing expense.
Why Use This Calculator?
Budgeting: Helps you understand how much house you can afford based on your income and desired monthly payment.
Comparison: Allows you to compare different loan scenarios (e.g., shorter vs. longer terms, different interest rates).
Financial Planning: Provides a key figure for your overall financial planning and savings goals.
Remember, this calculator provides an estimate. For precise figures, consult with a mortgage lender or financial advisor.
function calculateMortgage() {
var loanAmount = parseFloat(document.getElementById("loanAmount").value);
var annualInterestRate = parseFloat(document.getElementById("interestRate").value);
var loanTerm = parseInt(document.getElementById("loanTerm").value);
var monthlyPaymentElement = document.getElementById("monthlyPayment");
var resultElement = document.getElementById("result");
// Clear previous results and styles
monthlyPaymentElement.textContent = "$0.00";
monthlyPaymentElement.style.color = "#28a745"; // Reset to success green
resultElement.style.borderColor = "#004a99"; // Reset border color
// Input validation
if (isNaN(loanAmount) || loanAmount <= 0 ||
isNaN(annualInterestRate) || annualInterestRate < 0 ||
isNaN(loanTerm) || loanTerm <= 0) {
monthlyPaymentElement.textContent = "Invalid Input";
monthlyPaymentElement.style.color = "red";
resultElement.style.borderColor = "red";
return;
}
// Convert annual interest rate to monthly interest rate
var monthlyInterestRate = annualInterestRate / 100 / 12;
// Calculate the total number of payments
var numberOfPayments = loanTerm * 12;
var monthlyPayment = 0;
// Handle the edge case where interest rate is 0%
if (monthlyInterestRate === 0) {
monthlyPayment = loanAmount / numberOfPayments;
} else {
// Calculate monthly payment using the standard mortgage formula
// M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1]
var numerator = monthlyInterestRate * Math.pow(1 + monthlyInterestRate, numberOfPayments);
var denominator = Math.pow(1 + monthlyInterestRate, numberOfPayments) – 1;
monthlyPayment = loanAmount * (numerator / denominator);
}
// Format the monthly payment to two decimal places and add currency symbol
var formattedMonthlyPayment = "$" + monthlyPayment.toFixed(2);
monthlyPaymentElement.textContent = formattedMonthlyPayment;
monthlyPaymentElement.style.color = "#28a745"; // Success green
resultElement.style.borderColor = "#28a745"; // Success green border
}
// Optional: Add event listeners for range sliders to update number inputs
document.getElementById("loanAmount").addEventListener("input", function() {
document.getElementById("loanAmount").value = parseFloat(this.value).toFixed(0);
});
document.getElementById("interestRate").addEventListener("input", function() {
document.getElementById("interestRate").value = parseFloat(this.value).toFixed(1);
});
document.getElementById("loanTerm").addEventListener("input", function() {
document.getElementById("loanTerm").value = parseInt(this.value);
});
// Initial calculation on page load (optional, can be removed if not desired)
// window.onload = calculateMortgage;