Mortgage Payment Calculator
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Mortgage Payment Calculator
Your Estimated Payment
$0.00
Per Period
Understanding Your Mortgage Payment
A mortgage is a significant financial commitment, and understanding how your monthly (or periodic) payment is calculated is crucial for budgeting and financial planning. This calculator helps you estimate your regular mortgage payments based on the loan amount, interest rate, loan term, and payment frequency.
The Math Behind the Mortgage Payment
The standard formula for calculating the periodic payment (M) of a mortgage is derived from the formula for the present value of an annuity:
M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1]
Where:
- M = Your periodic payment (the amount you'll pay each period)
- P = The principal loan amount (the total amount borrowed)
- i = The periodic interest rate. This is calculated by dividing the annual interest rate by the number of payment periods in a year (e.g., Annual Rate / 12 for monthly payments).
- n = The total number of payments over the loan's lifetime. This is calculated by multiplying the number of years in the loan term by the number of payment periods in a year (e.g., Loan Term in Years * 12 for monthly payments).
How the Calculator Works:
- Loan Amount (Principal): This is the total sum you are borrowing to purchase your home.
- Annual Interest Rate: This is the yearly rate charged by the lender. The calculator converts this to a periodic rate.
- Loan Term (Years): The total duration of the loan, typically 15, 20, or 30 years. This is used to calculate the total number of payments.
- Payment Frequency: This determines how often you make payments (e.g., monthly, bi-weekly, weekly). This significantly impacts the total interest paid and how quickly you build equity. More frequent payments generally mean paying less interest over time.
The calculator takes these inputs, calculates the periodic interest rate and the total number of payments, and then applies the formula above to provide your estimated payment per period. It also includes checks to ensure valid numerical inputs are provided.
Why Use a Mortgage Calculator?
- Budgeting: Accurately estimate your largest monthly expense to ensure it fits your financial situation.
- Affordability: Determine how much house you can realistically afford.
- Comparison: Compare loan offers from different lenders or explore different loan terms and interest rates.
- Understanding Trade-offs: See how a slightly higher interest rate or a longer term can increase your total borrowing cost.
- Extra Payments: While this calculator focuses on the base payment, understanding it is the first step to figuring out how extra payments can accelerate your payoff.
Remember, this calculator provides an estimate. Your actual mortgage payment may include additional costs such as property taxes, homeowner's insurance, and private mortgage insurance (PMI), often bundled into an escrow payment (known as PITI – Principal, Interest, Taxes, Insurance).
function calculateMortgage() {
var principal = parseFloat(document.getElementById("principal").value);
var annualInterestRate = parseFloat(document.getElementById("interestRate").value);
var loanTermYears = parseFloat(document.getElementById("loanTerm").value);
var paymentFrequency = parseInt(document.getElementById("paymentFrequency").value, 10);
var errorMessageDiv = document.getElementById("error-message");
var resultDiv = document.getElementById("result");
errorMessageDiv.innerText = ""; // Clear previous errors
// Input validation
if (isNaN(principal) || principal <= 0) {
errorMessageDiv.innerText = "Please enter a valid Loan Amount greater than zero.";
return;
}
if (isNaN(annualInterestRate) || annualInterestRate < 0) {
errorMessageDiv.innerText = "Please enter a valid Annual Interest Rate (cannot be negative).";
return;
}
if (isNaN(loanTermYears) || loanTermYears <= 0) {
errorMessageDiv.innerText = "Please enter a valid Loan Term greater than zero.";
return;
}
if (isNaN(paymentFrequency) || paymentFrequency <= 0) {
errorMessageDiv.innerText = "Please select a valid Payment Frequency.";
return;
}
var monthlyInterestRate = annualInterestRate / 100 / paymentFrequency;
var numberOfPayments = loanTermYears * paymentFrequency;
var monthlyPayment = 0;
if (monthlyInterestRate === 0) {
// Handle zero interest rate case
monthlyPayment = principal / numberOfPayments;
} else {
// Standard mortgage payment formula
var numerator = principal * monthlyInterestRate * Math.pow(1 + monthlyInterestRate, numberOfPayments);
var denominator = Math.pow(1 + monthlyInterestRate, numberOfPayments) – 1;
monthlyPayment = numerator / denominator;
}
// Format the result
var formattedPayment = monthlyPayment.toLocaleString(undefined, {
minimumFractionDigits: 2,
maximumFractionDigits: 2
});
resultDiv.innerHTML = "$" + formattedPayment + "
Per Period";
}