A mortgage is a significant financial commitment, and understanding how your monthly payment is calculated is crucial. The primary components that determine your principal and interest payment are the loan amount, the annual interest rate, and the loan term.
This calculator helps you estimate your P&I (Principal and Interest) payment. Keep in mind that your actual total monthly housing cost will likely be higher, as it often includes property taxes, homeowner's insurance (often escrowed), and potentially Private Mortgage Insurance (PMI) or Homeowner Association (HOA) fees. These additional costs are not included in this calculation.
How it works: The formula used is the standard annuity formula for loan amortization. It calculates a fixed monthly payment that will pay off the loan over its entire term, considering the compounding interest.
Example: If you borrow $250,000 at an annual interest rate of 5% for 30 years, your estimated monthly Principal & Interest payment would be approximately $1,342.05. This amount covers both the repayment of the principal loan amount and the interest charged over the life of the loan.
function calculateMortgage() {
var loanAmount = parseFloat(document.getElementById("loanAmount").value);
var annualInterestRate = parseFloat(document.getElementById("annualInterestRate").value);
var loanTermYears = parseInt(document.getElementById("loanTermYears").value);
var resultDiv = document.getElementById("result");
// Clear previous results
resultDiv.innerHTML = "";
// Input validation
if (isNaN(loanAmount) || loanAmount <= 0) {
resultDiv.innerHTML = "Please enter a valid loan amount.";
return;
}
if (isNaN(annualInterestRate) || annualInterestRate < 0) {
resultDiv.innerHTML = "Please enter a valid annual interest rate.";
return;
}
if (isNaN(loanTermYears) || loanTermYears <= 0) {
resultDiv.innerHTML = "Please enter a valid loan term in years.";
return;
}
var monthlyInterestRate = annualInterestRate / 100 / 12;
var numberOfPayments = loanTermYears * 12;
var monthlyPayment = 0;
// Mortgage Payment Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1]
// Where:
// M = Monthly Payment
// P = Principal Loan Amount
// i = Monthly Interest Rate
// n = Total Number of Payments (Loan Term in Months)
if (monthlyInterestRate === 0) {
// Handle the case of 0% interest rate
monthlyPayment = loanAmount / numberOfPayments;
} else {
monthlyPayment = loanAmount * (monthlyInterestRate * Math.pow(1 + monthlyInterestRate, numberOfPayments)) / (Math.pow(1 + monthlyInterestRate, numberOfPayments) – 1);
}
// Format the result to two decimal places
var formattedMonthlyPayment = monthlyPayment.toFixed(2);
resultDiv.innerHTML = "Estimated Monthly Payment (P&I): $" + formattedMonthlyPayment + "";
}
// Function to update the display for the rate – not strictly necessary for calculation but good UX
function updateRateDisplay() {
var rateInput = document.getElementById("annualInterestRate");
if (rateInput.value !== "" && rateInput.value.indexOf('%') === -1) {
// You might want to prevent this if the input type is number, but for clarity of intent:
// For a number input, we don't add '%', but if it were text, you'd handle it.
// In this case, it's a number input, so no '%' is added. The label already indicates it.
}
}