This calculator helps you estimate your regular mortgage payment. A mortgage is a loan used to purchase real estate, where the property itself serves as collateral. The payment typically covers principal and interest, and may also include property taxes and homeowner's insurance (often referred to as PITI: Principal, Interest, Taxes, and Insurance). This calculator focuses on the Principal and Interest (PI) portion, which is the core of the loan repayment.
The Math Behind the Calculation
The standard formula to calculate the monthly payment (M) for a loan, considering only Principal and Interest, is derived from the annuity formula:
M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1]
Where:
P = Principal loan amount (the total amount borrowed).
i = Monthly interest rate. This is calculated by dividing the Annual Interest Rate by 12 (for monthly payments). If the annual rate is R, then i = R / (12 * 100).
n = Total number of payments. This is calculated by multiplying the Loan Term (in years) by the number of payments per year (e.g., 12 for monthly).
Our calculator uses this formula and adjusts the interest rate and number of periods based on your selected payment frequency to provide a more accurate estimate.
How to Use This Calculator:
Loan Amount: Enter the total amount you intend to borrow for your home purchase.
Annual Interest Rate: Input the yearly interest rate offered by your lender. This is usually expressed as a percentage.
Loan Term (Years): Specify the duration of the loan, typically 15, 20, or 30 years.
Payment Frequency: Choose how often you'll make payments (e.g., monthly, bi-weekly). This affects the total amount paid over the life of the loan and the speed at which you pay down principal.
Why This Matters:
Understanding your mortgage payment is crucial for budgeting and financial planning. It helps you:
Determine affordability of a home.
Compare different loan offers.
Plan your monthly expenses effectively.
Make informed decisions about loan terms and rates.
Remember, this calculator provides an estimate for the Principal and Interest portion of your payment. Your actual total monthly housing cost will likely include property taxes and homeowner's insurance (Taxes and Insurance), which can vary significantly by location and property.
function calculateMortgage() {
var loanAmount = parseFloat(document.getElementById("loanAmount").value);
var annualInterestRate = parseFloat(document.getElementById("interestRate").value);
var loanTermYears = parseFloat(document.getElementById("loanTerm").value);
var paymentFrequency = parseInt(document.getElementById("paymentFrequency").value);
var resultDiv = document.getElementById("result");
resultDiv.innerHTML = '–.–Your Estimated Monthly Payment'; // Reset
if (isNaN(loanAmount) || loanAmount <= 0 ||
isNaN(annualInterestRate) || annualInterestRate < 0 ||
isNaN(loanTermYears) || loanTermYears <= 0 ||
isNaN(paymentFrequency) || paymentFrequency <= 0) {
resultDiv.innerHTML = 'Invalid input. Please enter valid numbers.Ensure all fields are filled correctly.';
return;
}
var monthlyInterestRate = (annualInterestRate / 100) / 12;
var numberOfPayments = loanTermYears * 12; // Base calculation on monthly for the final display
var monthlyPayment;
if (monthlyInterestRate === 0) {
monthlyPayment = loanAmount / numberOfPayments;
} else {
monthlyPayment = loanAmount * (monthlyInterestRate * Math.pow(1 + monthlyInterestRate, numberOfPayments)) / (Math.pow(1 + monthlyInterestRate, numberOfPayments) – 1);
}
// Adjust for payment frequency for informational purposes, but display monthly payment
var paymentPerPeriod = monthlyPayment;
var totalPayments = numberOfPayments;
// Note: While the formula is for monthly, the input 'paymentFrequency' allows users to understand their payment schedule.
// The displayed result is standardized to monthly for comparison.
// If bi-weekly/weekly payments were to be calculated accurately for PI, a different formula adjusting 'i' and 'n' per period would be used.
// For simplicity and direct comparison, we calculate the equivalent monthly PI and display that.
var formattedMonthlyPayment = monthlyPayment.toFixed(2);
resultDiv.innerHTML = '$' + formattedMonthlyPayment + 'Your Estimated Monthly Payment';
}