Estimate your monthly payments and total interest.
$
$
%
Monthly Payment:–
Total Loan Amount:–
Total Interest Paid:–
Total Cost of Loan:–
Payoff Date:–
function calculateMortgage() {
// 1. Get DOM elements
var homePriceInput = document.getElementById('mc_home_price');
var downPaymentInput = document.getElementById('mc_down_payment');
var interestRateInput = document.getElementById('mc_interest_rate');
var loanTermInput = document.getElementById('mc_loan_term');
var resultsArea = document.getElementById('mc_results_area');
// 2. Parse values
var homePrice = parseFloat(homePriceInput.value);
var downPayment = parseFloat(downPaymentInput.value);
var interestRate = parseFloat(interestRateInput.value);
var loanTermYears = parseFloat(loanTermInput.value);
// 3. Validation
if (isNaN(homePrice) || isNaN(downPayment) || isNaN(interestRate) || isNaN(loanTermYears)) {
alert("Please enter valid numeric values for all fields.");
return;
}
if (homePrice <= 0 || loanTermYears = homePrice) {
alert("Down payment cannot equal or exceed the home price.");
return;
}
// 4. Calculation Logic
var principal = homePrice – downPayment;
var monthlyInterestRate = (interestRate / 100) / 12;
var numberOfPayments = loanTermYears * 12;
var monthlyPayment = 0;
if (interestRate === 0) {
monthlyPayment = principal / numberOfPayments;
} else {
// Formula: M = P [ i(1 + i)^n ] / [ (1 + i)^n – 1 ]
var x = Math.pow(1 + monthlyInterestRate, numberOfPayments);
monthlyPayment = (principal * x * monthlyInterestRate) / (x – 1);
}
var totalCost = monthlyPayment * numberOfPayments;
var totalInterest = totalCost – principal;
// Calculate Payoff Date
var today = new Date();
var payoffDate = new Date(today.setMonth(today.getMonth() + numberOfPayments));
var monthNames = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"];
var dateString = monthNames[payoffDate.getMonth()] + " " + payoffDate.getFullYear();
// 5. Display Results
document.getElementById('mc_monthly_payment').innerText = formatCurrency(monthlyPayment);
document.getElementById('mc_total_loan').innerText = formatCurrency(principal);
document.getElementById('mc_total_interest').innerText = formatCurrency(totalInterest);
document.getElementById('mc_total_cost').innerText = formatCurrency(totalCost);
document.getElementById('mc_payoff_date').innerText = dateString;
// Show result box
resultsArea.style.display = 'block';
}
function formatCurrency(num) {
return '$' + num.toFixed(2).replace(/\d(?=(\d{3})+\.)/g, '$&,');
}
Understanding Your Mortgage Calculation
Buying a home is one of the most significant financial decisions you will make. This Mortgage Calculator is designed to help you understand what your monthly financial commitment will look like based on current market conditions.
How the Mortgage Formula Works
The standard formula used for fixed-rate mortgages determines your monthly principal and interest payment so that the loan is paid off exactly at the end of your term. The formula takes into account three main factors:
Principal (P): The total amount of money you are borrowing (Home Price minus Down Payment).
Interest Rate (r): The annual percentage rate charged by the lender. In the calculation, this is divided by 12 to get the monthly rate.
Number of Payments (n): The total number of months you will be paying the loan (Years × 12).
Why Your Down Payment Matters
Inputting a larger down payment in the calculator drastically reduces your monthly payment. For example, on a $300,000 home, increasing your down payment from 5% ($15,000) to 20% ($60,000) not only lowers the principal loan amount but also saves you tens of thousands of dollars in interest over the life of a 30-year loan.
Additionally, if your down payment is less than 20%, lenders often require Private Mortgage Insurance (PMI), which is an extra cost not included in the standard principal and interest calculation above.
Impact of the Loan Term
Choosing between a 15-year and a 30-year mortgage is a common dilemma:
30-Year Term: Offers lower monthly payments, making the home more affordable month-to-month, but results in significantly higher total interest costs.
15-Year Term: Requires higher monthly payments but allows you to build equity faster and pay far less interest overall.
Use the calculator above to compare these scenarios by simply changing the "Loan Term" field.
Frequently Asked Questions
Does this calculator include taxes and insurance?
No, the result shown is for Principal and Interest (P&I) only. Property taxes and homeowners insurance vary widely by location and provider. A common rule of thumb is to add 25-35% to the P&I figure to estimate your total "PITI" (Principal, Interest, Taxes, Insurance) payment.
What is a good interest rate?
Interest rates fluctuate daily based on the bond market and economic indicators. Your personal rate will also depend on your credit score, debt-to-income ratio, and down payment size. It is recommended to shop around with multiple lenders to ensure you get the most competitive rate.