A mortgage is a significant financial commitment, and understanding how your monthly payment is calculated is crucial. This calculator helps estimate your principal and interest payment based on the loan amount, interest rate, and loan term. The standard formula used is the:
Mortgage Payment Formula
The formula for calculating the monthly mortgage payment (M) is:
M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1]
Where:
M = Your total monthly mortgage payment (principal and interest)
P = The principal loan amount (the total amount you borrow)
i = Your monthly interest rate (annual rate divided by 12)
n = The total number of payments over the loan's lifetime (loan term in years multiplied by 12)
How the Calculator Works:
1. Loan Amount (P): This is the total sum of money you are borrowing to purchase your home.
2. Annual Interest Rate: This is the yearly percentage charged by the lender. For the calculation, we convert this to a monthly interest rate by dividing it by 12. For example, a 4.5% annual rate becomes 0.045 / 12 = 0.00375 monthly.
3. Loan Term (Years): This is the duration of your mortgage. We convert this into the total number of monthly payments (n) by multiplying the years by 12. A 30-year mortgage has 360 payments (30 * 12).
4. The calculator then plugs these values into the formula above to compute your estimated monthly payment.
Important Considerations:
This calculator provides an estimate for the principal and interest portion of your mortgage payment. It does not include other costs that are often part of your total monthly housing expense, such as:
Property Taxes
Homeowner's Insurance
Private Mortgage Insurance (PMI), if applicable
Homeowner Association (HOA) fees
Always consult with your lender for a precise mortgage payment quote and to understand all associated costs.
function calculateMortgage() {
var loanAmountInput = document.getElementById("loanAmount");
var interestRateInput = document.getElementById("interestRate");
var loanTermInput = document.getElementById("loanTerm");
var monthlyPaymentOutput = document.getElementById("monthlyPayment");
var principal = parseFloat(loanAmountInput.value);
var annualRate = parseFloat(interestRateInput.value);
var loanTermYears = parseFloat(loanTermInput.value);
if (isNaN(principal) || isNaN(annualRate) || isNaN(loanTermYears) || principal <= 0 || annualRate < 0 || loanTermYears <= 0) {
monthlyPaymentOutput.textContent = "Please enter valid positive numbers.";
monthlyPaymentOutput.style.color = "#dc3545"; // Red for error
return;
}
var monthlyRate = annualRate / 100 / 12;
var numberOfPayments = loanTermYears * 12;
var monthlyPayment = (principal * monthlyRate * Math.pow(1 + monthlyRate, numberOfPayments)) / (Math.pow(1 + monthlyRate, numberOfPayments) – 1);
if (isNaN(monthlyPayment) || !isFinite(monthlyPayment)) {
monthlyPaymentOutput.textContent = "Calculation error. Please check inputs.";
monthlyPaymentOutput.style.color = "#dc3545"; // Red for error
} else {
monthlyPaymentOutput.textContent = "$" + monthlyPayment.toFixed(2);
monthlyPaymentOutput.style.color = "#28a745"; // Green for success
}
}