Buying a home is a significant financial undertaking, and understanding your mortgage payment is crucial. This calculator helps you estimate your principal and interest (P&I) payment based on the loan amount, interest rate, and loan term. This estimate does not include property taxes, homeowner's insurance, or private mortgage insurance (PMI), which would increase your total monthly housing expense.
How the Calculation Works
The formula used to calculate the monthly mortgage payment (M) is derived from the standard annuity formula:
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 calculated by dividing your annual interest rate by 12. For example, a 5.5% annual rate becomes 0.055 / 12 = 0.004583.
n = The total number of payments over the loan's lifetime. This is calculated by multiplying the loan term in years by 12. For a 30-year mortgage, n = 30 * 12 = 360.
Example Calculation:
Let's say you're looking to borrow $300,000 with an annual interest rate of 5.5% for a 30-year term.
This calculation results in an estimated monthly principal and interest payment of approximately $1,698.90.
Important Considerations:
This is an estimate: The actual payment may vary slightly due to lender calculations and rounding.
Taxes and Insurance (PITI): Your total monthly housing payment (often called PITI) will include Principal, Interest, Taxes, and Insurance. This calculator only provides the Principal & Interest portion.
PMI: If your down payment is less than 20%, you'll likely need to pay Private Mortgage Insurance (PMI), which adds to your monthly cost.
Escrow: Property taxes and homeowner's insurance are often collected monthly by your lender and held in an escrow account.
Adjustable vs. Fixed Rates: This calculator assumes a fixed-rate mortgage. Adjustable-rate mortgages (ARMs) have payments that can change over time.
Use this calculator as a starting point to understand the core cost of your mortgage. Always consult with a mortgage professional for precise figures and personalized advice.
function calculateMortgage() {
var loanAmount = parseFloat(document.getElementById("loanAmount").value);
var annualInterestRate = parseFloat(document.getElementById("annualInterestRate").value);
var loanTermYears = parseFloat(document.getElementById("loanTermYears").value);
var resultDiv = document.getElementById("result");
var monthlyPaymentDisplay = document.getElementById("monthlyPayment");
if (isNaN(loanAmount) || isNaN(annualInterestRate) || isNaN(loanTermYears) ||
loanAmount <= 0 || annualInterestRate < 0 || loanTermYears 0) {
monthlyPayment = loanAmount * (monthlyInterestRate * Math.pow(1 + monthlyInterestRate, numberOfPayments)) / (Math.pow(1 + monthlyInterestRate, numberOfPayments) – 1);
} else {
// If interest rate is 0, payment is just principal divided by number of payments
monthlyPayment = loanAmount / numberOfPayments;
}
if (!isNaN(monthlyPayment) && monthlyPayment >= 0) {
monthlyPaymentDisplay.textContent = "$" + monthlyPayment.toFixed(2);
resultDiv.style.display = 'block';
} else {
resultDiv.style.display = 'none';
}
}