This calculator helps you estimate your monthly principal and interest payment for a mortgage, a common type of loan used to purchase real estate. Understanding this payment is crucial for budgeting and financial planning when buying a home.
How the Calculation Works
The monthly mortgage payment is calculated using the following standard formula for an amortizing loan:
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 (e.g., 4.5% annual rate becomes 0.045 / 12 = 0.00375 monthly rate).
n = The total number of payments over the loan's lifetime. This is calculated by multiplying your loan term in years by 12 (e.g., a 30-year loan has 30 * 12 = 360 payments).
Example Calculation
Let's say you are taking out a mortgage with the following terms:
Loan Amount (P): $250,000
Annual Interest Rate: 4.5%
Loan Term: 30 Years
First, we convert these to the formula's variables:
This means your estimated monthly principal and interest payment for this loan would be approximately $1,267.60.
Important Considerations
The payment calculated by this tool is for Principal and Interest (P&I) only. Your actual total monthly housing expense will likely be higher and may include:
Property Taxes
Homeowner's Insurance Premiums
Private Mortgage Insurance (PMI) if your down payment is less than 20%
Homeowner Association (HOA) Dues, if applicable
It's recommended to consult with a UW Credit Union mortgage specialist to get a precise loan estimate that includes all potential costs and to explore different loan programs that best fit your financial situation.
function calculateMortgage() {
var loanAmount = parseFloat(document.getElementById("loanAmount").value);
var annualInterestRate = parseFloat(document.getElementById("annualInterestRate").value);
var loanTerm = parseInt(document.getElementById("loanTerm").value);
var resultValueElement = document.getElementById("result-value");
if (isNaN(loanAmount) || isNaN(annualInterestRate) || isNaN(loanTerm) ||
loanAmount <= 0 || annualInterestRate < 0 || loanTerm <= 0) {
resultValueElement.textContent = "Invalid input";
return;
}
var monthlyInterestRate = annualInterestRate / 100 / 12;
var numberOfPayments = loanTerm * 12;
var monthlyPayment;
if (monthlyInterestRate === 0) {
monthlyPayment = loanAmount / numberOfPayments;
} else {
monthlyPayment = loanAmount * (monthlyInterestRate * Math.pow(1 + monthlyInterestRate, numberOfPayments)) / (Math.pow(1 + monthlyInterestRate, numberOfPayments) – 1);
}
if (isNaN(monthlyPayment) || !isFinite(monthlyPayment)) {
resultValueElement.textContent = "Error";
return;
}
resultValueElement.textContent = "$" + monthlyPayment.toFixed(2);
}
// Initialize calculation on page load if values are present
document.addEventListener('DOMContentLoaded', function() {
calculateMortgage();
});